Geological characteristics and resource prospect of deep marine shale gas in the southern Sichuan Basin

There are abundant deep shale gas resources in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation of southern Sichuan Basin, which is a replacement area for increasing gas reserves and production in the Sichuan Basin. So far, however, the geological characteristics of deep shale gas reservoirs in this area have not been understood comprehensively. In order to evaluate the resource potential of deep shale gas in the southern Sichuan Basin, determine the favorable exploration areas and provide reference for the three-dimensional development of shale gas in this area, this paper systematically analyzes the sedimentary environment, organic geochemistry, reservoir physical properties, rock brittleness, gas-bearing characteristics and other geological characteristics of deep marine shale. Then, the volume of deep shale gas resources is calculated by dividing evaluation units and selecting shale gas resource evaluation methods and calculation parameters. Finally, favorable exploration areas of deep shale gas are determined. And the following research results are obtained. First, the deep shale of Wufeng Formation-Longmaxi Formation in the southern Sichuan Basin is deposited in the environment of deep-water shelf facies, its average effective thickness is 61 m, its distribution area accounts for 83% of the whole area, and its resource base is great. The total organic carbon is 3.1%. The organic matter is of sapropel type and is in the stage of high-over-mature dry gas generation of crude oil cracking, with better gas generation potential. Second, organic pores, inorganic pores and microfractures are developed in the deep shale of this area. The porosity is in the range of 2.3%-8.1%. The reservoir physical properties are good. The mineral compositions are mainly quartz and clay minerals, and the content of brittle minerals is 63%, so it has better fracability. Third, the gas content of deep shale gas reservoir is high and free gas accounts for 60%. The pressure coefficient is high. And the gas preservation conditions are good. Fourth, the geological resources of deep shale gas are 25.62×1012 m3, the recoverable resources are 6.14×1012 m3, and the resource abundance is 6.67×108 m3/km2, indicating great resource potential. In conclusion, the favorable exploration areas of deep shale gas in the Wufeng Formation-lower Long 1 submember of Longmaxi Formation in the southern Sichuan Basin are Zigong-Fushun, Luxian-Luzhou and Neijiang-Yongchuan whose burial depth is less than 4500 m. © 2021, Natural Gas Industry Journal Agency. All right reserved.