Abstract: The hydrocarbon storage capacity of shale reservoirs depends on their complex pore structures, which vary by lithofacies of shales. In order to clarify the control of shale lithofacies on the pore structure, the lithofaices of the shales in the Da'anzhai member of Jurassic Ziliujing formation in central Sichuan basin were determined based on total organic carbon and X-ray diffraction analyses, and the pore structure characteristics of the shales were identified by means of thin section observation, and analysis on scanning electron microscopy, low-temperature nitrogen adsorption and high-pressure mercury injection. The results show that six shale lithofacies (organic-rich clayey shale, organic-moderate clayey shale, organic-poor clayey shale, organic-moderate mixed shale, organic-poor mixed shale, and organic-poor calcareous shale) are mainly developed in the Da'anzhai member, with parallel plate-like and slit-like pores dominantly. Clayey shales mainly contain clay mineral interlayer pores, organic matter pores, and fractures induced by hydrocarbon generation pressurization; mixed shale mainly contains residual intergranular pores; and calcareous shale mainly contains a small amount of dissolution pores. For all these lithofacies, the clay mineral content is positively correlated with pore volume and specific surface area, and the TOC is positively correlated with the macropore volume of organic-rich clayey shale. The organic-rich clayey shale exhibits the largest macropore volume and trimodal pore-size distribution, making it the most favorable lithofacies for shale oil storage in the Da'anzhai member in central Sichuan basin.

Key words: central Sichuan basin, Lower Jurassic, Da'anzhai member, reservoir, shale lithofacies, pore structure, controlling factor