Lower Cambrian Shale Gas in Southern China: A Review of Distribution, Geochemical Characteristics, and Its Genesis...

The Lower Cambrian shale gas resources in southern China have huge potential. However, the reservoirs generally contain a relatively high content of nonhydrocarbon gases, which has a significant impact on the exploration and development of shale gas. The source of nonhydrocarbon gases in the Lower Cambrian shales has become a research hotspot. Based on recently published data, this paper systematically summarizes the geological and distribution characteristics, gas-bearing properties, and genesis mechanism of Lower Cambrian shale gas in southern China. The Lower Cambrian shales in southern China are widely distributed in the Yangtze Platform. The overall total organic carbon (TOC) content is relatively high. The kerogen type is mainly type I, and the equivalent vitrinite reflectance (EqRo) value ranges from 2.5 to 6.0%. The mineral components of the Lower Cambrian shales are mainly quartz, clay minerals, and carbonate minerals, and the main lithofacies types are mainly siliceous shale and mixed (siliceous-clay) shale. The gas-in-place (GIP) content of the Lower Cambrian shales varies greatly in different regions. Currently, shale gas, mainly composed of CH₄, has only been discovered in the southwestern Sichuan, western Hubei, northeastern Chongqing, and northern Guizhou areas, while in other Yangtze regions, the GIP content of shales is very low or almost nonexistent. The distribution of δ¹³C₁, δ¹³C₂, and δ¹³C₃ in the Lower Cambrian shale gas shows a significant inversion. Among the nonhydrocarbon gases, N₂ mainly originates from atmospheric and/or pyrolysis, CO₂ is mainly of organic origin, while He exhibits typical shell-source characteristics, which may be from the ancient basement and/or the radioactive decay accumulation of U and Th in the shale. On this basis, an evolution model of the Lower Cambrian shale gas was constructed. Hydrocarbon generation in the Lower Cambrian shales can continue until EqRo = 3.5%, after which N₂ is mainly produced. CO₂ is mainly formed in the low mature stage, and the He content shows continuous generation throughout the shale evolution process.