Reservoir Characteristics and Gas Content of Black Shale of Wufeng-Longmaxi Formation in Qianjiang Area, Southeast Chongqing, China

Abstract

The shale gas exploration in Changning, Weiyuan, Fuling Jiaoshiba, and other areas has achieved great success in the Sichuan Basin, and the basin margin of the Sichuan Basin will be an important target in the future. In view of the lack of understanding of shale gas reservoir characteristics of the basin margin area of the Sichuan Basin, this paper conducts a study on reservoir characteristics and gas-bearing properties of this area using qualitative and quantitative techniques. The results show that the thickness of the black shale reservoir in the Qianjiang area is up to 33 m, the kerogen sapropelic type (I) is dominant, and Ro ranges from 2.2 wt% to 3.39 wt%. The mineral composition is relatively complex, mainly composed of quartz and clay minerals. Quartz content is in the range between 9.4 wt% and 79 wt% ( $\text{average}=47.48\text{wt}\%$ ), while clay content is in the range between 10.02 wt% and 43.6 wt% ( $\text{average}=29.41\text{wt}\%$ ). There are various types of reservoir spaces, mainly including organic pores dominated by matrix pores, intergranular and intragranular pores, intergranular and intragranular dissolution pores, secondary dissolution pores, and microfractures, among which organic pores are the main pore types. The pore volume of nanopores is 0.009897-0.017177 cm3/g ( $\text{average}=0.01304\text{c}{\text{m}}^3/\text{g}$ ), and the specific surface area ranges from 12.1 m2/g to 35.1 m2/g ( $\text{average}=20.47{\text{m}}^2/\text{g}$ ). Micropores and mesopores are the main pore sizes for the development of organic pores. Quartz particles promote the development of reservoir space, while clay minerals are not conducive to the development of pores. The total gas content ranges from 0.5 m3/t to 4.43 m3/t ( $\text{average}=1.69{\text{m}}^3/\text{t}$ ). Its enrichment is controlled by many factors. The increase of organic matter abundance is the material basis for shale gas enrichment. The development of organic pores provides high-quality storage space for shale gas preservation. The development of different mineral components restricts the enrichment of shale gas.