Geochemical Characteristics and Gas-Bearing Properties of Shale Gas in the Chang 7 Member of the Yanchang Formation, Ordos Basin

Research on shale reservoir characteristics and the controlling factors of gas content is critical for predicting shale gas “sweet spots”. Based on integrated geochemical, mineralogical, and petrophysical analyses, this study investigates the mineral composition of shale and the influencing factors of gas content in the Chang 7 Member of the Ordos Basin. The results show that (1) the shale clasts are mainly composed of quartz and feldspar, with the quartz contents in the range of 10.6%–29.7%, the feldspar contents in the range of 5.2%–38.2%, and the clay mineral contents in the range of 25.0%–70.5%. (2) The shale pore types include intergranular pores, granular dissolved pores and organic matter pores. The shale pores are mainly mesoporous and the pore sizes are mainly 6–9 nm with an average of 7.2 nm, accounting for 50% of the total pores population. (3) The shale kerogen maceral assemblage is composed primarily of the sapropel group, accounting for 77.2%–94.9% of the total macerals; vitrinite reflectance ranges from 0.80% to 1.13%, with a mean value of 0.83%. (4) The gas content of shale reservoirs is mainly controlled by temperature, pressure, total organic carbon (TOC) content, vitrinite reflectance, and reservoir physical properties. Among them, medium-to-large pores, TOC content, and pressure are the main controlling factors of shale gas content. The clay mineral content, vitrinite reflectance, and gas content constitute secondary controlling factors.