Characterization and Evolution of Nanoporosity in Superdeeply Buried Shales: A Case Study of the Longmaxi and Qiongzhusi Shales from MS Well #1, North Sichuan Basin, China

Abstract

The nanopore characteristics of shale samples from the superdeeply buried Longmaxi Shale (drillcore recovered from 6604–6920 m below ground level), Wufeng Shale (6920–6926 m), and Qiongzhusi Shale (7960–8044 m) were studied from MS Well #1, Sichuan Province, China, which was completed in March 2016 and is the deepest onshore well yet drilled in Asia. To gain a better understanding of the influence of burial depth on the pore system of shales and to aid in the study of nanopore characteristics, the samples were analyzed by FESEM and N2 gas adsorption. Samples of Sichuan Basin shales recovered from depths ranging from 0 to 5000 m were selected as a control group. The results show similar nanopore characteristics in all 32 superdeeply buried shale samples from the three formations. The dominant pore types in the superdeeply buried shales are organic matter pores and interparticle pores, along with minor intraparticle pores. The dominant pore morphology is slit-like in shape. Low-pressure N2 adsorption analysis shows that the isotherms of all samples are type IV with an H3 hysteresis pattern. The quenched solid density functional theory (QSDFT) pore size distribution is dominantly in the range of 4–16 nm, and the BET surface area ranges between 8.63 and 16.13 m2/g. In comparison with nonsuperdeeply buried shales, superdeeply buried shales in MS Well #1 have a more dispersed pore-size distribution, lower micropore volume and micropore surface area, and higher mesopore volume and mesopore surface area. Thus, the mesopore/micropore volume and mesopore/micropore surface area ratios of the superdeeply buried shales are several orders of magnitude higher than those of the nonsuperdeeply buried shales. Compaction related to burial depth may compress the pores to reduce the pore sizes and change the pore shapes from round or elliptical-shaped to slit-shaped. Given their relatively small pore sizes, micropores are most easily destroyed during the superdeep burial stage.