Characteristics of Source Rocks and Oil–Source Correlation in the Seventh Member of the Yanchang Formation (Chang 7 Member), Pingbei Area, Ordos Basin

To clarify the hydrocarbon generation potential of dark mudstones and the source of crude oil in the seventh member of the Yanchang Formation (Chang 7 Member) in the Pingbei area of the Ordos Basin, and to support tight oil exploration in this region, this study focuses on the source rocks and crude oils from the Chang 7 Member. Comprehensive analyses including total organic carbon (TOC), rock pyrolysis, vitrinite reflectance (Ro), and saturated hydrocarbon gas chromatography–mass spectrometry (GC-MS) were conducted to systematically investigate the characteristics of source rocks and the geochemical properties of crude oils, and to perform oil–source correlation. The results indicate that the dark mudstones in the Chang 7 Member of the Pingbei area meet the geological conditions of effective source rocks: they exhibit high organic matter abundance with an average TOC content of 1.69% and strong heterogeneity, among which the Chang 73 sub-member has an average organic carbon content of 2.8%, conforming to the standard of high-quality source rocks; the organic matter type is dominated by Type II1, mixed with a small amount of Type II2 and Type III, characterized by both aquatic biological and terrestrial organic matter inputs; the Ro values range from 0.76% to 0.87%, indicating a mature stage corresponding to the peak period of liquid hydrocarbon generation. The crude oils in the study area can be classified into two types (Type A and Type B): Type A crude oil is distributed in deep reservoirs of the Chang 7 Member, while Type B crude oil is present in both shallow and deep layers. Oil–source correlation shows that Type A crude oil is highly consistent with the dark mudstones of the Chang 7 Member in terms of Pr/Ph ratio, rearranged hopane enrichment degree, and pentacyclic triterpane distribution pattern, clearly indicating that the Chang 7 Member dark mudstones are the main source rocks for Type A crude oil. In contrast, Type B crude oil is geochemically consistent with crude oils from the sixth member of the Yanchang Formation (Chang 6 Member) in oilfields surrounding the Pingbei area; it is derived from the Chang 6 Member source rocks in the peripheral regions.