Sedimentary and Reservoir Characteristics of Black Shale in the Jurassic Da’anzhai Member in the Yuanba Area of Northeastern Sichuan

Da’anzhai member of Ziliujing Formation in Jurassic is a hot zone for lacustrine shale exploration and development in Sichuan Basin at present. In this paper, through rock observation and logging analysis of special elements, field emission scanning electron microscope observation, rock porosity experiment, X-ray whole rock diffraction (XRD), cryogenic nitrogen adsorption and other experimental analysis, the sedimentary environment and reservoir characteristics of lacustrine shale in the second sub-member of Jurassic Da’anzhai Member in Yuanba area, northeast Si-chuan Basin were systematically analyzed. The research results show that: The Sr/Cu, Sr/Ba, and V/(V+Ni) of the Da’anzhai Member shale range from 2.50 to 8.34, 0.15 to 0.47, and 0.65 to 0.79, indicating the warm and humid environment and anoxic freshwater lacustrine sedimentary facies, shallow lacustrine sedimentary facies and semi-deep lacustrine sedimentary facies. There are four lithologic assemblages: bedded mudstone intercalated shell limestone, sand-lime-mud ternary mixing, laminated-bedded mudstone intercalated shell limestone, thick bedded plaster mixing. Shale minerals are mainly clay minerals, quartz and calcite, with a small amount of feldspar, do-lomite and pyrite. The pore types are mainly inorganic pores, especially intergranular clay pores, while the content of organic matter pores is relatively small. Laminate-bedded mudstone intercalated limestone facies and bedded mudstone intercalated limestone facies have the characteristics of high brittleness, high porosity and high gas content due to their good source-reservoir configuration. At the same time, cleavage fractures and intercalated laminae are developed, which become the main strata in the Development of Da’anzhai Member.