CONTROL OF SHALE LITHOFACIES ON THE SHALE OIL RESERVOIR IN QIANJIANG SAG

The rock mineral compositions, porosity-permeability, pore structure and geochemical characteristics of the shale reservoir of Qianjiang Formation in Qianjiang Sag, Jianghan Basin, are studied on the basis of core observation and rock slice identification, combined with XRD technology, argon ion polishing scanning electron microscopy, low temperature nitrogen adsorption experiment, high pressure Hg injection test and rock pyrolysis analysis. The results show that the shale of Qianjiang Formation is mainly composed of dolomite and clay minerals, developed with massive calcareous mudstone facies, laminated calcareous mudstone facies, massive dolomitic mudstone facies, laminated dolomitic mudstone facies, massive argillaceous dolomite facies and laminated argillaceous dolomite facies. The carbonate mineral intercrystalline pores and clay mineral interlayer pores are mainly developed in the shale reservoir, with the pore size of 2-200 nm, the porosity mostly lower than 20%, and the permeability of 0.1×10^-3-100×10^-3 μm², belonging to low porosity and low-extra low permeability reservoir. Clay minerals are conducive to the development of micropores and mesopores in shale reservoirs, while the dolomites contribute to the development of macropores, and biogenic dolomites help the accumulation of organic carbon and occurrence of residual hydrocarbon. Generally, the laminated shale has higher pore size, better pore connectivity and oil content than the massive shale dose. Especially the laminated argillaceous dolomite facies, with porosity of 5%-15%, permeability of 1×10^-3-10×10^-3 μm², main pore size of 50-200 nm, connected pore size of 50-100 nm, TOC content of 1%-3%, S₁ content of 5×10^-3-35×10^-3 and OSI value mostly 400×10^-3, is the dominant lithofacies of shale oil reservoir in Qianjiang Formation.