Abstract: In order to reveal the pore development characteristics and controlling factors of lacustrine carbonate reservoir in the Qianjiang Formation in Qianjiang Sag, Jianghan Basin, the rock lithofacies and reservoir physical properties were systematically studied by means of XRD, thin section, porosity, and high-pressure mercury injection, taking the third member of the typical well system as the research object. The results show that: (1) eight rock types were identified: granular carbonate, micrite carbonate, granular mixed stone, fine grained mixed stone, felsic-rich clastic rock, clay-rich shale, sulphate rock, and salt rock. (2) Four major lithofacies assemblages are formed, which are felsic-rich clastic rock and shale lithofacies assemblage from north to south, granular and fine-grained mixed lithofacies with shale lithofacies assemblage,grain carbonate rock and mudstone carbonate rock intercalation shale facies association, salt rock intercalation shale lithofacies association. (3) Total porosity across different rock types is comparable, averaging between 5.5% and 7.3%, but pore sizes vary significantly. Granular carbonate and felsic-rich clastic rock exhibit larger pore sizes, with high-pressure mercury intrusion revealing an average pore volume ratio of >10% and average throat radius >140 nm for micron-sized pores; in contrast, micrite carbonate and fine-grained mixed rocks have smaller throats, with pore volume ratios <5% and average throat radius <10 nm. It is pointed out that granular carbonate rock and felsic-rich clastic rock have great reservoir property and large pore size, which is conducive to the accumulation, seepage, and production of hydrocarbons. They are the beneficial lithofacies types in carbonate reservoir, and are also the most favorable targets for shale oil exploration in the study area.

Key words: Lacustrine carbonate, Lithofacies, Reservoir space, Qianjiang Formation, Jianghan Basin