Geochemical comparison and hydrocarbon potential of the Chang 7 and Chang 9 members of the Upper Triassic Yanchang Formation in the Longdong Area, Ordos Basin, China

In recent years, significant breakthroughs have been made in the exploration of both conventional and unconventional oils in the Chang 9 Member of the Upper Triassic Yanchang Formation in the Longdong area. However, the origin of hydrocarbons remains contentious, largely due to insufficient characterization of the source rocks in both the Chang 7 and Chang 9 Members. In this study, the geochemical characteristics and hydrocarbon generation potential of these two members were comprehensively analyzed. The Chang 7 Member is characterized by the development of thick mudstones with high total organic carbon (TOC = 0.25–19.89 wt.%) and significant hydrocarbon generation potential (S 1 +S 2 = 0.29–93.12 mg/g; HI = 72–714 mg/g TOC). In contrast, the mudstones in the Chang 9 Member are relatively thin and exhibit lower organic abundance (TOC = 0.11–4.79 wt.%) and subordinate generation potential (S 1 +S 2 = 0.02–6.53 mg/g; HI = 33–262 mg/g TOC). Biomarker and isotope analyses indicate that the superior hydrocarbon potential and Type II₁ kerogen of the Chang 7 Member are attributable to its highly anoxic lacustrine facies and a mixture of plankton and terrestrial plants. Conversely, the Type II₂ kerogen in the Chang 9 Member, characterized by slightly weaker generation potential, reflects a weakly anoxic deltaic environment with a higher proportion of terrestrial plants. This study provides a comprehensive understanding of the Chang 7 and Chang 9 source rocks and offers an effective theoretical basis for further research on the petroleum system of the Yanchang Formation in the Longdong area.