Evaluation of Hydrocarbon Generation Potential and Paleoenvironmental Implications of Dark Mudstones in the Upper Assemblage of the Yanchang Formation, Central–Eastern Ordos Basin

To evaluate the hydrocarbon generation potential and depositional environmental characteristics of dark mudstones in the upper assemblage of the Yanchang Formation in the central–eastern Ordos Basin, 39 samples were analyzed using organic geochemistry, organic petrography, biomarker analysis, stable carbon isotopes, and trace-element geochemistry. The samples comprise 18 from the Chang 6 interval, 13 from the Chang 4+5 interval, and four each from the Chang 2+3 and Chang 1 intervals. Valid Tmax data were obtained for 35 samples, whereas vitrinite reflectance, organic maceral composition, biomarker, stable carbon isotope, and trace-element analyses were conducted on 24, 18, 23, 13, and 13 samples, respectively. The results show that the Chang 6 dark mudstones are dominated by fair-to-good source rocks and are generally mature. The Chang 4+5 interval is dominated by non-source rocks, followed by fair-to-excellent source rocks, and is mainly at the low-mature to mature stage. The Chang 2+3 interval is dominated by non-source rocks, with excellent source rocks being subordinate, and is at the low-mature to mature stage. The Chang 1 interval comprises fair-to-excellent source rocks and is generally at the low-mature stage. Organic matter in the upper assemblage is predominantly characterized by type II1–II2 kerogen. Paleoenvironmental proxies indicate that the Chang 6 and Chang 4+5 intervals were mainly deposited under freshwater to brackish-water conditions, whereas the Chang 2+3 and Chang 1 intervals were deposited predominantly in freshwater environments. Redox conditions were generally weakly reducing to oxic during Chang 6 deposition, generally oxic to suboxic during Chang 4+5 and Chang 2+3 deposition, and weakly reducing to oxic during Chang 1 deposition. The upper assemblage was deposited overall under a warm and humid paleoclimate, accompanied by increasing terrestrial higher-plant input during lake-basin contraction. Relationships between TOC and environmental parameters indicate that organic matter enrichment in the Chang 6 interval is associated with low salinity, weakly reducing to oxic conditions, and a warm and humid paleoclimate, whereas the Chang 4+5 and Chang 1 intervals show a stronger influence of terrestrial higher-plant input. Oil–source comparison reveals a certain regional geochemical affinity between some Chang 6 and Chang 4+5 source rocks and crude oils from the corresponding intervals, with a relatively stronger similarity observed for Chang 6. However, contributions from the Chang 7 and Chang 9 source rocks and multi-source mixed charging cannot be excluded. Given the limited sample numbers for the Chang 1 and Chang 2+3 intervals, the corresponding interpretations primarily reflect the characteristics represented by the available samples. Overall, the dark mudstones of the upper assemblage possess hydrocarbon generation potential and provide geochemical constraints for petroleum exploration in the study area.