Depositional environment and diagenesis of Asmari Formation in the well-A (oilfield at western Lorestan basin) and Syah-koh Anticline (Zagros fold–thrust belt, west Iran): microfacies, sequence stratigraphy and geochemistry

The Asmari Formation (Oligocene-Miocene), as one of the main carbonate petroleum reservoirs of the Middle East, represents strongly variable reservoir quality on regional and local scale. One outcrop analogue (Syah-koh Anticline) and a well-A (oilfield at western Lorestan basin), both situated in western Iran, were investigated to address the reservoir characteristics. The 12 microfacies identified reflect deposition on a broad carbonate ramp reflecting an outer, mid and inner ramp setting. Three third-order depositional sequences are deduced based on interpreted Gamma-ray, sonic logs and stratal stacking patterns. The identified sequences and existence of a thicker evaporite succession in well-A points to more restricted depositional conditions with reduced connection to the open sea from the Syah-koh Anticline towards well-A. The diagenetic modifications revealed micritization, dissolution, cementation, fracturing, dolomitization and compaction. Petrographic and δ 13 C and δ 18 O analysis of cements points towards marine, meteoric and burial diagenetic alteration, whereby the cements with δ 13 C and δ 18 O values similar to those of enclosing micrite (0 to -2‰ V-PDB) reflect a marine diagenetic environment. The micrite phases enriched in aforementioned isotopes resulted from evaporation which is supported by widespread associated dolomitization. The J trend present in the δ 13 C and δ 18 O cross plot reflects the meteoric diagenetic overprint. The cements with highly depleted δ 18 O signatures (<-10‰ V-PDB) are linked to burial diagenetic processes. The isotopic values are in agreement with the luminescence characteristics of the cements including dull for marine, alternating bright and dull zones for meteoric and bright for burial cements.