Main controlling factors and types of continental shale oil and gas enrichment in Sichuan Basin

Continental shale gas has achieved commercial production in the Lower Jurassic Daanzhai Member in the Fuling and Yuanba areas in the Sichuan Basin; however, the accumulation conditions are different from the marine facies Lower Silurian Longmaxi shale gas, and the exploration potential remains to be explored. The X-ray diffraction analysis of mineral composition, geochemical analysis, argon ion polishing-scanning electron microscopy, mercury intrusion-adsorption combined measurement, pulse permeability, nano-CT and gas content were applied to study three key factors controlling the enrichment of lacustrine shale gas, including lithologic combination, maturity and pressure conditions. The shales deposited on a storm beach of the carbonate lake slope of the lake facies are interbedded with the coquina limestone of unequal thickness, which is the most favorable lithologic combination with favorable configuration conditions for shale gas generation (organic matter abundance greater than 1.4%), reservoir (porosity greater than 3%), permeability (horizontal fracture developed) and fracturing (brittleness index greater than 0.6). Maturity controls the development of organic pores and the formation of shale oil and gas (Ro=1.3% as the maturity boundary). Overpressure (pressure coefficient greater than 1.2) is the key to the enrichment and high yield of continental shale oil and gas. The accumulation of continental shale oil and gas can be divided into two types: ultra-high pressure, high maturity shale interbedded with limestone and ultra-high pressure, low maturity shale interbedded with limestone. The most favorable target for shale gas exploration in the lacustrine facies of the Sichuan Basin includes the Daanzhai Member in Yuanba and the Dongyuemiao section in Fuling. The Daanzhai Member in the northern Fuling and Langzhong-Pingchang, the Dongyuemiao Section in Jiannan, and the second section of Qianfoya Formation in Yuanba are secondary favorable target for shale oil and gas exploration.