Characteristics and significance of sedimentary facies of Wufeng-Longmaxi formation shale in Weirong Shale Gas Field, southern Sichuan Basin

The sedimentary microfacies of organic-rich shale in the Wufeng-Longmaxi formations in the Weirong Shale Gas Field of the southern Sichuan Basin were studied based on core observation, organic carbon content, biological identification, in situ gas content and well logging data. The shale from the Wufeng Formation to the first member of Longmaxi Formation mainly developed in a deep-water shelf environment. They were divided into six microfacies mostly depending on mineral composition, including clayey siliceous, siliceous-rich biological, siliceous clay, calcareous clay, calcium-rich clay and clay-rich deep-water shelf micro-focies. Vertically, the first four facies are characterized by high TOC content (≥ 3%), high porosity (≥ 5%) and high gas content (≥ 2 m³/t). The high abundance of organic matter provided a material basis for shale gas generation, and a large number of organic pores generated during hydrocarbon generation provided space for shale gas occurrence. Moreover, siliceous-rich biological shale is the most favorable facies zone and dessert zone for shale gas exploration and development, due to its high content of brittle minerals, high porosity and permeability, and easy fracturing. From the view of horizontal distribution, the west block of Weirong Shale Gas Field has a greater gas exploration potential than the east block because of greater thickness of the four favorable facies.