Pore Structure Change in the Continental Shale Oil Reservoir and Its Main Influencing Factors: A Case Study of the Chang 7 Member in the Ordos Basin

Abstract

Shale oil in the Chang 7 member is the main target for further exploration in the Ordos Basin. However, the lack of research on the characteristics of the Chang 7 member’s mudstone and shale reservoir has seriously affected the next stage of exploration for shale oil in the Chang 7 member. This study analyzed in detail the changes in the pore structure of different types of reservoirs and their influencing factors in detail through experiments such as gas adsorption, nuclear magnetic resonance, and high-pressure mercury intrusion. The results showed that black shale is mainly composed of micropores, with a micropore ratio of up to 79.88%. Dark mudstone and silty mudstone are mainly composed of mesopores, with a mesopore ratio greater than 30%. Argillaceous sandstone is mainly composed of large pores, accounting for 78.57%. From black shale to muddy sandstone, the proportion of micropores decreases, while the proportion of macropores increases. The micropore volume is mainly controlled by the organic matter content and the inorganic mineral content related to the organic matter content. Inorganic minerals such as quartz and feldspar mainly control the degree of development of mesopores and macropores. For rocks with a strong hydrocarbon-generating ability (black shale and dark mudstone), the ratio between the total pore volume and micropore volume is the main factor restricting their oil content. For rocks with a weak hydrocarbon-generating capacity (silty mudstone and argillaceous sandstone), the macropore volume and total pore volume are the main controlling factors for oil content. The results of this study have important guiding significance for the next step of exploration and development for shale oil in the Chang 7 member and, to a certain degree, for the exploration of continental shale oil in other basins.