Abstract:
Objective The Chang 7 and Chang 6 Members of the Yanchang Formation in the Jingbian Area of the Ordos Basin constitute significant source rock intervals. However, systematic studies on their hydrocarbon generation potential and contributions to petroleum accumulation remain limited, which constrains further exploration efforts in this region. Therefore, this study integrates geochemical analytical methods to systematically characterize the distribution and the geochemical properties of the Chang 7 and Chang 6 source rocks in the Jingbian area, as well as the geochemical signatures of the crude oils from the Yan 9 and Chang 2 reservoirs in the Jingbian Oilfield. Methods By comparing the geochemical parameters between the local Chang 6 and Chang 7 source rocks and the Zhangjiatan Shale in the center of the basin, the potential sources of the oils in the Yan 9 and Chang 2 reservoirs were identified. Results The results demonstrate that the Chang 7 source rocks are high quality, with widespread distribution and considerable thickness. They are characterized by Type I-II
2 kerogen and are in the mature stage. The biomarker characteristics indicate mixed organic matter inputs, deposited in a reducing environment with low salinity. The low Pr/Ph and high C
27/C
29 regular sterane ratios further suggest that the organic matter is predominantly derived from aquatic algae and other lower organisms, consistent with a reducing, fresh to brackish water depositional environment. In contrast, the Chang 6 source rocks are of medium to high quality, containing Type II
2-III kerogen and are generally in the low-to-mature stage. The moderate Pr/Ph and low C
27/C
29 regular sterane ratios reflect a predominance of terrigenous higher plant input, deposited under a weakly reducing environment. The oil-source correlation results reveal that the Yan 9 and Chang 2 oils are of the same origin, primarily derived from the Chang 7 source rocks, with a minor contribution from the Chang 6 Member. Conclusion (1) the Chang 7 Member is the primary, high-quality source rock in the Jingbian area, superior to the Chang 6 Member in terms of thickness, lateral distribution, organic richness, and hydrocarbon generation potential. (2) Distinct biomarker signatures differentiate the Chang 7 (aquatic/algal, reducing), Chang 6 (terrigenous, weakly reducing), and distal Zhangjiatan (highly aquatic, strongly reducing) source rocks. (3) The Yan 9 and Chang 2 crude oils share a common origin and are primarily sourced from the local Chang 7 source rocks, with a possible minor contribution from the Chang 6 Member. The oils show no direct genetic link to the Zhangjiatan Shale in the basin center, supporting a predominantly local hydrocarbon sourcing model for the Jingbian area. Significance This study provides crucial geochemical evidence for a local hydrocarbon system within the Jingbian area, resolving previous ambiguities regarding oil sources and assessing the resource potential in the Jingbian Area, Ordos Basin.