Geological and geochemical characteristics of marine-continental transitional shale from the Upper Permian Longtan formation, Northwestern Guizhou, China

Organic-rich shale deposited in a marine-continental transitional facies was well developed in the upper Yangtze area during the Late Permian. It is considered to contain a large amount of shale hydrocarbon resources. This paper takes the Upper Permian Longtan shale in the central Guizhou uplift as an example to characterize the transitional shale reservoir. Based on organic geochemistry data, X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM) observation, the geochemical characteristics of this transitional shale were systematically studied and the shale gas potential was investigated. The results indicate that the Longtan shale has fair to good hydrocarbon potential with high total organic carbon (TOC) and contains type III gas-prone organic matter. Its high thermal maturity favors the generation of gas (Ro, 1.10%–2.74%). XRD analyses show that the mineral components of the transitional shale are dominated by high clay content (58.7 wt%). The clay minerals are dominated by illite (44%) and mixed illite/smectite (43%). SEM observation shows that intra-particle pores, inter-particle pores and microfracture were well developed in the Longtan shale, which can provide reservoir space for the accumulation of shale gas. Organic matter pores appear rare due to its organic type. In summary, the Longtan shale has high organic content, high maturation, and high gas content, all of which demonstrate its great potential as a shale-gas resource. But the high clay content of the Longtan shale may be a challenge to shale-gas production.