Abstract: Deep coal-rock gas in China generally refers to the combination of free gas and adsorbed gas stored in coal seams deeper than 1500m and with a favorable preservation conditions. It is a type of natural gas resource with condition characteristics similar to shale gas but different from shallow coalbed methane. China shows great potential for deep coal-rock gas resources. An early understanding of its formation conditions and distribution characteristics is crucial for guiding and accelerating the reserve and production growth of deep coal-rock gas.Based on limited exploration well and trial production data, this study proposes that China’s onshore coal-rock gas mainly develops in coal seams formed in tidal flat-lagoons, lakeside swamps, and delta swamps; Micro- to nano-scale pores and cleavage fractures serve as the main storage spaces for coal-rock gas; Deep coal-rock gas accumulates into reservoirs by means of free and adsorbed states, with a relatively high proportion of free gas being an important characteristic that distinguishes it from shallow coalbed methane; Deep coal-rock gas can be divided into two categories based on gas sources: self-generated and self-stored, and externally-sourced and self-stored, with the latter exhibiting a greater proportion of free gas; Effective sealing layers and deep burial depth are essential conditions for the formation of deep coal-rock gas.Furthermore, this study identifies two major types of coal-rock in China: medium- and high-rank coal (R_o ≥ 1.3%) and low- and medium-rank coal (R_o < 1.3%). In medium- and high-rank coal, the enrichment of coal-rock gas benefits from the high gas generation potential, low formation water content, stable structure, well-developed cleats, and effective sealing conditions from the roof and floor. In low- and mediumrank coal, the enrichment of coal-rock gas is attributed to the cumulative enrichment of low gas generation rate, continuous distribution of high-quality coal seams, coupling of structure and gas sources. In addition, natural gas accumulated in adjacent tight sandstone (limestone) is also an important contributor to low- and medium-rank coal-rock gas. China’s deep coal-rock gas resources are abundant, yet exploration remains in its early stages. In the future, deep coal-rock gas production will rise rapidly, and is expected to reach its peak around 2040.

Key words: Deep coal-rock gas, Free gas, Adsorbed gas, Accumulation conditions, Distribution characteristics, Development prospects