A Novel Redox Indicator based on Relative Abundances of C31 And C32 Homohopanes in the Eocene Lacustrine Dongying Depression, East China

A suite of oils and bitumens from the Eocene Shahejie Formation (Es) in the Dongying Depression, East China was geochemically characterized to illustrate the impact of source input and redox conditions on the distributions of pentacyclic terpanes. The fourth member (Es4) developed under highly reducing, sulfidic hypersaline conditions, while the third member (Es3) formed under dysoxic, brackish to freshwater conditions. Oils derived from Es4 are enriched in C 32 homohopanes (C 32 H), while those from Es3 are prominently enriched in C 31 homohopanes (C 31 H). The C 32 H/C 31 H ratio shows positive correlation with homohopane index (HHI), gammacerane index (G/C 30 H), and negative correlation with pristane/phytane (Pr/Ph) ratio, and can be used to evaluate oxic/anoxic conditions during deposition and diagenesis. High C 32 H/C 31 H ratio (> 0.8) is an important characteristic of oils derived from sulfidic, hypersaline anoxic environments, while low values (< 0.8) indicate non-sulfidic, dysoxic conditions. Extremely low C 32 H/C 31 H ratios (< 0.4) indicate strong oxic conditions of coal depsoition. Advantages to use C 32 H/C 31 H ratio as redox condition proxy compared to the HHI and gammacerane indexes are wider valid maturity range, less sensitive to biodegradation influence and better differentiation of reducing from oxic environments. Preferential cracking of C 35 -homohopanes leads HHI to be valid in a narrow maturity range before peak oil generation. No C 35 homohopane can be reliably detected in the Es4 bitumens when vitrinite reflectance is > 0.75%, which explains the rare occurrence of high HHI values in Es4 source rocks. Gammacerane is thermally more stable and biologically more refractory than C 30 hopane, leading G/C 30 H ratio more sensitive to maturation and biodegradation than C 32 H/C 31 H ratio. Meanwhile, both HHI and gammacerane index cannot differentiate level of oxidation. The C 32 H/C 31 H ratio can be applied globally as a novel redox proxy in addition to the Dongying Depression.