Environmental reconstruction and formation mechanisms for lacustrine source rocks of the third member of the Shahejie Formation in the Raoyang Sag, Bohai Bay Basin, eastern China. Issue 8 (17th November 2019)

• Record Type: Journal Article
• Authors: Yin, J.; Guo, L.; Meng, J.; Hao, F.; Chen, X.; Cao, Y.; Zou, H.
• Is Part Of: Australian journal of earth sciences. Volume 66:Issue 8(2019)
• Page Start: 1145
• Page End: 1162
• Publication Date: 2019-11-17
• Subjects: Lacustrine source rock -- stable isotope -- trace metals -- biomarkers -- paleoproductivity -- redox conditions; Earth sciences -- Australia -- Periodicals; Earth sciences -- Periodicals; Geology -- Australia -- Periodicals; Geology -- Periodicals; 559.405
• DOI: 10.1080/08120099.2019.1611662
• Languages: English
• ISSNs: 0812-0099

Abstract

Abstract: The third (E s 3 ) member of the Eocene Shahejie Formation is the main hydrocarbon source rock interval in the Raoyang Sag of the Bohai Bay Basin. The lower E s 3 (Es3 L ) and upper E s 3 (Es3 U ) submembers display clear differences in source rock quality. The formation mechanisms of the source rocks are investigated via geochemical methods using 60 samples from three wells to reconstruct the paleo-environment during deposition. The major element parameters exhibit changes in paleoclimate from humid to arid. Indicators such as Sr/Ba, B/Ga, the gammacerane index and isotopic data suggest fresh–brackish and hydrological open lakes with unstable water column stratification likely occurred during deposition of the Es3 L submember and saline and hydrologically closed lakes with stable water column stratification likely during deposition of the Es3 U submember. Carbon isotope values of organic matter, trace elements and biomarker parameters suggest that the Es3 L submember had moderate productivity, with a significant contribution from terrigenous organic matter whereas the Es3 U submember had slightly enhanced productivity, with no or minor contributions from terrigenous organic matter. Furthermore, the pristane/phytane ratio and the enrichment of Mo and U indicate that the euxinic bottom-water conditions (sulfidic) of the lakes during deposition of the Es3 U submember were best for preserving organic matter. Comparison of the models of source rock deposition of the Es3 L and Es3 U submembers indicates that the redox conditions play an important role in the formation of organic-rich source rocks in the Raoyang Sag.