Geochemical Characteristics of the Chang 7 Source Rocks of the Triassic Yanchang Formation in Ordos Basin, China: Implications for Organic Matter Accumulation and Shale Oil Potential

The Chang 7 member of the Upper Triassic Yanchang Formation in Ordos Basin is considered to hold main source rocks for conventional and unconventional oil gas. lamination or lithology alteration vertical lateral directions, even over a short distance, common feature lacustrine rocks. differences geochemical characteristics black shales, dark mudstones interbedded sandstones have been scarcely reported, their influences on petroleum generation potential shale are not clear. To this end, 22 core samples were collected from Lower Middle (C7-3 C7-2) members well CYX Qingcheng area. By conducting series analyses including TOC, Rock-Eval pyrolysis yields, bitumen extraction quantification, separation quantification saturates, aromatics, resins asphaltenes, along with biomarker analyses, several results found. Firstly, C7-3 C7-2 thermally mature entered into stage hydrocarbon generation. good very especially shales. Secondly, terrigenous input more abundant C7-2, whereas phytoplankton, algae microbial lipids C7-3. Moreover, high TOC content basically corresponds low wax indexes, terrigenous/aquatic ratios (TARs), Pr/nC17 Ph/nC18 C27/C29 regular sterane ratios, which suggests that favorable OM accumulation. Third, molecular composition saturated fractions shales production index demonstrate migration organic-rich organic-lean counterparts. could be as “sweet spot” since saturation (OSI) was highest among all studied they enriched light aliphatic fractions.