Thrust anticlines responding to gravitational instability deep water offshore, Niger Delta Basin

Thrust anticlines responding to gravitational instability deep water offshore, Niger Delta Basin | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Thrust anticlines responding to gravitational instability deep water offshore, Niger Delta Basin Abubakar Maunde This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8087043/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The deep-water slope area of the Niger Delta is increasingly becoming one of the world’s hydrocarbon potential areas as the progress of exploration off the shelf and down the slope has intensified. High-resolution three-dimensional (3-D) seismic data from the outer fold and thrust belt region of Niger delta are used to investigate the geometry and progressive development of thrust anticlines through a detailed structural restoration. The results show that the outer fold and thrust belt region of the Niger Delta is characterised by closely spaced synthetic forethrust faults, antithetic backthrust faults and verging thrust anticlines with a well-developed NW-SE linear trend perpendicular to the gentle seabed bathymetric slope. These structures respond to accommodate up-dip extension due to instabilities at the shelf by sliding down the detachment to form toe thrusts and associated folds. Identifying the geometry and progressive development of thrust anticlines is key to understanding how thrust anticlines evolve and grow. It also provides information on the timing of fault activity, a parameter directly affecting sub-surface fluid flow in geological reservoirs. 3D seismic data Thrust anticline Niger Delta Restoration Unfolding Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 1. Introduction In the deep-water Niger Delta settings, gravity tectonism became the primary deformational process after the cessation of rifting. This process has induced a continuum of internal deformation that is expressed in the form of imbricate thrust faults, thrust anticlines, detachment folds, shale diapirs, roll-over anticlines and megasplay faults (Damuth, 1994 ; Erickson, 1996 ; Wu and Bally, 2000 ; Connors et al., 1998 ; Suppe et al., 2004 ; Corredor et al., 2005 ; Bilotti and Shaw, 2005 ; Briggs et al., 2006 ). The gravitational collapsed of overburden rocks often generates folds that are accompanied by outer-arc extensional and inner-contractional rock fractures (Price and Cosgrove, 1990 ; Frehner, 2011), which are typically localised and not basement-linked. Such fractures accommodate a significant part of the local tectonic strain that occurs during the development of folds and broader, horizontally shortened, structural traps (Yeats, 1986; Schlische, 1995 ; Mitra, 2002; Brandes and Tanner, 2014). Overburden rocks in the deep-water Niger Delta exhibit a wide range of complexities in their structural and stratigraphic characteristics, due to differences in hydrodynamic conditions prevalent in the depositional setting. As petroleum exploration becomes increasingly focused on the deep-water Niger Delta settings, insights into the geometry and progressive evolution of the thrust anticlines that are located in the toe regions of the gravity tectonic system and their suitability as hydrocarbon traps have become important. Despite the growing number of studies addressing the regional structural styles in the Niger Delta (e.g., Damuth, 1994 ; Erickson, 1996 ; Wu and Bally, 2000 ; Connors et al., 1998 ; Suppe et al., 2004 ; Corredor et al., 2005 ; Bilotti and Shaw, 2005 ; Briggs et al., 2006 ), the published literature has thus far overlooked the geometries and progressive development of the local thrust anticlines offshore Niger Delta, in great part due to the relative lack of extensive high resolution 3D seismic reflection data acquired in its outer wedge region. This results in a relative underrepresentation of the true structural evolution of the deep-water Niger Delta as a whole. With the advancement of high resolution three-dimensional (3D) seismic data, recent investigations have mostly focused on studying the regional structural styles, channels interaction with growth structures, sedimentary depocenters associated with compressional tectonics and detachment levels (e.g., Bilotti and Shaw, 2005 ; Corredor et al., 2005 ; Briggs et al., 2006 ; Heinio and Davies, 2006; Jolly et al., 2016 ). However, despite the growing number of studies there is currently limited documentation on the geometry and progressive development of the thrust anticlines and their suitability as hydrocarbon traps in the Niger Delta Basin. Therefore, this paper aims at evaluating the geometry and progressive evolution of thrust anticlines through the detail mapping and structural restoration of thrust anticlines using high-resolution 3D seismic data from the outer folds and thrust belt region of the deep-water Niger Delta basin (Figs. 1 and 2 ). 2. Structural styles of the deep-water Niger Delta basin Offshore, deep-water region of the Niger-delta basin is presently undergoing thin-skinned gravitational collapse driven by differential loading of the advancing delta, resulting in downslope translation of the delta front and slope deposits on major detachment levels within the marine shales of the Akata Formation (Damuth, 1994 ; Bilotti and Shaw, 2005 ; Corredor et al., 2005 ; Briggs et al., 2006 ). Damuth ( 1994 ) and Connors et al. ( 1998 ) classified the Niger Delta basin into three major (3) structural zones; 1) upper extensional, 2) intermediate translational, and 3) lower compressional zones. Subsequently, Corredor et al. ( 2005 ) studied the structural styles in the deep-water fold and thrust belt of the Niger delta and further sub-divided the Niger delta basin into five (5) main structural provinces; 1) an extensional province beneath the continental shelf, 2) a mud diapir zone located beneath the upper continental slope, 3) the inner fold and thrust belt, 4) a transitional detachment fold zone beneath the lower continental slope, and 5) the outer fold and thrust belt province - area of interest for this study (Fig. 2 ). Corredor et al. ( 2005 ) explained the reversal in vergence from one-fold or thrust to the next or reversal in vergence within a single structure using the critical taper theory and established that individual thrust faults detach at different levels within the stratigraphic succession i.e. presence of multiple detachment levels (Briggs et al., 2006 ). Morley ( 2007 ) established that the thrust fault in the thin-skinned deformation have overall basinward verging sequence with some out-of-sequence thrusting developed as a result of reactivation of older in-sequence thrust. Erickson ( 1996 ) studied the influence of mechanical stratigraphy on folding versus faulting and established that the wavelength, amplitude and asymmetry of thrust related fold structures depend on the strength and thickness of the deformed competent layer. Suppe et al. ( 2004 ) and Corredor et al. ( 2005 ) studied structural styles in the outer fold and thrust belt and interpreted them to be fault-propagation fold and detachment fold as well as simple and pure- shear fault bend folds. The Niger Delta exhibits comparable rock properties, but an anomalously low taper angle compared with most orogenic fold and thrust belts and this low taper shape leads to the extensive development of backthrust zones as well as relatively undeformed regions that separate the deep-water fold and thrust belts (Billoti and Shaw, 2005) (Fig. 2 ). Briggs et al. ( 2006 ) studied multiple detachment levels and their control on fold styles in the compressional domain of the deep-water west Niger Delta and demonstrated that regional variations in thrust and fold styles are related to the characteristics of the detachment surface resulting in contrasting styles of thrust propagation and fold growth in the deep-water west Niger delta. According to them, detachments are located within the Dahomey unit, the transition between the Agbada and Top Akata Formations and within the Akata Formation. Changes in vergence of hanging wall anticline show that interaction exists between detaching Fore-thrust and back thrust in the deep-water Niger Delta 3. 3-D seismic data and methods 3.1. 3-D seismic data The 3-D seismic base map of the study area is located off the western margin of deep-water fold and thrust belts region of the Niger Delta basin (Figs. 1 and 2 ). It covers an approximated area of 32.6 km by 18 km in a water depth ranging between 2000 and 3000 m (Figs. 1 and 2 ). The 3-D seismic data were acquired using dual airgun-sources and six (6) streamer cables. Each streamer cables is 6000 m-long separated by a spacing of 50 m from each other, giving a coverage of 120-fold and a maximum lateral resolution of 12.5 m. The data were recorded with a 2 ms vertical sampling interval, and a 12.5 x 12.5 m bin size spacing. The seismic data was zero-phase post stack time migrated with a dominant frequency of 40 Hz. Based on an assumed interval velocities ranging between 2400 and 2000 m/s - and the dominant frequency content of the seismic data (40 Hz), the vertical resolution of the seismic data was estimated to be 15 m in the shallow sedimentary section, and 12.5 m in the deepest parts of the section. The overall quality/resolution of the 3-D seismic data was good and thus permitted a detailed visualisation and mapping of fault traces and seismic units. However, some of the reflections were weak in the footwall regions beneath the major thrust planes (Fig. 4 ). 3.2. Methods 3.2.1 Mapping of seismic horizons and units Seismic horizons and units were identified and mapped using the Petrel™ seismic interpretation software. Three (3) major seismic units (S1 to S3) were mapped and correlated with stratigraphic information from Tuttle et al. ( 1999 ) (Fig. 3 ). These seismic units include Unit S1 (Akata Formation), Unit S2 (Agbada Formation) and the shallower Unit S3 (Benin Formation) (Fig. 3 ; Table 1 ). Table 1 Internal seismic character of the interpreted seismic units’ deep water offshore Niger Delta basin. S/N Stratigraphic Formations/Age (Tuttle et al., 1999 ). Main lithologies (Tuttle et al., 1999 ). Interpreted seismic units Internal seismic character 1 Benin Formation (Pleistocene to Recent) Sandstones, grits, claystone and streaks of lignite. Unit S3 Characterised by a package of high-frequency, moderate- to high-amplitude and continuous seismic reflections. 2 Agbada Formation (Miocene to Pleistocene) Deltaic sandstones with shale. Unit S2 Unit S2d Characterized by moderately to high amplitude, fairly continuous folded reflector with a gentle relief hanging wall anticlinal crest of about 172.5 m high, the surface thinned and overlapped the fold crest. Unit S2c Characterized by moderately to high amplitude, fairly continuous folded reflector with a fairly high relief hanging wall anticlinal crest of about 575 m high, the surface also thinned and overlapped the fold crest. Unit S2b Characterized by moderately to high amplitude, fairly continuous folded and faulted reflector with high relief hanging wall anticlinal crest of about 684m high, the surface shows tectonic thickening at the fold crest Unit S2a Characterized by moderately to high amplitude, fairly continuous folded and faulted reflector with a high relief hanging wall anticlinal crest of about 687 m high, the surface also shows tectonic thickening at the fold crest. 3 Akata Formation (Eocene to Miocene) Marine dark grey shales and silts, with rare streaks of sand. Unit S1 Chaotic internal reflections with low amplitude. The top and bottom of the unit (H2 and H1) are marked by a strong seismic reflector that is highly continuous with strong amplitude and sparsely faulted. In the Niger Delta basin, the Agbada Formation Unit S2) is the most structurally complex formation that is largely affected by syn-sedimentary structural features (e.g., thrust anticlines), and thus was identified by it internally deformed wedge of sediments (Fig. 4 ). The Akata Formation (Unit S1) was identified by thick package of sediments that was devoid of internal reflections (Fig. 4 ). In the deep-water region of the Niger Delta, the Benin Formation (Unit S3) thin and disappears seawards (Morgan, 2004 ; Rouby et al., 2011 ). The Agbada Formation (Unit S2) is the main interval of interest for this paper, as this unit was deformed by thrust anticlines. Also, the Agbada Formation (Unit S2) comprise reservoir intervals inside which petroleum derived from Akata Formation (Unit S1) is stored (Tuttle et al., 1999 ) (Fig. 4 ). Moreover, Intra-Agbada horizon (HA) was mapped in detail throughout the 3-D seismic volume. From the Intra-Agbada structural map and seismic section in Figs. 4 and 5 , three (3) major thrust anticlines (A to C) were identified. Subsequently, four (4) sub-seismic horizons (HA to HD) and four (4) sub-seismic units (S2a to S2d) through the target Thrust Anticline B were mapped to quantify their structural geometry and progressive evolution in the study area (Figs. 4 to 6 ). 3.2.2 Structural restorations From the 3D seismic volume, three (3) seismic inline sections (i.e. 7884, 7984 and 8084) were selected through the target Thrust Anticline B for the purpose of 2-D structural restoration. These seismic inlines (7884, 7984 and 8084) approximately slice through the crest of the target Thrust Anticline B (Fig. 7 ). The picks of the five mapped horizons (H2, HA to HD) across the target Thrust Anticline B were imported into 2-D Move™ software to produce line cross sections for the selected three (3) seismic inline sections (7884, 7984 and 8084). A velocity of 2000 m/s was assumed to perform time-depth conversion on the line sections using Move™ software after which polygons for the cross-sections were created (Fig. 7 ). 2-D Move makes use of the following equation. $$\:z=\left(\frac{{v}_{o}}{k}\right)\:\times\:\left[\left({e}^{kt}\right)-1\right]\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:(\text{E}\text{q}.\:1)$$ Where \(\:z\) is the depth in meters (m), \(\:{v}_{o}\) is the initial velocity in meter per second (m/s), \(\:k\) is the rate of change in velocity with increasing depth (assumed to be constant) and \(\:t\) is the one-way travel time in seconds (s). Therefore, two (2) structural restoration operations including move on fault and unfolding operations were applied to the selected line cross-sections 7884, 7984 and 8084 (Fig. 7 ). 3.2.2.1 Move on fault operation Move on fault operation aimed at removing the deformation caused by faulting and is based on a decision about how the rocks deformed by faulting (Buddin et al 1997 ). Trishear algorithm was used to restore the fault offset sequentially. The procedure used was to move along forethrust fault F1 first to restore it offset and then move along backthrust fault F2 in a section where the two faults exist (Fig. 8 ). Therefore, the Move on fault operation using the trishear algorithm was applied to the target thrust faults F1 and F2 for all the seismic lines cross sections through the Thrust Anticline B (Fig. 8 ). 3.2.2.2 Unfolding operation Unfolding operation aimed at reconstructing the progressive evolution of the anticlinal structure with time. In the seismic section, the interpreted thrust faults F1 and F2 are restricted to Unit S2 (Fig. 4 ) i.e. not offsetting the entire sedimentary units. This signifies deformation of the underlying units by flexure, and thus flexural slip algorithm was deemed suitable for this deformation type. Therefore, sequential unfolding operation using the flexural slip algorithm was applied to the target Thrust Anticline B for all the seismic inlines cross sections (Figs. 9 to 11 ). Flexural slip unfolding tool was used to unfold the interpreted seismic units (S2a to S2d) sequentially, thus moving the unfolded units below the template bed to their pre-deformation positions. Firstly, unfolding was done using the upper Unit S2d as the template bed to which unfolding was done, subsequently followed by unfolding to Units S2c, S2b and S2a for all the three (3) seismic line sections (Figs. 9 to 11 ). 3.2.3 Surface maps of restored horizons After the move on fault and unfolding restoration operations, Data from the three (3) seismic inline sections (i.e., 7884, 7984 and 8084) were imported into Petrel™ software from 2-D Move™ software as general lines and points for 3-D surface maps generation (Figs. 12 b to 15 b). This was done by adding up the point data from 2-D Move™ for the respective surface across the three (3) seismic inlines to form a single 3-D point data with which the surface was created to determine the geometry and progressive development of the Thrust Anticline B (Figs. 12 to 15 ). Therefore, Figs. 12 to 15 highlighted the 3-D geometric expression of the interpreted seismic surfaces/horizons before and after structural restoration. 3.3 Uncertainties in structural restoration Restoration operations are prone to uncertainties which are inherent in the choice of algorithms and parameters assumed - these must be recognized and considered during the analysis stage. These uncertainties can come from the values of interval velocities used in depth conversion. Variations in the assumed values can be very sensitive to the final output. For instance an increase in the average interval velocity will cause the horizons to appear deeper whereas, reduction in the average interval velocity will cause the horizons to appear shallower. In addition to these uncertainties resulting from parameters, using an algorithm that is not particularly suited for the structural scenario introduces some form of error. Nevertheless, the technique used in this paper to quantify the structural restoration of thrust anticlines is robust considering the high quality/resolution of the seismic volume (Figs. 4 and 5 ). 4. Seismic - stratigraphic correlation Three (3) seismic units including, Units S1 to S3 were mapped and correlated with stratigraphic information of Niger Delta Basin (Fig. 3 ; Table 1 ). 4.1 Unit S1: Akata Formation Unit S1 correlates with the Akata Formation and comprises thick marine shales sequences that are believed to contain source rocks (Corredor et al., 2005 ; Doust and Omatsola, 1990 ) (Fig. 3 ). Unit S1 is characterised by its chaotic internal seismic reflections with low amplitude. The top of the unit (horizon H2) is marked by a strong seismic reflector and thus serves as a detachment surface from which thrust anticlines in the study area are detached (Fig. 4 ). 4.2 Unit S2: Agbada Formation Unit S2 correlates with the Agbada Formation and comprises alternating deltaic sandstones with shale deposited in fluvial-deltaic environments. The unit is the main interval of interest for this paper - it forms a package of high-amplitude, high-frequency seismic reflections that is locally deformed by closely spaced thrust anticlines (Figs. 4 and 5 ). For the purposes of structural restoration in this work, Unit 2 was sub-divided into four (4) sub-units that include Units S2a, S2b, S2c and S2d (See Table 1 for their internal seismic character). Unit S2 is conformable above the bright reflections that mark the top of Unit S1 (Akata Formation). It also comprises the reservoir intervals from which oil is sourced from the Akata Formation (Unit S1) (Nwachukwu and Odjegba, 2001 ). 4.2 Unit S3: Benin Formation Unit S3 correlates with the Benin Formation, and comprises sandstones, grits, claystone and streaks of lignite (Avbovbo, 1978 ). In seismic data, Unit S3 form a package of high-frequency, continuous and moderate to high-amplitude seismic reflections. The unit overlies the Agbada Formation (Unit S2) and is bounded at its top by the seafloor horizon (Figs. 3 and 4 ). 5. Geometric expression of thrust anticlines Seismic imaging shows that the study area is characterised by closely spaced synthetic forethrust faults, antithetic backthrust faults and verging thrust anticlines with a well-developed NW-SE linear trend perpendicular to the gentle seabed bathymetric slope (Figs. 4 and 5 ). These thrust anticlines are restricted within Unit S2 (Agbada Formation) and thus accommodate updip, gravity-driven extension on the continental shelf in deep-water Niger Delta (Corredor et al., 2005 ). The thrust anticlines are asymmetrical and offset different parts of the Agbada Formation (Unit S2) and flatten onto detachment planes at the top of Akata surfaces (Horizon H2; Fig. 4 ). Three (3) major thrust anticlines (A to C) which can be traced for up to 6.5 km along dip were identified in the study area (Figs. 4 and 5 ). These thrust anticlines are restricted and terminated within the upper parts of Agbada Formation (Unit S2), with no structures extending to the seabed surface (i.e. the thrust anticlines are completely buried with a package of thin Unit S3 (Benin Formation; Fig. 4 ). The crests of the thrust anticlines are separated by 8-11.5 km (Figs. 4 and 5 ). The forethrusts of the Thrust Anticlines (A and C) dominantly shows a seaward-vergence with Thrust Anticline B showing out of thrusts sequence i.e. verging-landward due to reactivation of the older in sequence thrusts as a result of low taper angle of the delta (Corredor et al., 2005 ). The backthrust faults intersect the forethrust faults and dominantly show a seaward- vergence (Fig. 4 ). Therefore, the seaward-vergence of the fold crests indicates the overall tectonic transport direction that formed the structures (Figs. 4 and 5 ). Thrust Anticline A is the most landward of the thrust anticlines. It is linear in map view and formed above a seaward-verging forethrust. An antithetic backthrust, which intersects the forethrust, deforms the back limb of the anticline (Fig. 4 ). Thrust Anticline A comprises a much narrower, shorter wavelength anticline compared to seawards Thrust Anticline C, with a much broader, longer wavelength (Figs. 4 and 5 ). Thrust Anticline B is a landward-verging structure compared to Thrust Anticlines A and C. it consists of two closely spaced thrusts. The seaward-verging forethrust dominates the structure. A second backthrust cuts the backlimb of the frontal thrust and dies out upwards, with no faults reaching the sea floor (Figs. 4 and 5 ). Thrust Anticline B comprises a much narrower, shorter wavelength anticline compared to Thrust Anticlines A and C (Figs. 4 and 5 ). Thrust Anticline C is the most distal of seawards structure and extends outside the seismic dataset (Figs. 4 and 5 ). The seaward-verging forethrust dominates the structure. Thrust Anticline C comprises a much broader, longer wavelength in the study area (Fig. 5 ). 6. Progressive evolution of thrust anticlines The modes of evolution and growth of the interpreted thrust anticlines were investigated using the geometric expression of the restored surfaces/horizons on the seismic line sections 7884, 7984 and 8084 (Figs. 8 to 15 . 6.1 Restored seismic line section 7884 The restored seismic line section (7884) through the crest of the Thrust Anticline B revealed that during the deposition of the Unit S2a, the Unit S1 was relatively flat with a buckle synclinal geometry at its centre (Fig. 9 a). During the deposition of Unit S2b, a symmetric anticline started to develop towards the southwest (Fig. 9 b). During the subsequent deposition of Unit S2c, the anticline growth increased showing westward vergence. The anticline continues growing to present day after deposition of Unit S2d (Fig. 9 c, d). The forethrust fault propagates through the Units S2a and S2b and remains active during the deposition of Unit S2c, with no thrust intersecting the Unit S2c surface (Horizon HC) i.e. the thrust terminated at the base of Unit S2c (Fig. 9 d). The backthrust intersect the forethrust within the Unit S2a and terminated at the base of Unit S2b (Horizon HA; Fig. 9 b-e). 6.2 Restored seismic line section 7984 The restored dip-line section 7984 across the Thrust Anticline B revealed that during the deposition of Unit S2a, the Unit S1 surface (H1) was relatively flat with a buckle synclinal geometry (Fig. 10 a). During the deposition of Unit S2b, the Unit S2a was almost flat while it was slightly folded together with the Unit S2b forming an anticline with westward vergence and associated syncline developing during Unit S2c deposition. The growth of the anticlinal crest continued with the Unit S2d deposition. At this point, the Units S2c, S2b and S2a have already formed a higher relief anticlinal structure, this growth continued to the present day as evident from thinning of syn-growth sequences above fold crests (Figs. 4 and 10 ). The forethrust fault propagates through the Units S2a and S2b and remains active during the deposition of Unit S2c, with no thrust intersecting the Unit S2c surface (HC) i.e. the thrust terminated at the base of Unit S2c (Fig. 10 c-e). This seismic line section (7984) was picked toward the flank of the fold where the backthrust signature disappears (Fig. 10 ). The forethrust also shows eastward vergence. 6.3 Restored seismic line section 8084 The restored seismic line section (8084) across the thrust anticline B revealed that during the deposition of Unit S2a, the Unit S1 was relatively flat. Also, Unit S2a was flat at the time of Unit S2b deposition. However, a gentle anticline develops during the of deposition of Unit S2c (Fig. 11 ). The anticline developed progressively during the deposition of Unit S2d, leading to a higher relief structure as evident from thinning of syn-growth sequences above fold crest (Figs. 5 and 11 ). The forethrust fault propagates through the Units S2a and S2b, and remains active during the deposition of Unit S2c, with no thrust intersecting the Unit S2c surface (HC) i.e. the thrust terminated at the base of Unit S2c (Fig. 11 ). The dip-line section (8084) was picked at the flank of the fold where the backthrust signature disappears (Fig. 11 ). Therefore, the restored results show that the Thrust Anticline B originated as a buckle fold and subsequently cut by several basinward and landward dipping thrusts that penetrated the fold at different depth levels (Figs. 4 , 9 to 11 ). 7. Discussion 7.1 Structural styles in the outer fold and thrust belt region of the Niger Delta The outer fold and thrust belt region of the Niger Delta basin comprises seaward (basinward) and landward (hinterland)-verging anticlines with their associated thrust faults (Corredor et al, 2005 ) (Figs. 4 and 5 ). These structures resulted from the effect of gravity gliding and spreading upon basinward dipping detachment and sloping seabed (Fig. 4 ). The thrust anticlines respond to accommodate up-dip extension due to instabilities at the shelf by sliding down the detachment to form toe thrusts and associated folds that are majorly seaward-verging except for some landward verging folds with associated back-thrust formed due to the low taper angle in the delta (Bilotti et al., 2005) or changes in the properties of the detachment surface. The amplitude and size of these thrust anticlines are related to the amount of up dip extensions. The rate of growth of the anticline over time is closely related to the rate of up-dip extension which may be further related to the rate of deposition at the shelf. The top Akata surface (Horizon H2) shows location of thrust base which suggests that it played a major role in controlling the distinct deformation styles of thrust and thrust related folds (Figs. 4 and 6 f). This corresponds to one of the three possible detachment surfaces according to Briggs et al., 2006 which was reported to be mobile and highly over pressured. Therefore, the evolution of the interpreted thrust anticline in the outer fold and thrust belt region of the deep-water Niger delta is linked to the combined effect of gravity gliding and spreading, and thus, does not involve the basement (thin skinned tectonics). 7.2 Structural evolution of thrust anticlines in the outer fold and thrust belt of Niger Delta basin The sequential restoration of the seismic line sections through the thrust anticline was used to reconstruct its evolution with time. The deformation of the anticline evolved through multiple phases of deformation that began approximately during the of deposition of Unit S2a (Figs. 9 to 11 ). At this early stage, the thrust fault F1 was active, and the horizon H2 which has been interpreted here as a possible detachment surface had features of buckle folding (Figs. 9 b to 11 b). At the time of Unit S2b deposition, horizon HA had formed a symmetric fold of about 800 m high toward the southwest (Figs. 9 b and 13 ), while horizon HB showed evidence of thickening signifying deposition while faulting was occurring. Simultaneous deposition and deformation continued with faulting during the deposition of Unit S2c, the relief of the fold increased to about 1,100 m, asymmetric and developed a north eastwards vergence (Figs. 9 c and 13 ). Deposition and deformation continued through time causing crest thickening in the fold during the present-day deposition of Unit S2d (Figs. 9 d, e, 14 and 15 ). Therefore, the interpreted seismic units are syn-kinematic in nature showing differential growth with deposition (Figs. 9 to 11 ). The detachment layer/surface is the horizon H2 (top Akata Formation; Fig. 4 ) which corresponds to one of the detachment surfaces according to Briggs et al., 2006 . A schematic model in Fig. 16 highlights the structural evolution of the interpreted thrust anticlines. The thrust fold started to evolve as a symmetric buckle fold in response to gravity instability and subsequent gliding on the detachment which simultaneously led to the formation of toe thrusts down-dip to accommodate up-dip extensions (Fig. 16a). Basinward-verging, thrust fault F3 was firstly formed whereas continuous compression led to the development of basinward-verging thrust faults F4 (Figs. 16b). Moreover, landward verging thrust fault F1 developed alongside thrust fault F4, due to either reduction in critical taper angle or change in the properties of the detachment surface (Fig. 16b). Figure 16 As deposition continued, the fold tightened and another zone of weakness developed which resulted to the initiation of thrust fault F2 (Fig. 16c). These thrusts continued to propagate with deposition and hence changing the geometry of the structure from a buckle fold to a northeast verging (landward) fault propagation fold with symmetric and asymmetric growth packages at southwest and northeast of the fold respectively (Fig. 16c). 8. Conclusions The conclusions derived from the 3-D seismic interpretation and structural restoration can be summarised as follows: The outer fold and thrust belt region of the Niger Delta is characterised by closely spaced synthetic forethrust faults, antithetic backthrust faults and verging thrust anticlines with a well-developed NW-SE linear trend perpendicular to the gentle seabed bathymetric slope. The top Akata surface (horizon H2) provide suitable detachment surfaces for gravity collapse. The studied thrust anticlines are thrust propagation fold that has been growing over time as evident by the folding of recent sediments above the anticlinal structures. Declarations Declaration of competing interest The author declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Declaration of data availability The data supporting the findings of this study are available within the paper. Acknowledgements Petroleum Technology Development Fund (PTDF), Nigeria was acknowledged and thanked for funding the research study. We acknowledge the permission approved by DPR and CGG-Veritas for the use of the seismic data included in this article and Schlumberger for the provision of seismic interpretation software (Petrela). Author Contribution Maunde analyse and prepare the manuscripts References Avbovbo, A. 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Wu, S., and Bally, A.W., 2000, Slope tectonics— Comparisons and contrasts of structural styles of salt and shale tectonics of the northern Gulf of Mexico with shale tectonics of offshore Nigeria in Gulf of Guinea, in W. Mohriak and M. Talwani, eds., Atlantic rifts and continental margins: Washington, D.C., American Geophysical Union, p. 151–172. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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1","display":"","copyAsset":false,"role":"figure","size":301756,"visible":true,"origin":"","legend":"\u003cp\u003ea) Map of Nigeria highlighting the major rock distributions and location of the study area (red box) in the deep-water offshore Niger Delta basin.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/0721af3c6493724a15620ad0.jpeg"},{"id":96914096,"identity":"5808786d-e337-467d-a049-bb68f2f802b1","added_by":"auto","created_at":"2025-11-27 14:05:25","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":351300,"visible":true,"origin":"","legend":"\u003cp\u003ea) Map of the Niger Delta region highlighting the major structural provinces and location of the study area (red box) in the deep-water outer fold and thrust belt region. Figure is modified from Connors et al. (1998). b) Schematic NE-SW cross section highlighting the structural zones and location of the study area within the deep-water outer fold and thrust belt region of the Niger Delta. Figure is modified from Corredor et al. (2005). (See Fig. 2a for line profile A-B location.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/eaeb400b8e67ce890bc84033.jpeg"},{"id":96914263,"identity":"d9d515af-65fd-4887-b994-1d3711ed2d82","added_by":"auto","created_at":"2025-11-27 14:05:39","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":404936,"visible":true,"origin":"","legend":"\u003cp\u003eSeismic-stratigraphic correlation amongst interpreted seismic units, the main stratigraphic Formations and dominant lithologies recognised in the Niger Delta Basin. Stratigraphic Formations and main lithologies are based on Tuttle et al. (1999). Unit S1 represent Akata Formation. Unit S2 is Agbada Formation and Unit S3 represent Benin Formation. H1 to H3 and S1 to S3 are interpreted seismic horizons and units respectively.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/28b5232e346e09c72a95e3da.jpeg"},{"id":96913389,"identity":"28e16103-f4bd-4ca1-97a8-2e897edc7022","added_by":"auto","created_at":"2025-11-27 14:00:20","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":656574,"visible":true,"origin":"","legend":"\u003cp\u003eInterpreted seismic section across the deep-water outer fold and thrust belt region of the Niger Delta basin highlighting the major structural styles (thrust anticlines), interpreted seismic units (Units S1 to S3) and the three Thrust Anticlines major A to C in the study area.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/1acf8de236334e01b4f49ef0.jpeg"},{"id":96740598,"identity":"0a716dcd-95e6-4e39-97d8-30f8fd8d8df0","added_by":"auto","created_at":"2025-11-25 14:56:41","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":423945,"visible":true,"origin":"","legend":"\u003cp\u003eTWTT structural map of the Intra Agbada Formation (Horizon - HA) highlighting the structural expression of the Thrust Anticlines (A to C) in the study area.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/c37ad50e37828d6494ce5f55.jpeg"},{"id":96740600,"identity":"4c6fc40c-0c4c-4879-b213-79177d346f35","added_by":"auto","created_at":"2025-11-25 14:56:41","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":631901,"visible":true,"origin":"","legend":"\u003cp\u003eTWTT structural maps of the interpreted horizons through Thrust Anticline B highlighting the geometric expression of the studied Thrust Anticline B. a) 3D map view of the interpreted horizons (H2, HA to HD) and thrust faults (F1 and F2) through Thrust Anticline B. b) Structural map of Horizon HD. c) Structural map of Horizon HC. d) Structural map of Horizon HB. e) Structural map of Horizon HA and finally, f) Structural map of Horizon H2 through the Thrust Anticline B. See Fig. 4 for the interpreted seismic Horizons (H2, HA to HD) through Thrust Anticline B.\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/f380a347a73fbce4ea049271.jpeg"},{"id":96913745,"identity":"c0cc70dc-8d87-47de-9ee6-050e626f13af","added_by":"auto","created_at":"2025-11-27 14:04:10","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":393885,"visible":true,"origin":"","legend":"\u003cp\u003eCross sections and their corresponding polygons highlighting the geometric expression of the Thrust Anticline B. a) Cross section and polygon for seismic inline 7884. c) Cross section and polygon for seismic inline 7984. d) Cross section and polygon for seismic inline 8084. Horizons (H2, HA to HD) are interpreted seismic horizons through the target Thrust Anticline B. See Fig. 4 for the interpreted seismic Horizons (H2, HA to HD).\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/bacc5c0f1d2c0d04944caf29.jpeg"},{"id":96740604,"identity":"26522c3c-80a5-4647-8015-70180b307693","added_by":"auto","created_at":"2025-11-25 14:56:41","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":386357,"visible":true,"origin":"","legend":"\u003cp\u003eCross sections highlighting the geometric expression of the Thrust Anticline B before (present day) and after (restored) faults (F1 and F2) restoration. a) present day and restored faults sections for seismic line 7884. b) present day and restored fault sections for seismic line 7984. c) present day and restored fault sections for seismic inline 8084.\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/6fdf6c2bfed7ead71dede76b.jpeg"},{"id":96913402,"identity":"0407acd2-9950-4e52-8444-e4474068dfb9","added_by":"auto","created_at":"2025-11-27 14:00:39","extension":"jpeg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":860566,"visible":true,"origin":"","legend":"\u003cp\u003eSequential unfolding operations for seismic inline section 7884 highlighting the geometric expression of the Thrust Anticline B with time of seismic units’ deposition, starting from deposition of older Unit S2a to the time of deposition of present-day geometry. a) Fold geometry after deposition of seismic Unit S2a. b) Fold geometry after deposition of seismic Unit S2b. c) Fold geometry after deposition of seismic Unit S2c. d) Fold geometry after deposition of seismic Unit S2d and e) Present day fold geometry.\u003c/p\u003e","description":"","filename":"floatimage9.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/c8ac4785b162f85650c20f47.jpeg"},{"id":96915161,"identity":"672a2d47-5db2-4807-943f-f59a130fe4f2","added_by":"auto","created_at":"2025-11-27 14:06:56","extension":"jpeg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":803495,"visible":true,"origin":"","legend":"\u003cp\u003eSequential unfolding operations for seismic inline section 7984 highlighting the geometric expression of the Thrust Anticline B with time of seismic units’ deposition, starting from deposition of older Unit S2a to the time of deposition of present-day geometry. a) Fold geometry after deposition of seismic Unit S2a. b) Fold geometry after deposition of seismic Unit S2b. c) Fold geometry after deposition of seismic Unit S2c. d) Fold geometry after deposition of seismic Unit S2d and e) Present day fold geometry.\u003c/p\u003e","description":"","filename":"floatimage10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/e408d37c273e78fc12ba4f8a.jpeg"},{"id":96914189,"identity":"2a375c9b-2df6-4805-b4f2-6d06468baef3","added_by":"auto","created_at":"2025-11-27 14:05:33","extension":"jpeg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":886892,"visible":true,"origin":"","legend":"\u003cp\u003eSequential unfolding operations for seismic inline section 8084 highlighting the geometric expression of the Thrust Anticline B with time of seismic units’ deposition, starting from deposition of older Unit S2a to the time of deposition of present-day geometry. a) Fold geometry after deposition of seismic Unit S2a. b) Fold geometry after deposition of seismic Unit S2b. c) Fold geometry after deposition of seismic Unit S2c. d) Fold geometry after deposition of seismic Unit S2d and e) Present day fold geometry.\u003c/p\u003e","description":"","filename":"floatimage11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/1a6dd5662c276af588839a9b.jpeg"},{"id":96913752,"identity":"f4e0442f-885f-476e-8c26-0802c92e7662","added_by":"auto","created_at":"2025-11-27 14:04:11","extension":"jpeg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":258864,"visible":true,"origin":"","legend":"\u003cp\u003eTWTT structural maps highlighting geometric expression of a) present day surface and b) restored surface of the seismic Unit S2a through Thrust Anticline B.\u003c/p\u003e","description":"","filename":"floatimage12.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/8968405ced0adac65d7f2d2d.jpeg"},{"id":96914938,"identity":"2baed33d-e0b9-4ba3-9477-1ed605cba8c2","added_by":"auto","created_at":"2025-11-27 14:06:39","extension":"jpeg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":210222,"visible":true,"origin":"","legend":"\u003cp\u003eTWTT structural maps highlighting geometric expression of a) present day surface and b) restored surface of the seismic Unit S2b through Thrust Anticline B.\u003c/p\u003e","description":"","filename":"floatimage13.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/83b12df154aa655b9129cab2.jpeg"},{"id":96740627,"identity":"6370fef3-6854-4d9d-8e30-639b3c99feba","added_by":"auto","created_at":"2025-11-25 14:56:42","extension":"jpeg","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":247866,"visible":true,"origin":"","legend":"\u003cp\u003eTWTT structural maps highlighting geometric expression of a) present day surface and b) restored surface of the seismic Unit S2c through Thrust Anticline B.\u003c/p\u003e","description":"","filename":"floatimage14.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/8d9d2e763b5fb17e29121475.jpeg"},{"id":96913755,"identity":"defc04fd-76a1-4075-bd69-1f938afe74b5","added_by":"auto","created_at":"2025-11-27 14:04:11","extension":"jpeg","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":259278,"visible":true,"origin":"","legend":"\u003cp\u003eTWTT structural maps highlighting geometric expression of a) present day surface and b) restored surface of the seismic Unit S2d through Thrust Anticline B.\u003c/p\u003e","description":"","filename":"floatimage15.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/ae2dd71b8b620959a7e81a6d.jpeg"},{"id":96740622,"identity":"5324d0d4-eb9f-47c1-a523-8613078a797a","added_by":"auto","created_at":"2025-11-25 14:56:42","extension":"jpeg","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":323728,"visible":true,"origin":"","legend":"\u003cp\u003eLegend not included with this version.\u003c/p\u003e","description":"","filename":"floatimage16.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/26cf03b56152482ed226af27.jpeg"},{"id":97671589,"identity":"50eaf9d4-75a4-477f-b8a1-89d886640912","added_by":"auto","created_at":"2025-12-08 09:32:46","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8271847,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8087043/v1/f097e07b-ad1d-49f6-8b11-3a84cc63fe71.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Thrust anticlines responding to gravitational instability deep water offshore, Niger Delta Basin","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn the deep-water Niger Delta settings, gravity tectonism became the primary deformational process after the cessation of rifting. This process has induced a continuum of internal deformation that is expressed in the form of imbricate thrust faults, thrust anticlines, detachment folds, shale diapirs, roll-over anticlines and megasplay faults (Damuth, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Erickson, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Wu and Bally, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Connors et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Suppe et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Corredor et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Bilotti and Shaw, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Briggs et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe gravitational collapsed of overburden rocks often generates folds that are accompanied by outer-arc extensional and inner-contractional rock fractures (Price and Cosgrove, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Frehner, 2011), which are typically localised and not basement-linked. Such fractures accommodate a significant part of the local tectonic strain that occurs during the development of folds and broader, horizontally shortened, structural traps (Yeats, 1986; Schlische, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Mitra, 2002; Brandes and Tanner, 2014).\u003c/p\u003e\u003cp\u003eOverburden rocks in the deep-water Niger Delta exhibit a wide range of complexities in their structural and stratigraphic characteristics, due to differences in hydrodynamic conditions prevalent in the depositional setting. As petroleum exploration becomes increasingly focused on the deep-water Niger Delta settings, insights into the geometry and progressive evolution of the thrust anticlines that are located in the toe regions of the gravity tectonic system and their suitability as hydrocarbon traps have become important.\u003c/p\u003e\u003cp\u003eDespite the growing number of studies addressing the regional structural styles in the Niger Delta (e.g., Damuth, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Erickson, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Wu and Bally, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Connors et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Suppe et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Corredor et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Bilotti and Shaw, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Briggs et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), the published literature has thus far overlooked the geometries and progressive development of the local thrust anticlines offshore Niger Delta, in great part due to the relative lack of extensive high resolution 3D seismic reflection data acquired in its outer wedge region. This results in a relative underrepresentation of the true structural evolution of the deep-water Niger Delta as a whole.\u003c/p\u003e\u003cp\u003eWith the advancement of high resolution three-dimensional (3D) seismic data, recent investigations have mostly focused on studying the regional structural styles, channels interaction with growth structures, sedimentary depocenters associated with compressional tectonics and detachment levels (e.g., Bilotti and Shaw, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Corredor et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Briggs et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Heinio and Davies, 2006; Jolly et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). However, despite the growing number of studies there is currently limited documentation on the geometry and progressive development of the thrust anticlines and their suitability as hydrocarbon traps in the Niger Delta Basin. Therefore, this paper aims at evaluating the geometry and progressive evolution of thrust anticlines through the detail mapping and structural restoration of thrust anticlines using high-resolution 3D seismic data from the outer folds and thrust belt region of the deep-water Niger Delta basin (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"2. Structural styles of the deep-water Niger Delta basin","content":"\u003cp\u003eOffshore, deep-water region of the Niger-delta basin is presently undergoing thin-skinned gravitational collapse driven by differential loading of the advancing delta, resulting in downslope translation of the delta front and slope deposits on major detachment levels within the marine shales of the Akata Formation (Damuth, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Bilotti and Shaw, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Corredor et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Briggs et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eDamuth (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1994\u003c/span\u003e) and Connors et al. (\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) classified the Niger Delta basin into three major (3) structural zones; 1) upper extensional, 2) intermediate translational, and 3) lower compressional zones. Subsequently, Corredor et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) studied the structural styles in the deep-water fold and thrust belt of the Niger delta and further sub-divided the Niger delta basin into five (5) main structural provinces; 1) an extensional province beneath the continental shelf, 2) a mud diapir zone located beneath the upper continental slope, 3) the inner fold and thrust belt, 4) a transitional detachment fold zone beneath the lower continental slope, and 5) the outer fold and thrust belt province - area of interest for this study (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eCorredor et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) explained the reversal in vergence from one-fold or thrust to the next or reversal in vergence within a single structure using the critical taper theory and established that individual thrust faults detach at different levels within the stratigraphic succession i.e. presence of multiple detachment levels (Briggs et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Morley (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) established that the thrust fault in the thin-skinned deformation have overall basinward verging sequence with some out-of-sequence thrusting developed as a result of reactivation of older in-sequence thrust. Erickson (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) studied the influence of mechanical stratigraphy on folding versus faulting and established that the wavelength, amplitude and asymmetry of thrust related fold structures depend on the strength and thickness of the deformed competent layer.\u003c/p\u003e\u003cp\u003eSuppe et al. (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) and Corredor et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) studied structural styles in the outer fold and thrust belt and interpreted them to be fault-propagation fold and detachment fold as well as simple and pure- shear fault bend folds. The Niger Delta exhibits comparable rock properties, but an anomalously low taper angle compared with most orogenic fold and thrust belts and this low taper shape leads to the extensive development of backthrust zones as well as relatively undeformed regions that separate the deep-water fold and thrust belts (Billoti and Shaw, 2005) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eBriggs et al. (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) studied multiple detachment levels and their control on fold styles in the compressional domain of the deep-water west Niger Delta and demonstrated that regional variations in thrust and fold styles are related to the characteristics of the detachment surface resulting in contrasting styles of thrust propagation and fold growth in the deep-water west Niger delta. According to them, detachments are located within the Dahomey unit, the transition between the Agbada and Top Akata Formations and within the Akata Formation. Changes in vergence of hanging wall anticline show that interaction exists between detaching Fore-thrust and back thrust in the deep-water Niger Delta\u003c/p\u003e"},{"header":"3. 3-D seismic data and methods","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1. 3-D seismic data\u003c/h2\u003e\u003cp\u003eThe 3-D seismic base map of the study area is located off the western margin of deep-water fold and thrust belts region of the Niger Delta basin (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). It covers an approximated area of 32.6 km by 18 km in a water depth ranging between 2000 and 3000 m (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe 3-D seismic data were acquired using dual airgun-sources and six (6) streamer cables. Each streamer cables is 6000 m-long separated by a spacing of 50 m from each other, giving a coverage of 120-fold and a maximum lateral resolution of 12.5 m. The data were recorded with a 2 ms vertical sampling interval, and a 12.5 x 12.5 m bin size spacing. The seismic data was zero-phase post stack time migrated with a dominant frequency of 40 Hz.\u003c/p\u003e\u003cp\u003eBased on an assumed interval velocities ranging between 2400 and 2000 m/s - and the dominant frequency content of the seismic data (40 Hz), the vertical resolution of the seismic data was estimated to be 15 m in the shallow sedimentary section, and 12.5 m in the deepest parts of the section. The overall quality/resolution of the 3-D seismic data was good and thus permitted a detailed visualisation and mapping of fault traces and seismic units. However, some of the reflections were weak in the footwall regions beneath the major thrust planes (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e3.2. Methods\u003c/h2\u003e\u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\u003ch2\u003e3.2.1 Mapping of seismic horizons and units\u003c/h2\u003e\u003cp\u003eSeismic horizons and units were identified and mapped using the Petrel\u0026trade; seismic interpretation software. Three (3) major seismic units (S1 to S3) were mapped and correlated with stratigraphic information from Tuttle et al. (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1999\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e). These seismic units include Unit S1 (Akata Formation), Unit S2 (Agbada Formation) and the shallower Unit S3 (Benin Formation) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eInternal seismic character of the interpreted seismic units\u0026rsquo; deep water offshore Niger Delta basin.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eS/N\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eStratigraphic Formations/Age (Tuttle et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1999\u003c/span\u003e).\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMain lithologies (Tuttle et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1999\u003c/span\u003e).\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003eInterpreted seismic units\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eInternal seismic character\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBenin Formation\u003c/p\u003e\u003cp\u003e(Pleistocene to Recent)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSandstones, grits, claystone and streaks of lignite.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003eUnit S3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCharacterised by a package of high-frequency, moderate- to high-amplitude and continuous seismic reflections.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"3\" rowspan=\"4\"\u003e\u003cp\u003eAgbada Formation\u003c/p\u003e\u003cp\u003e(Miocene to Pleistocene)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\" morerows=\"3\" rowspan=\"4\"\u003e\u003cp\u003eDeltaic sandstones with shale.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\" morerows=\"3\" rowspan=\"4\"\u003e\u003cp\u003eUnit S2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eUnit S2d\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCharacterized by moderately to high amplitude, fairly continuous folded reflector with a gentle relief hanging wall anticlinal crest of about 172.5 m high, the surface thinned and overlapped the fold crest.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eUnit S2c\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCharacterized by moderately to high amplitude, fairly continuous folded reflector with a fairly high relief hanging wall anticlinal crest of about 575 m high, the surface also thinned and overlapped the fold crest.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eUnit S2b\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCharacterized by moderately to high amplitude, fairly continuous folded and faulted reflector with high relief hanging wall anticlinal crest of about 684m high, the surface shows tectonic thickening at the fold crest\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eUnit S2a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eCharacterized by moderately to high amplitude, fairly continuous folded and faulted reflector with a high relief hanging wall anticlinal crest of about 687 m high, the surface also shows tectonic thickening at the fold crest.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAkata Formation\u003c/p\u003e\u003cp\u003e(Eocene to Miocene)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMarine dark grey shales and silts, with rare streaks of sand.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003eUnit S1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eChaotic internal reflections with low amplitude. The top and bottom of the unit (H2 and H1) are marked by a strong seismic reflector that is highly continuous with strong amplitude and sparsely faulted.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eIn the Niger Delta basin, the Agbada Formation Unit S2) is the most structurally complex formation that is largely affected by syn-sedimentary structural features (e.g., thrust anticlines), and thus was identified by it internally deformed wedge of sediments (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The Akata Formation (Unit S1) was identified by thick package of sediments that was devoid of internal reflections (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In the deep-water region of the Niger Delta, the Benin Formation (Unit S3) thin and disappears seawards (Morgan, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Rouby et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe Agbada Formation (Unit S2) is the main interval of interest for this paper, as this unit was deformed by thrust anticlines. Also, the Agbada Formation (Unit S2) comprise reservoir intervals inside which petroleum derived from Akata Formation (Unit S1) is stored (Tuttle et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1999\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eMoreover, Intra-Agbada horizon (HA) was mapped in detail throughout the 3-D seismic volume. From the Intra-Agbada structural map and seismic section in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, three (3) major thrust anticlines (A to C) were identified. Subsequently, four (4) sub-seismic horizons (HA to HD) and four (4) sub-seismic units (S2a to S2d) through the target Thrust Anticline B were mapped to quantify their structural geometry and progressive evolution in the study area (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e to \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section3\"\u003e\u003ch2\u003e3.2.2 Structural restorations\u003c/h2\u003e\u003cp\u003eFrom the 3D seismic volume, three (3) seismic inline sections (i.e. 7884, 7984 and 8084) were selected through the target Thrust Anticline B for the purpose of 2-D structural restoration. These seismic inlines (7884, 7984 and 8084) approximately slice through the crest of the target Thrust Anticline B (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe picks of the five mapped horizons (H2, HA to HD) across the target Thrust Anticline B were imported into 2-D Move\u0026trade; software to produce line cross sections for the selected three (3) seismic inline sections (7884, 7984 and 8084). A velocity of 2000 m/s was assumed to perform time-depth conversion on the line sections using Move\u0026trade; software after which polygons for the cross-sections were created (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\n\u003ch3\u003e2-D Move makes use of the following equation.\u003c/h3\u003e\n\u003cp\u003e\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:z=\\left(\\frac{{v}_{o}}{k}\\right)\\:\\times\\:\\left[\\left({e}^{kt}\\right)-1\\right]\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:(\\text{E}\\text{q}.\\:1)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eWhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:z\\)\u003c/span\u003e\u003c/span\u003e is the depth in meters (m), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{o}\\)\u003c/span\u003e\u003c/span\u003e is the initial velocity in meter per second (m/s), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:k\\)\u003c/span\u003e\u003c/span\u003e is the rate of change in velocity with increasing depth (assumed to be constant) and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:t\\)\u003c/span\u003e\u003c/span\u003e is the one-way travel time in seconds (s).\u003c/p\u003e\u003cp\u003eTherefore, two (2) structural restoration operations including move on fault and unfolding operations were applied to the selected line cross-sections 7884, 7984 and 8084 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e\u003cdiv id=\"Sec9\" class=\"Section4\"\u003e\u003cdiv class=\"Heading\"\u003e3.2.2.1 Move on fault operation\u003c/div\u003e\u003cp\u003eMove on fault operation aimed at removing the deformation caused by faulting and is based on a decision about how the rocks deformed by faulting (Buddin et al \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1997\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eTrishear algorithm was used to restore the fault offset sequentially. The procedure used was to move along forethrust fault F1 first to restore it offset and then move along backthrust fault F2 in a section where the two faults exist (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Therefore, the Move on fault operation using the trishear algorithm was applied to the target thrust faults F1 and F2 for all the seismic lines cross sections through the Thrust Anticline B (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section4\"\u003e\u003cdiv class=\"Heading\"\u003e3.2.2.2 Unfolding operation\u003c/div\u003e\u003cp\u003eUnfolding operation aimed at reconstructing the progressive evolution of the anticlinal structure with time. In the seismic section, the interpreted thrust faults F1 and F2 are restricted to Unit S2 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e) i.e. not offsetting the entire sedimentary units. This signifies deformation of the underlying units by flexure, and thus flexural slip algorithm was deemed suitable for this deformation type. Therefore, sequential unfolding operation using the flexural slip algorithm was applied to the target Thrust Anticline B for all the seismic inlines cross sections (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e to \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFlexural slip unfolding tool was used to unfold the interpreted seismic units (S2a to S2d) sequentially, thus moving the unfolded units below the template bed to their pre-deformation positions. Firstly, unfolding was done using the upper Unit S2d as the template bed to which unfolding was done, subsequently followed by unfolding to Units S2c, S2b and S2a for all the three (3) seismic line sections (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e to \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003cdiv class=\"Heading\"\u003e3.2.3 Surface maps of restored horizons\u003c/div\u003e\u003cp\u003eAfter the move on fault and unfolding restoration operations, Data from the three (3) seismic inline sections (i.e., 7884, 7984 and 8084) were imported into Petrel\u0026trade; software from 2-D Move\u0026trade; software as general lines and points for 3-D surface maps generation (Figs.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e12\u003c/span\u003eb to \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e15\u003c/span\u003eb). This was done by adding up the point data from 2-D Move\u0026trade; for the respective surface across the three (3) seismic inlines to form a single 3-D point data with which the surface was created to determine the geometry and progressive development of the Thrust Anticline B (Figs.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e12\u003c/span\u003e to \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e15\u003c/span\u003e). Therefore, Figs.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e12\u003c/span\u003e to \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e15\u003c/span\u003e highlighted the 3-D geometric expression of the interpreted seismic surfaces/horizons before and after structural restoration.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Uncertainties in structural restoration\u003c/h2\u003e\u003cp\u003eRestoration operations are prone to uncertainties which are inherent in the choice of algorithms and parameters assumed - these must be recognized and considered during the analysis stage. These uncertainties can come from the values of interval velocities used in depth conversion. Variations in the assumed values can be very sensitive to the final output. For instance an increase in the average interval velocity will cause the horizons to appear deeper whereas, reduction in the average interval velocity will cause the horizons to appear shallower. In addition to these uncertainties resulting from parameters, using an algorithm that is not particularly suited for the structural scenario introduces some form of error. Nevertheless, the technique used in this paper to quantify the structural restoration of thrust anticlines is robust considering the high quality/resolution of the seismic volume (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Seismic - stratigraphic correlation","content":"\u003cp\u003eThree (3) seismic units including, Units S1 to S3 were mapped and correlated with stratigraphic information of Niger Delta Basin (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e4.1 Unit S1: Akata Formation\u003c/h2\u003e\u003cp\u003eUnit S1 correlates with the Akata Formation and comprises thick marine shales sequences that are believed to contain source rocks (Corredor et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Doust and Omatsola, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e1990\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Unit S1 is characterised by its chaotic internal seismic reflections with low amplitude. The top of the unit (horizon H2) is marked by a strong seismic reflector and thus serves as a detachment surface from which thrust anticlines in the study area are detached (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e4.2 Unit S2: Agbada Formation\u003c/h2\u003e\u003cp\u003eUnit S2 correlates with the Agbada Formation and comprises alternating deltaic sandstones with shale deposited in fluvial-deltaic environments. The unit is the main interval of interest for this paper - it forms a package of high-amplitude, high-frequency seismic reflections that is locally deformed by closely spaced thrust anticlines (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). For the purposes of structural restoration in this work, Unit 2 was sub-divided into four (4) sub-units that include Units S2a, S2b, S2c and S2d (See Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e for their internal seismic character).\u003c/p\u003e\u003cp\u003eUnit S2 is conformable above the bright reflections that mark the top of Unit S1 (Akata Formation). It also comprises the reservoir intervals from which oil is sourced from the Akata Formation (Unit S1) (Nwachukwu and Odjegba, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2001\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e4.2 Unit S3: Benin Formation\u003c/h2\u003e\u003cp\u003eUnit S3 correlates with the Benin Formation, and comprises sandstones, grits, claystone and streaks of lignite (Avbovbo, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1978\u003c/span\u003e). In seismic data, Unit S3 form a package of high-frequency, continuous and moderate to high-amplitude seismic reflections. The unit overlies the Agbada Formation (Unit S2) and is bounded at its top by the seafloor horizon (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e"},{"header":"5. Geometric expression of thrust anticlines","content":"\u003cp\u003eSeismic imaging shows that the study area is characterised by closely spaced synthetic forethrust faults, antithetic backthrust faults and verging thrust anticlines with a well-developed NW-SE linear trend perpendicular to the gentle seabed bathymetric slope (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). These thrust anticlines are restricted within Unit S2 (Agbada Formation) and thus accommodate updip, gravity-driven extension on the continental shelf in deep-water Niger Delta (Corredor et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The thrust anticlines are asymmetrical and offset different parts of the Agbada Formation (Unit S2) and flatten onto detachment planes at the top of Akata surfaces (Horizon H2; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThree (3) major thrust anticlines (A to C) which can be traced for up to 6.5 km along dip were identified in the study area (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). These thrust anticlines are restricted and terminated within the upper parts of Agbada Formation (Unit S2), with no structures extending to the seabed surface (i.e. the thrust anticlines are completely buried with a package of thin Unit S3 (Benin Formation; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The crests of the thrust anticlines are separated by 8-11.5 km (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe forethrusts of the Thrust Anticlines (A and C) dominantly shows a seaward-vergence with Thrust Anticline B showing out of thrusts sequence i.e. verging-landward due to reactivation of the older in sequence thrusts as a result of low taper angle of the delta (Corredor et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The backthrust faults intersect the forethrust faults and dominantly show a seaward- vergence (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Therefore, the seaward-vergence of the fold crests indicates the overall tectonic transport direction that formed the structures (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThrust Anticline A is the most landward of the thrust anticlines. It is linear in map view and formed above a seaward-verging forethrust. An antithetic backthrust, which intersects the forethrust, deforms the back limb of the anticline (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Thrust Anticline A comprises a much narrower, shorter wavelength anticline compared to seawards Thrust Anticline C, with a much broader, longer wavelength (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThrust Anticline B is a landward-verging structure compared to Thrust Anticlines A and C. it consists of two closely spaced thrusts. The seaward-verging forethrust dominates the structure. A second backthrust cuts the backlimb of the frontal thrust and dies out upwards, with no faults reaching the sea floor (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Thrust Anticline B comprises a much narrower, shorter wavelength anticline compared to Thrust Anticlines A and C (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThrust Anticline C is the most distal of seawards structure and extends outside the seismic dataset (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The seaward-verging forethrust dominates the structure. Thrust Anticline C comprises a much broader, longer wavelength in the study area (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e"},{"header":"6. Progressive evolution of thrust anticlines","content":"\u003cp\u003eThe modes of evolution and growth of the interpreted thrust anticlines were investigated using the geometric expression of the restored surfaces/horizons on the seismic line sections 7884, 7984 and 8084 (Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e to \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e15\u003c/span\u003e.\u003c/p\u003e\u003cdiv id=\"Sec19\" class=\"Section2\"\u003e\u003ch2\u003e6.1 Restored seismic line section 7884\u003c/h2\u003e\u003cp\u003eThe restored seismic line section (7884) through the crest of the Thrust Anticline B revealed that during the deposition of the Unit S2a, the Unit S1 was relatively flat with a buckle synclinal geometry at its centre (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea). During the deposition of Unit S2b, a symmetric anticline started to develop towards the southwest (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb). During the subsequent deposition of Unit S2c, the anticline growth increased showing westward vergence. The anticline continues growing to present day after deposition of Unit S2d (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec, d).\u003c/p\u003e\u003cp\u003eThe forethrust fault propagates through the Units S2a and S2b and remains active during the deposition of Unit S2c, with no thrust intersecting the Unit S2c surface (Horizon HC) i.e. the thrust terminated at the base of Unit S2c (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ed). The backthrust intersect the forethrust within the Unit S2a and terminated at the base of Unit S2b (Horizon HA; Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb-e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec20\" class=\"Section2\"\u003e\u003ch2\u003e6.2 Restored seismic line section 7984\u003c/h2\u003e\u003cp\u003eThe restored dip-line section 7984 across the Thrust Anticline B revealed that during the deposition of Unit S2a, the Unit S1 surface (H1) was relatively flat with a buckle synclinal geometry (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea). During the deposition of Unit S2b, the Unit S2a was almost flat while it was slightly folded together with the Unit S2b forming an anticline with westward vergence and associated syncline developing during Unit S2c deposition. The growth of the anticlinal crest continued with the Unit S2d deposition. At this point, the Units S2c, S2b and S2a have already formed a higher relief anticlinal structure, this growth continued to the present day as evident from thinning of syn-growth sequences above fold crests (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe forethrust fault propagates through the Units S2a and S2b and remains active during the deposition of Unit S2c, with no thrust intersecting the Unit S2c surface (HC) i.e. the thrust terminated at the base of Unit S2c (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ec-e).\u003c/p\u003e\u003cp\u003eThis seismic line section (7984) was picked toward the flank of the fold where the backthrust signature disappears (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). The forethrust also shows eastward vergence.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec21\" class=\"Section2\"\u003e\u003ch2\u003e6.3 Restored seismic line section 8084\u003c/h2\u003e\u003cp\u003eThe restored seismic line section (8084) across the thrust anticline B revealed that during the deposition of Unit S2a, the Unit S1 was relatively flat. Also, Unit S2a was flat at the time of Unit S2b deposition. However, a gentle anticline develops during the of deposition of Unit S2c (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). The anticline developed progressively during the deposition of Unit S2d, leading to a higher relief structure as evident from thinning of syn-growth sequences above fold crest (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e and \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe forethrust fault propagates through the Units S2a and S2b, and remains active during the deposition of Unit S2c, with no thrust intersecting the Unit S2c surface (HC) i.e. the thrust terminated at the base of Unit S2c (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). The dip-line section (8084) was picked at the flank of the fold where the backthrust signature disappears (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eTherefore, the restored results show that the Thrust Anticline B originated as a buckle fold and subsequently cut by several basinward and landward dipping thrusts that penetrated the fold at different depth levels (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e, \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e to \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e"},{"header":"7. Discussion","content":"\u003cdiv id=\"Sec23\" class=\"Section2\"\u003e\u003ch2\u003e7.1 Structural styles in the outer fold and thrust belt region of the Niger Delta\u003c/h2\u003e\u003cp\u003eThe outer fold and thrust belt region of the Niger Delta basin comprises seaward (basinward) and landward (hinterland)-verging anticlines with their associated thrust faults (Corredor et al, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). These structures resulted from the effect of gravity gliding and spreading upon basinward dipping detachment and sloping seabed (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The thrust anticlines respond to accommodate up-dip extension due to instabilities at the shelf by sliding down the detachment to form toe thrusts and associated folds that are majorly seaward-verging except for some landward verging folds with associated back-thrust formed due to the low taper angle in the delta (Bilotti et al., 2005) or changes in the properties of the detachment surface.\u003c/p\u003e\u003cp\u003eThe amplitude and size of these thrust anticlines are related to the amount of up dip extensions. The rate of growth of the anticline over time is closely related to the rate of up-dip extension which may be further related to the rate of deposition at the shelf. The top Akata surface (Horizon H2) shows location of thrust base which suggests that it played a major role in controlling the distinct deformation styles of thrust and thrust related folds (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef). This corresponds to one of the three possible detachment surfaces according to Briggs et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e which was reported to be mobile and highly over pressured. Therefore, the evolution of the interpreted thrust anticline in the outer fold and thrust belt region of the deep-water Niger delta is linked to the combined effect of gravity gliding and spreading, and thus, does not involve the basement (thin skinned tectonics).\u003c/p\u003e\u003cp\u003e\u003cem\u003e7.2 Structural evolution of thrust anticlines in the outer fold and thrust belt of Niger Delta basin\u003c/em\u003e\u003c/p\u003e\u003cp\u003eThe sequential restoration of the seismic line sections through the thrust anticline was used to reconstruct its evolution with time. The deformation of the anticline evolved through multiple phases of deformation that began approximately during the of deposition of Unit S2a (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e to \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). At this early stage, the thrust fault F1 was active, and the horizon H2 which has been interpreted here as a possible detachment surface had features of buckle folding (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb to \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb). At the time of Unit S2b deposition, horizon HA had formed a symmetric fold of about 800 m high toward the southwest (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb and \u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e13\u003c/span\u003e), while horizon HB showed evidence of thickening signifying deposition while faulting was occurring.\u003c/p\u003e\u003cp\u003eSimultaneous deposition and deformation continued with faulting during the deposition of Unit S2c, the relief of the fold increased to about 1,100 m, asymmetric and developed a north eastwards vergence (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec and \u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e13\u003c/span\u003e). Deposition and deformation continued through time causing crest thickening in the fold during the present-day deposition of Unit S2d (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ed, e, \u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e14\u003c/span\u003e and \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e15\u003c/span\u003e). Therefore, the interpreted seismic units are syn-kinematic in nature showing differential growth with deposition (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e to \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). The detachment layer/surface is the horizon H2 (top Akata Formation; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e) which corresponds to one of the detachment surfaces according to Briggs et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eA schematic model in Fig.\u0026nbsp;16 highlights the structural evolution of the interpreted thrust anticlines. The thrust fold started to evolve as a symmetric buckle fold in response to gravity instability and subsequent gliding on the detachment which simultaneously led to the formation of toe thrusts down-dip to accommodate up-dip extensions (Fig.\u0026nbsp;16a). Basinward-verging, thrust fault F3 was firstly formed whereas continuous compression led to the development of basinward-verging thrust faults F4 (Figs.\u0026nbsp;16b). Moreover, landward verging thrust fault F1 developed alongside thrust fault F4, due to either reduction in critical taper angle or change in the properties of the detachment surface (Fig.\u0026nbsp;16b).\u003c/p\u003e\u003cp\u003e\u003cb\u003eFigure\u0026nbsp;16\u003c/b\u003e\u003c/p\u003e\u003cp\u003eAs deposition continued, the fold tightened and another zone of weakness developed which resulted to the initiation of thrust fault F2 (Fig.\u0026nbsp;16c). These thrusts continued to propagate with deposition and hence changing the geometry of the structure from a buckle fold to a northeast verging (landward) fault propagation fold with symmetric and asymmetric growth packages at southwest and northeast of the fold respectively (Fig.\u0026nbsp;16c).\u003c/p\u003e\u003c/div\u003e"},{"header":"8. Conclusions","content":"\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eThe conclusions derived from the 3-D seismic interpretation and structural restoration can be summarised as follows:\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThe outer fold and thrust belt region of the Niger Delta is characterised by closely spaced synthetic forethrust faults, antithetic backthrust faults and verging thrust anticlines with a well-developed NW-SE linear trend perpendicular to the gentle seabed bathymetric slope.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThe top Akata surface (horizon H2) provide suitable detachment surfaces for gravity collapse.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThe studied thrust anticlines are thrust propagation fold that has been growing over time as evident by the folding of recent sediments above the anticlinal structures.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe author declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of data availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data supporting the findings of this study are available within the paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePetroleum Technology Development Fund (PTDF), Nigeria was acknowledged and thanked for funding the research study. We acknowledge the permission approved by DPR and CGG-Veritas for the use of the seismic data included in this article and Schlumberger for the provision of seismic interpretation software (Petrela).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMaunde analyse and prepare the manuscripts\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAvbovbo, A. A., 1978, Tertiary lithostratigraphy of the Niger Delta: AAPG Bulletin, v. 62, p. 295\u0026ndash;300\u003c/li\u003e\n \u003cli\u003eBilotti F, Shaw J.H., 2005, Deep-water Niger Delta folds and thrust belt modelled as a critical-taper wedge: The influence of elevated basal fluid pressure on structural styles. AAPG BULLETIN Vol. 89 Issue. 11 Pages 1475-1491\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eBrandes, Ch., Tannerr, D.C. 2014. Fault-related folding: A review of kinematic models and their application. Earth-Sci. Rev. 138, 352-370. https://doi.org/10.1016/j.earscirev.2014.06.008\u003c/li\u003e\n \u003cli\u003eBriggs S.E, Davies R.J, Cartwright J.A and Morgan R., 2006, Multiple detachment levels and their control on fold styles in the compressional domain of the deepwater west Niger Delta. Journal of Basin research Vol.18, Issue 4. Pages 435-450\u003c/li\u003e\n \u003cli\u003eBuddin T. S, Kane S .J, Williams G. D and Egan S. S., 1997, A sensitivity analysis of 3-dimensional restoration techniques using vertical and inclined shear constructions Tectonophysics 269 33\u0026ndash;50\u003c/li\u003e\n \u003cli\u003eConnors, C. D., Denson, D.B., Kristiansen, G., and Angstadt, D.M., 1998, Compressive anticlines of the mid-outer slope, central Niger Delta (abs.): AAPG Bulletin, v. 82, p. 1903.\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eCorredor, F., Shaw, J.H., \u0026amp; Bilotti, F., 2005, Structural styles in the deep-water fold and thrust belts of the Niger Delta. AAPG BULLETIN Vol. 89, No.6 pp 753-780.\u003c/li\u003e\n \u003cli\u003eDamuth, J. E., 1994, Neogene gravity tectonics and depositional processes on the deep Niger Delta continental margin: Marine and Petroleum Geology, v. 11, no. 3, p. 320\u0026ndash;346\u003c/li\u003e\n \u003cli\u003eDoust, H. and Omatsola, E., 1990, Niger Delta. In: Divergent/passive Margins (Ed. By J.D. Edwards \u0026amp; P.A Santogrossi), AAPG Memoir 48, p. 201-238.\u003c/li\u003e\n \u003cli\u003eErickson, S.G., 1996, Influence of mechanical stratigraphy on folding vs. faulting. J. Struct. Geol., 18, 443-450\u003c/li\u003e\n \u003cli\u003eFrehner, M., 20011. The neutral lines in buckle folds. Journal of Structural Geology 33, 1501-1508\u003c/li\u003e\n \u003cli\u003eHeinio, P., Davis, R.J., 2007, Knickpoint migration in submarine channels in response to fold growth, western Niger Delta.\u003c/li\u003e\n \u003cli\u003eJolly, B.A., Lonergan, L., Whittaker, A.C., 2016. Growth history of fault-related folds and interaction with seabed channels in the toe-thrust region of the deep-water Niger delta. Mar. Pet. Geol. 70 (2016), 58\u0026ndash;76.\u003c/li\u003e\n \u003cli\u003eMitra, S., 2022. Fold-accommodation faults. AAPG Bull. 86, 671-693. \u0026nbsp; https://doi.org/10.1306/61EEDB7A-173E-11D7-8645000102C1865D\u003c/li\u003e\n \u003cli\u003eMorgan, R., (2004): Structural controls on the positioning of submarine channels on the lower slopes of the Niger Delta. In: Davies, R.J., Cartwright, J.A., Stewart, S.A., Lappin, M., Underhill, J.R. (Eds.), 3D Seismic Technology: Application to the Exploration of Sedimentary Basins, Geological Society London Memoirs, Vol. 29, pp. 45-51.\u003c/li\u003e\n \u003cli\u003eMorley CK., 2007, Development of crestal normal faults associated with deepwater fold growth. Journal Of Structural Geology Vol.29, Issue.7 Pages 1148 1163\u003c/li\u003e\n \u003cli\u003eNwachukwu, J.I., and Odjegba, 2001: Compaction in reservoir sandstone of the Niger Delta, Journal of mining and geology, v. 37, No. 2, p. 113\u0026ndash;120.\u003c/li\u003e\n \u003cli\u003ePrice, N.J., Cosgrove, J.W., 1990. Analysis of Geological Structures. Cambridge University Press, Cambridge, ISBN 0-521-26581-9.\u003c/li\u003e\n \u003cli\u003eRouby, D., Nalpas, T., Jermannaud, P., Robin, C., Guillocheau, F., And Raillard, S., (2011): Gravity driven deformation controlled by the migration of the delta front: the Plio-Pleistocene of the eastern Niger Delta. Tectonophysics Vol. 513, pp.54-67.\u003c/li\u003e\n \u003cli\u003eSchlische, R.W., 1995. Geometry and origin of fault-related folds in extensional settings. AAPG Bull. 79, 1661-1678. \u0026nbsp; https://doi.org/10.1306/7834DE4A-1721-11D7-8645000102C1865D\u003c/li\u003e\n \u003cli\u003eSuppe, J., Connors, C.D., and Zhang, Y., 2004, Shear fault-bend folding, in K. R. McClay, ed., Thrust tectonics and hydrocarbon systems: AAPG Memoir 82, p. 303 323.\u003c/li\u003e\n \u003cli\u003eTuttle M.L.W., Charpentier R.R. And Brownfield M.E., (1999); The Niger Delta Petroleum system: Niger Delta Province, Nigeria Cameroon, and Equatorial Guinea, Africa, USGS Open-File Report 99-50-H.\u003c/li\u003e\n \u003cli\u003eWu, S., and Bally, A.W., 2000, Slope tectonics\u0026mdash; Comparisons and contrasts of structural styles of salt and shale tectonics of the northern Gulf of Mexico with shale tectonics of offshore Nigeria in Gulf of Guinea, in W. Mohriak and M. Talwani, eds., Atlantic rifts and continental margins: Washington, D.C., American Geophysical Union, p. 151\u0026ndash;172.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"3D seismic data, Thrust anticline, Niger Delta, Restoration, Unfolding","lastPublishedDoi":"10.21203/rs.3.rs-8087043/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8087043/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe deep-water slope area of the Niger Delta is increasingly becoming one of the world\u0026rsquo;s hydrocarbon potential areas as the progress of exploration off the shelf and down the slope has intensified. High-resolution three-dimensional (3-D) seismic data from the outer fold and thrust belt region of Niger delta are used to investigate the geometry and progressive development of thrust anticlines through a detailed structural restoration. The results show that the outer fold and thrust belt region of the Niger Delta is characterised by closely spaced synthetic forethrust faults, antithetic backthrust faults and verging thrust anticlines with a well-developed NW-SE linear trend perpendicular to the gentle seabed bathymetric slope. These structures respond to accommodate up-dip extension due to instabilities at the shelf by sliding down the detachment to form toe thrusts and associated folds. Identifying the geometry and progressive development of thrust anticlines is key to understanding how thrust anticlines evolve and grow. It also provides information on the timing of fault activity, a parameter directly affecting sub-surface fluid flow in geological reservoirs.\u003c/p\u003e","manuscriptTitle":"Thrust anticlines responding to gravitational instability deep water offshore, Niger Delta Basin","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-25 14:56:36","doi":"10.21203/rs.3.rs-8087043/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"4e5d6ee6-9104-4171-ab7a-00b588636234","owner":[],"postedDate":"November 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-12-05T09:54:42+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-25 14:56:36","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8087043","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8087043","identity":"rs-8087043","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}