Factors influencing microstructure and porosity in shales of the Wufeng-Longmaxi formations in northwestern Guizhou, China

Thickness marine shale of Paleozoic Wufeng-Longmaxi Formations is deposited in the northwestern Guizhou province, which borders on the shale gas production area in southeast Sichuan Basin. However, few studies have been performed to characterize shale gas reservoir of the Wufeng-Longmaxi Formations in this area. In this study, 80 shale samples collected from the LY1 well were designed to conduct a series of analyses, aiming to obtain microstructure features and reveal their controlling factors. Shales within the Upper Longmaxi Formation characterized by low total organic carbon (TOC) content, high clay mineral content and low porosity are considered to be unfavorable interval for shale gas exploration. Whereas, a high-quantity shale (TOC>2%, clay<40%) interval with thickness of about 27 m exists within Lower Longmaxi Formation and Wufeng Formation. Pores in the studied shales are dominated by organic matter (OM) pores, followed intraparticle (intraP) within carbonates and intercrystalline pores within pyrite framboids. OM pores could be divided into three types, as OM pores within solid bitumen, in-situ amorphous kerogen and graptolite. OM pores are extremely developed in solid bitumen and in-situ amorphous kerogen, and the former have larger diameter than the latter. Rare pores are developed in graptolite. The total porosity and specific surface area of the shales within the Lower Longmaxi Formation and Wufeng Formation are twice more than those within the Upper Longmaxi Formation. Samples with higher TOC values commonly have higher porosities and specific surface areas, suggesting TOC content determines shale physical property. Clay mineral content exhibits a negative influence on microstructure and porosity but determines water saturation. Water saturation has significant negative relationships with micro-to mesopore volumes, porosity and TOC content, illustrating water molecular stores mainly in hydrophilic pores associated with clay minerals rather than OM-hosted nanopores. The gas generation, reservoir quantity and gas content of the Longmaxi-Wufeng shales in northwestern Guizhou are also compared with those of the famous Fuling and Weiyuan districts in eastern Sichuan Basin, and we considered that the shale gas prospect in northwestern Guizhou is excellent.