Geological characteristics and high production control factors of shale gas reservoirs in Silurian Longmaxi Formation, southern Sichuan Basin, SW China

The marine shale gas resources in the south of the Sichuan Basin in China have great potential. At present, the high-quality shale gas resources at buried depth of 2000-3500 m have been effectively developed, and strategic breakthroughs have been made in deeper shale gas resources at buried depth of 3500-4500 m. To promote the effective production of shale gas in this area, this study examines key factors controlling high shale gas production and presents the next exploration direction in the southern Sichuan Basin based on summarizing the geological understandings from the Lower Silurian Longmaxi Formation shale gas exploration combined with the latest results of geological evaluation. The results show that: (1) The relative sea depth in marine shelf sedimentary environment controls the development and distribution of reservoir. The relative deep water area in deep-water shelf developed class I reservoirs with good continuity and larger thickness. The relative depth of sea in marine shelf sedimentary environment can be determined by redox conditions. Through research, the mass ratio of uranium to thorium greater than 1.25 indicates deep water in anoxic reduction environment, and the mass ratio of uranium to thorium of 0.75-1.25 indicates semi-deep water in weak reduction and weak oxidation environment, and the mass ratio of uranium to thorium less than 0.75 indicates relatively shallow water in strong oxidation environment. (2) The propped fractures in shale reservoirs are generally 10-12 m high, if the classⅠreservoirs are more than 10 m in continuous thickness, then all the propped section would be high quality reserves; in this case, the longer the continuous thickness of classⅠ reservoirs is drilled, the higher the production is. (3) The shale gas reservoirs at 3500-4500 m depth in southern Sichuan are characterized by high formation pressure, high pressure coefficient, well preserved porosity, good pore structure and high proportion of free gas, making them the most favorable new field for shale gas exploration; and the pressure coefficient greater than 1.2 is a necessary condition for shale gas wells to obtain high production. (4) High production wells in the deep shale gas reservoirs are those in areas where Long11- Long13 sublayer is more than 10 m thick, with 1500 m long horizontal section, class I reservoirs drilling rate of over 90%, and fractured by dense cutting + high sand adding intensity + large displacement + large liquid volume. (5) The relative deep-water area in the deep-water shelf and the area of 3500-4500 m buried depth show a good coincidence in the southern Sichuan area, and the overlapping area is the most favorable exploration and development zone for shale gas in the southern Sichuan in the future. With advancement in theory and technology, annual shale gas production in the southern Sichuan is expected to reach 450×108 m3. © 2020, The Editorial Board of Petroleum Exploration and Development. All right reserved.