Pore Characterization and Its Controlling Factors in the Wufeng-Longmaxi Shale of North Guizhou, Southwest China

Abstract

Pore types and structure in shales have a significant impact on shale gas accumulation and shale reservoir quality. Based on data from the Wufeng-Longmaxi formation, this paper presents an in-depth investigation of pore types and structure in shales, including a detailed analysis of pore features and controlling factors of pore development. During this study, the researchers used eight testing techniques, including scanning electron microscopy (SEM) after Ar-ion milling, total organic carbon (TOC), thermal maturity, thin-section identification, X-ray diffraction, mercury intrusion, pulse decay porosity and permeability, and low-temperature N₂ adsorption. The study identified five types of reservoir spaces in Wufeng-Longmaxi shales, including organic pores, interparticle pores, intercrystalline pores, dissolved pores, and microfractures, among which organic pores and interparticle pores were the most common types. The test results suggested that the pore diameters were predominantly 5–20 nm and 5–10 μm. The findings also indicated that the pore throats were narrow and the pore connectivity was relatively good. The porosity and permeability were generally low, with the former measured at 1.27% on average and the latter at 0.0116 mD. The controlling factors of pore development in the Wufeng-Longmaxi shale were mainly thermal maturity, TOC, and mineral components. TOC had a positive correlation with the specific surface area. TOC was associated with samples with the lowest clay content, and the Brunauer–Emmett–Teller (BET) specific surface area was correlated with TOC content. The clay mineral content was negatively correlated with porosity, indicating a clear controlling role in pore development by TOC and clay mineral content. In addition, a favorable thermal maturity could contribute to the pore development in the study area.