Deep Coal-rock Gas in China: A Review of Distribution, Geological Characteristics, and Its Enrichment Conditions

Deep coal-rocks in China are widely distributed and characterized by greater thickness, and coal-rock methane (CRM) resource potentials have been suggested to be huge. In recent years, the exploration and development of deep CRM have achieved major breakthroughs, attracting widespread attention. Based on recently published data, this paper systematically summarizes the exploration and development history, geological and distribution characteristics of the coal-rocks, and the main controlling factors of deep CRM enrichment in China. Deep CRM is mainly widely distributed in the northern and southwestern areas of China and primarily developed in three sets of strata: the Carboniferous, Permian, and Jurassic systems. Deep coal-rocks exhibit high total organic carbon contents, with organic matter types mainly classified as types II2 and III, generally in the mature to highly mature stages, demonstrating strong gas generation capacity. The maceral composition is dominated by vitrinite, and the industrial composition is primarily fixed carbon, which is classified as high-quality coal. The distribution of sedimentary facies, high thermal evolution degree, development of pores and microfractures, and favorable preservation conditions are significant factors contributing to the high CRM content. The primary occurrence states of deep CRM consist of adsorbed and free states. Under geological conditions, the adsorbed CRM content is mainly controlled by temperature, showing a clear negative correlation, while the reservoir pressure has a lesser impact on CRM content. Based on the analysis of the controlling factors for the enrichment and high production of deep CRM in China, it is concluded that CRM reservoirs can be classified into two accumulation models: self-sourcing and self-reservoiring and endogenous sourcing with external reservoiring.