The fine-grained gravity flow sedimentary model and organic matter enrichment in Chang 7 sub-member of the Longdong Area from Ordos Basin, China

Abstract

Fine-grained sediments, capable of being transported into deep lacustrine basins via gravity flows, can form potentially high-quality shale oil reservoirs. Therefore, it is crucial to investigate the sedimentary characteristics of various fine-grained gravity flow sedimentation processes within lacustrine basins. This study examines the sedimentary features of fine-grained gravity flow deposits in the 7 3 sub-members of the Yanchang formation in the Longdong area of the Ordos Basin, utilizing core and thin-section observations, and well-logging characters. The findings reveal that the Chang 7 3 sub-member experienced a deep-lacustrine environment, where nine distinct lithofacies assemblages emerged under different sedimentation processes: the fine-grained quiescent sedimentation assemblage (LA1), the fine-grained mudflow sedimentation assemblage (LA2), the fine-grained debris flow sedimentation assemblage (LA3), the fine-grained slump sedimentation assemblage (LA4), the fine-grained turbidity currents sedimentation assemblage (LA5), the fine-grained concentrated density flow sedimentation assemblage (LA6), the fine-grained transitional flow sedimentation assemblage (LA7), the fine-grained surge-like turbidity flow sedimentation assemblage (LA8), and the fine-grained quasi-steady turbidity current sedimentation assemblage (LA9). These lithofacies assemblages were generated under three primary conditions: tranquil environments, slump events, and flood events. A comprehensive sedimentary model has been constructed based on the distribution of the siltstone bodies, illustrating the sedimentary processes and the spatial evolution of these lithofacies assemblages. The fine-grained sediments deposited in Chang 7 3 exhibit a notably high total organic carbon (TOC) content. The extensive distribution and the presence of multiple interbedded layers of organic matter-rich shale and fine-grained siltstone create a conducive environment for the generation and preservation of shale oil within the Chang 7 3 sub-member in the Longdong area.