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 N 2 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 N 2 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 m 2 /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.

The Impacts of Matrix Compositions on Nanopore Structure and Fractal Characteristics of Lacustrine Shales from the Changling Fault Depression, Songliao Basin, China

“…Shales display strong heterogeneity at the micro scale and becomes weaker heterogeneity in a larger field of view, therefore, representative elementary area (REA) were taken at a magnification of ×20,000 [39]. REA images of the Shahezi shales were quantitatively studied using Image-Pro Plus software [40]. The parameters of pores image processing, including pore width, length, perimeter, area and fractal dimension, are listed in Table 2.…”

In situ Raman spectroscopic quantification of CH4–CO2 mixture: application to fluid inclusions hosted in quartz veins from the Longmaxi Formation shales in Sichuan Basin, southwestern China

“…It is also one of the most important shale gases playing in China at present (Jia et al 2012;Zou et al 2015;Jin et al 2016;Ran et al 2016). Many studies are focused on the current fluid pressure in the Longmaxi shales (e.g., Zou et al 2015;Jin et al 2016;Jiao et al 2018;Wang et al 2019), whereas the variation of paleo-fluid pressure has rarely been discussed. Shale gas is different from conventional natural gas in that the formation itself is both the source and the reservoir.…”

Variation of Organic Pore Structure With Maceral Types in the Longmaxi Shale, Sichuan Basin

“…Although thermal maturity may be one of the most important controlling factors for pore structure in organic matters, as discussed above, other factors should also be focused on, such compacted, the depth of buried shale, and coefficient (Jiao et al, 2017;Liu et al, 2019). The pressure coefficient of well ys118 (1.65 by Wang et al, 2018) is higher than that of well jy2 HF (1.5 by Feng et al, 2018).…”