Abstract
Marine–terrestrial transitional Permian shales occur throughout South China and have suitable geological and geochemical conditions for shale gas accumulation. However, the Permian shales have not made commercial exploitation, which causes uncertainly for future exploration. In this study, high-pressure methane (CH4) adsorption experiments were carried out on the Permian shales in the Lower Yangtze area, and the influences of total organic carbon (TOC) content and temperature on adsorption parameters were investigated. The characteristics and main controlling factors of methane storage capacity (MSC) of the Permian shales are discussed. The results show that the maximum adsorption and the adsorbed phase density of these Permian samples are positively correlated with TOC contents but negatively correlated with temperatures. The pores of organic matter in shale, especially a large number of micropores and mesopores, can provide important sites for methane storage. Due to underdeveloped pore structure and poor connectivity, the methane adsorption capacities of the Permian shales are significantly lower than those of marine shales. Compared with the Longmaxi shales, the lower porosity and lower methane adsorption of the Permian shales are reasonable explanations for their lower gas-in-place (GIP) contents. It is not suitable to apply the index system of marine shales to the evaluation of marine–terrestrial transitional shales. The further exploration of Permian shales in the study area should be extended to overpressure stable reservoirs with high TOC contents (e.g., >5%), high porosity (e.g., >3%), and deep burial (e.g., >2000 m).
Highlights
IntroductionThe predicted recoverable shale gas resources of the Yangtze Plate account for approximately 70% of the terrestrial resources of China [1], which occur in two sets marine shales (the Lower Silurian Longmaxi Formation and the Lower Cambrian Qiongzhusi Formation) and in the marine–terrestrial transitional Permian strata system [2–8]
The predicted recoverable shale gas resources of the Yangtze Plate account for approximately 70% of the terrestrial resources of China [1], which occur in two sets marine shales and in the marine–terrestrial transitional Permian strata system [2–8]
It will be reduced to zero when the bulk methane density is equal to the adsorbed phase density if the experimental pressure continues to increase
Summary
The predicted recoverable shale gas resources of the Yangtze Plate account for approximately 70% of the terrestrial resources of China [1], which occur in two sets marine shales (the Lower Silurian Longmaxi Formation and the Lower Cambrian Qiongzhusi Formation) and in the marine–terrestrial transitional Permian strata system [2–8]. There are many differences between the marine–terrestrial transitional shales and the marine shales in terms of gas generation, reservoirs, and preservation conditions [5–8]. In essence, it is mainly reflected in the different organic matter types. The Permian in the Lower Yangtze area is mainly composed of organic-rich shale, occurring in a large regional extent, and have moderate burial depths (generally 1000–4000 m) and large thicknesses (about 100–600 m), which are excellent geological conditions for shale gas generation and preservation [17,23–26]. The relevant reports of the high-pressure adsorption of Permian shales are very limited [25], and it is difficult to accurately predict the GIP of Permian shales under geological conditions In this context, systematic methane adsorption experiments were performed on the homologous Permian shales collected from relatively shallow reservoirs. The investigation of shale storage capacity is helpful to understand its potential as the carrier for CO2 geological storage [34,35]
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