Shale gas potential of Yanguanjie Formation and Longtan Formation in central and southeastern Hunan Province

In order to reveal the shale gas potential and exploration direction of the marine-continental transitional shale in central and southeastern Hunan, samples were collected from the Lower Carboniferous Yanguanjie Formation and Upper Permian Longtan Formation, measurements were conducted on TOC, R_ran, kerogen carbon isotope, maceral composition, X-ray diffraction, scanning electron microscopy, porosity, permeability and methane sorption capacity. The results indicate that Yanguanjie Formation and Longtan Formation shale are at the mature stage, and mainly dominated by type Ⅲ kerogen. The Longtan shale generally has higher TOC content, while Yanguanjie shale has usually low TOC content. The minerals are mainly dominated by clay mineral and quartz, some Yanguanjie shale has high content of calcite. Pores in the two set of shales are not well developed, and their main pores are organic matter pore, dissolution pore and microfracture. The porosity of the Yuanguanjie and Longtan shales varies from 0.41% to 2.73%, and the permeability is between 0.08×10^-3 μm² and 0.98×10^-3 μm². The porosity is positively affected by TOC content. Unstable minerals such as feldspar and carbonate could provide some pore space, but have limited influence on shale physical property. However, the porosity is negatively correlated with clay mineral content. Methane sorption capacity of shales is generally between 1.67 and 2.5 cm³/g. The desorbed gas content of Longtan Formation shales from well 2015H-D3 is generally higher than 0.5 cm³/g, with a highest value of 2.35 cm³/g, indicating that Longtan Formation in central and southeastern Hunan Province has a certain shale gas potential. However, the shale gas potential of the Yanguanjie Formation is relatively weaker compared to Longtan For-mation.