Methane adsorption characteristics and influence factors of Mesozoic shales in the Kuqa Depression, Tarim Basin, China

Shale gas is an important unconventional energy source that has great potential for exploration and development. The Tarim Basin is the primary gas-producing basin with abundant shale gas resource in western China. Rock pyrolysis analysis, rock quantitative analysis, helium porosity measurements, scanning electronic microscope image analysis and isothermal adsorption tests at 30 degrees C were performed on outcrop samples of continental Mesozoic shales from the Kuqa Depression to determine the methane adsorption of the shale and its influencing factors. The methane adsorption capacity varied greatly in shales of different ages. Specifically, the maximum methane adsorption capacity of the Triassic shale was 0.74-4.71 mL/g and averaged 1.56 mL/g, whereas the maximum methane adsorption capacity of Jurassic shale was 1.12-14.7 mL/g and averaged 4.82 mL/g. The carbonaceous shale had high methane adsorption capacity. The methane adsorption capacity increased with increasing total organic carbon, and the free hydrocarbon content was negatively correlated with the maximum methane adsorption capacity. There was a positive relationship between the maximum methane adsorption capacity and the organic maturity in the Jurassic shale, but no correlation was observed in the Triassic shale. This lack of correlation suggests a variation in the influence of maturity on the methane adsorption in shales of different ages. There was no relationship between the clay mineral content and the maximum methane adsorption capacity, which indicates that the clay mineral content did not affect methane adsorption. The porosity had a nonlinear negative correlation with the maximum methane adsorption capacity in the Jurassic shale. The porosity of the Jurassic shale is primarily controlled by intragranular and dissolution pores in brittle minerals, and the dissolution pores provide space for free gas but contributed less to the increase in the specific surface area.