An resources potential analysis of Wufeng-Longmaxi Formation shale gas in the southwestern margin of Sichuan Basin

Wufeng-Longmaxi Formation in the southwestern margin of Sichuan Basin is an important field of shale gas exploration in the periphery of the Sichuan Basin. In order to study the parameter characteristics of shale reservoir and potential of shale gas resource in Wufeng-Longmaxi Formation and analyze its resource potential, the authors studied the parameters of shale by such means as geochemistry, mineral composition, scanning electron microscope, specific surface-pore diameter, and isothermal adsorption. The thickness of the organic-rich shale (TOC>1%) is generally 54-162 m, the thickness of high quality part (TOC>2%) is 26-70 m. The average organic carbon content is 2.99%-3.78%, the silicon content is 27.96% -37.0%, the feldspar content is 2.87% -4.85%, the carbonate rock content is 44.2%-54.2%, and the clay content is 9.9%-27.80%. It has the characteristics of low silicon content and high carbonate rock content and generally contains pyrite. The porosity of shale in the buried environment has a positive correlation with the gas-bearing property, and the better the gas bearing property, the higher the porosity. The main pore types are organic pore, clay mineral intergranular pore and pyrite intergranular pore. The evolution of the pores could be divided into three stages:the formation of the organic pore at the burial stage, the destruction of the organic pore at the gas reservoir destruction stage and supergene karstification. The Ro is 2.38%-3.37%, the paleogeothermal gradient is 32.57-37.55℃/km, and the terrestrial heat flow value is 84.32-97.21 mW/m². Influenced by the late Hercinian magmatic activity, the degree of thermal evolution, the paleogeothermal gradient and the terrestrial heat flow value become relatively high, and the differences between different areas could be relatively large. In Wufeng-Longmaxi Formation on the southwestern margin of Sichuan Basin, the shale deposit is favorable, the shale thickness is large, and the parameters of shale are good. The influence factors of shale gas in Longmaxi Group in this area are favorable shale deposit, good preservation condition and relatively moderate degree of thermal evolution (Ro < 3.0%). Therefore, shale gas exploration should be expanded from"structural anticline"to"stable syncline"in such places as Mugan, Gaoqiao, Yongsheng and Da Gudui synclines, where there probably exists great potential of shale gas resource.