Lithological Combination Model of the Continental Shale Series and Its Controls on Differential Crude Oil Enrichment:A Case Study of the Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin
To clarify the lithological combination modes of continental shale strata and their influence on crude oil enrichment,targets for further exploration are determined.Based on core observations,log interpretations,and reservoir physical property tests,along with microscopic observations and geochemical analyses,we ana-lyzed the superposition relationship and characteristics of hydrocarbon source rock and reservoirs in the Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin.Additionally,based on the reservoir anatomy of different blocks,we discussed how various lithological combination modes control the differential enrichment of crude oil.In this study,the lithologic combination model for the Chang 7 Member is categorized into eight types,labeled A through H.Among them,Types A to C are gravity flow types,characterized by the develop-ment of thick gravity flow sand bodies,black shale,and dark mudstone.Types A and C are primarily distribu-ted in the Longdong area,while Type B is distributed throughout the entire basin.Types D to G are delta front types,characterized primarily by the development of delta front sands and dark mudstone,and are mainly dis-tributed in Northern Shaanxi.Type H is the shale type,where Chang 73 to Chang 7,consist entirely of shale,with thin layers of shale in between.The organic matter abundance in gravity flow type(A-C)source rocks is significantly higher than in delta front type(D-G)source rocks.This is the primary reason why crude oil en-richment is greater in gravity flow sedimentary areas compared to delta front sedimentary areas.B-type,G-type,and H-type reservoirs have the poorest physical properties,which contributes to variations in crude oil enrich-ment within the same sedimentary area and across different blocks.Lithological changes associated with different lithologic combination modes control the degree of crude oil enrichment in various sub-members and blocks.The degree of crude oil enrichment is highest in A-type lithologic combinations,with large-scale reservoirs found in Chang 7,and Chang 72.G-type lithologic combinations have the lowest degree of crude oil enrichment,with few reservoirs present from Chang 71 to Chang 73.
lithological combination modelenrichment control mechanismcrude oil of shale seriesChang 7 MemberOrdos Basin
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