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

doi:10.19657/j.geoscience.1000-8527.2023.104

Abstract

Abstract:

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 analyzed 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 development of thick gravity flow sand bodies, black shale, and dark mudstone. Types A and C are primarily distributed 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 distributed in Northern Shaanxi. Type H is the shale type, where Chang 7₃ to Chang 7₁ 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 enrichment 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 enrichment 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 7₁ 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.

Key words: lithological combination model, enrichment control mechanism, crude oil of shale series, Chang 7 Member, Ordos Basin

CLC Number:

HE Xin, CHEN Shijia, HU Cong, ZHANG Haifeng, MOU Feisheng, LU Yifan, DAI Linfeng, FU Xiaoyan, HAN Meimei. 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[J]. Geoscience, 2024, 38(05): 1325-1337.

Figures/Tables 13

Fig.1 Geographical location of the study area and the comprehensive histogram of the Chang 7 Member of the Triassic Yancheng Formation (based on reference [39])

Fig.2 Typical well logging curves and core photos of the interlayer type lithologic combination mode in the Chang 7 Member

Table 1 Types and distribution characteristics of the lithologic association in the Chang 7 Member of the Ordos Basin

组合类型	组合特征	主要发育区域	主要油藏层位
A	上储下源,长7₁、长7₂以重力流砂体为主,长7₃为源岩	华池西、庆城和正宁一带	长7₁、长7₂
B	上储下源,长7₁以重力流砂体为主,长7₂和长7₃为厚层泥岩夹薄层砂岩	华池东、姬塬、吴起和正宁往南一带	长7₁
C	长7₁和长7₂砂体变薄,为中厚层砂体,中间夹有中厚层暗色泥岩,长7₃发育中厚层泥岩	合水、庆城	长7₁、长7₂
D	上下为源,中间为储,长7₂以细砂岩为主,长7₁和长7₃为源岩	新安边、环县东北一带	长7₂为主、部分地区长7₁发育
E	薄泥厚砂,源储互层,长7₃和长7₂为主要源岩	新安边往北、吴起	长7₁、长7₂
F	长7₃段烃源岩不发育,以中厚层砂岩为主,长7₁和长7₂发育中厚层砂岩夹薄层暗色泥岩	吴起、周家湾	长7₃
G	薄泥厚砂,中间发育泥质砂岩和砂质泥岩	庆阳,镇原、安塞、志丹	油藏零星分布
H	长7₁、长7₂和长7₃均以泥岩为主,中间夹薄层砂岩	姬塬,华池、正宁等	/

Fig.3 Types of lithologic association models for the Chang 7 Member in the Ordos Basin

Fig.4 Sandstone-mudstone well-tie section of the Chang 7 Member in the Ordos Basin

Fig.5 Plane distribution diagram of the lithologic combination patterns in the Chang 7 Member of the Ordos Basin

Fig.6 Core photos and microscopic characteristics of black shale and dark mudstone

Fig.7 Organic geochemical characteristics of hydrocarbon source rocks under different lithologic association modes

Fig.8 Reservoir core photos under different lithologic combination modes

Fig.9 Cross-plot of reservoir porosity and permeability for different lithologic combination modes (1 mD=10-3 μm2)

Fig.10 Overlay diagram of oil test results and lithologic combination patterns in the Chang 7 Member

Fig.11 Typical well reservoir profile for the partial lithologic combination mode

Fig.12 Single-well oil testing results for typical wells in different lithologic combination modes

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