鄂尔多斯盆地靖边地区延长组长7—长6段烃源岩评价及油源对比OA

[Objective]The Chang 7 and Chang 6 Members of the Yanchang Formation in the Jingbian area of the Ordos Basin are important source rock intervals.However,systematic studies on their hydrocarbon generation potential and contributions to hydrocarbon accumulation remain limited,which has hindered further exploration efforts in this region.Therefore,this study applies 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.In addition,the geochemical signatures of the crude oils from the Yan 9 and Chang 2 reservoirs in the Jingbian Oilfield were investigated.[Methods]By comparing the geochemical parameters of the Chang 7 and Chang 6 source rocks in the study area with those of the Zhangjiatan Shale in the basin center,the potential sources of the Yan 9 and Chang 2 reservoir oils were identified through oil-source correlation.[Results]The Chang 7 source rocks are moderate to excellent-quality source rocks,characterized by wide distribution,large thickness,Type Ⅰ-Ⅱ2 kerogen,and a mature thermal evolution stage.Biomarker characteristics indicate mixed organic matter inputs and deposition under reducing conditions with relatively low salinity.Low Pr/Ph ratios and high C27/C29 regular sterane ratios suggest that the organic matter was predominantly derived from aquatic algae and other lower organisms and accumulated in a reducing freshwater to brackish-water depositional environment.In contrast,the Chang 6 source rocks are moderate to good-quality source rocks,dominated by Type Ⅱ2-Ⅲ kerogen,and generally at the low-maturity to mature stage.Moderate Pr/Ph ratios and relatively low C27/C29 regular sterane ratios indicate a predomint innput of terrigenous higher plantsand deposition under weakly reducing conditions.Oil-source correlation results reveals that the Yan 9 and Chang 2 crude oils have the same originand were primarily sourced from the Chang 7 source rocks,with a minor contribution from the Chang 6 source rocks.The distinct geochemical characteristics of the Zhangjiatan Shale compared with the Yan 9 and Chang 2 crude oils further suggest that the Zhangjiatan Shale was not the major hydrocarbon source for these oils.[Conclusions](1)The Chang 7 Member is the primary and high-quality source rock in the Jingbian area,exhibiting advantages over the Chang 6 Member in terms of thickness,lateral distribution,organic matter abundance,and hydrocarbon generation potential.(2)Distinct biomarker signatures differentiate the Chang 7(aquatic/algal-derived organic matter under reducing conditions),Chang 6(terrigenous organic matter under weakly reducing conditions),and Zhangjiatan Shale(highly aquatic organic matter under strongly reducing conditions)source rocks.(3)The Yan 9 and Chang 2 crude oils share a common origin and were primarily sourced from local Chang 7 source rocks,with possible minor contributions from the Chang 6 source rocks.These oils show no direct genetic relationship with the Zhangjiatan Shale in the basin center,supporting a predominantly local hydrocarbon charging model in the Jingbian area.[Significance]This study provides critical geochemical evidence for understanding the local hydrocarbon system,resolving uncertainties regarding oil sources,and evaluating exploration potential in the Jingbian area of the Ordos Basin.