Abstract: Many studies have been conducted on conventional oil and gas in the Sanshui Basin, but insuffcient efforts have been made on unconventional oil and gas, especially shale gas. In this paper, based on the structural and regional sedimentary settings, and using multiple techniques such as outcrops observation, samples testing, and scanning electron microscope (SEM), the distribution, organic geochemical features and reservoir properties of the Palaeogene Buxin Formation dark shale were investigated to identify the accumulation conditions in this area. It is shown that the Buxin Formation shale was developed under the control of lacustrine sedimentary environment during the formation of the lake basin, with consistent depocenter and subsidence center, shale thickness of about 50-70 m (58 m on average), and burial depth of about 500-3000 m, which are favorable for preservation of organic matters. Dominated by Type Ⅱ-Ⅲ kerogens, the Buxin Formation shale is in the maturity stage, with TOC of 0.56%-3.29% (1.25% on average) and Ro of 0.71%-1.94% (1.31% on average), indicating that it is basically qualifed for gas accumulation. Microscopically, micropores in authigene clay minerals are dominant, with the pore-throat radius of 1-8 μm, and total pore space is developed, providing effective reservoir space for shale gas. The Buxin Formation can be divided into three members from the bottom to the top. The 2^nd Member composed of deep lacustrine sediments was deposited during the period when dark shale in the basin was formed and has the most favorable organic geochemistry parameters, relatively good reservoir properties, and greatest hydrocarbon generation potentials. In contrast, the 3^rd Member and 1^st Member have inferior properties in a descending order.

Key words: Buxin Formation, accumulation conditions, development prospects, Sanshui Basin, shale gas