Characteristics of reservoirs for inter-salt shale oil of Qianjiang Formation, Qianjiang Sag, Jianghan Basin: a case study of the Eq₃⁴-10 rhythm

The inter-salt shale oil of the Eocene Qianjiang Formation in the Jianghan Basin is a hot and difficult research target in recent years. In order to clarify the lithofacies and physical properties of the inter-salt shale reservoirs, based on core observation, thin section observation, X-ray diffraction, high-pressure mercury intrusion and shale porosity and permeability analyses, a case study was carried out for the Qianjiang Formation in the Qianjiang Sag of Jianghan Basin. Results showed that there were three types of lithofacies, including carbonaceous laminar argillaceous dolomites, carbonaceous lamellar dolomitic/lime mudstones and carbonaceous lamellar dolomitic mudstones filled with glauberites. The porosities of the first two lithofacies were significantly higher than that of the latter. The pore size distribution of different lithofacies showed that the carbonaceous laminar argillaceous dolomites were dominated by pores with radius greater than 41 nm, and the median pore-throat radius was 219 nm, while the other two facies were dominated by pores with pore radius less than 41 nm and the median pore-throat radius was 21 nm. The carbonaceous laminar argillaceous dolomites were indicated to be homogeneous in lithology. As reservoir, they have good reservoir space, larger pore-throat structure and higher oil content, and was the dominant facies in the inter-salt shale oil reservoir and is promising to be the most favorable shale oil exploration target in the study area.