Natural gas types and coal-rock gas classification in the whole petroleum system of coal measures

Abstract

There are various types of natural gas resources in coal measures, making them major targets for natural gas exploration and development in China. In view of the particularity of the whole petroleum system of coal measures and the reservoir-forming evolution of natural gas in coal, this study reveals the formation, enrichment characteristics and distribution laws of coal-rock gas by systematically reviewing the main types and geological characteristics of natural gas in the whole petroleum system of coal measures. First, natural gas in the whole petroleum system of coal measures is divided into two types, conventional gas and unconventional gas, according to its occurrence characteristics and accumulation mechanism, and into six types, distal detrital rock gas, special rock gas, distal/proximal tight sandstone gas, inner-source tight sandstone gas, shale gas, and coal-rock gas, according to its source and reservoir lithology. The natural gas present in coal-rock reservoirs is collectively referred to as coal-rock gas. Existing data indicate significant differences in the geological characteristics of coal-rock gas exploration and development between shallow and deep layers in the same area, with the transition depth boundary generally 1 500–2 000 m. Based on the current understanding of coal-rock gas and respecting the historical usage conventions of coalbed methane terminology, coal-rock gas can be divided into deep coal-rock gas and shallow coalbed methane according to burial depth. Second, according to the research concept of “full-process reservoir formation” in the theory of the whole petroleum system of coal measures, based on the formation and evolution of typical coal-rock gas reservoirs, coal-rock gas is further divided into four types: primary coal-rock gas, regenerated coal-rock gas, residual coal-rock gas, and bio coal-rock gas. The first two belong to deep coal-rock gas, while the latter two belong to shallow coal-rock gas. Third, research on the coal-rock gas reservoir formation and evolution shows that shallow coal-rock gas is mainly residual coal-rock gas or bio coal-rock gas formed after geological transformation of primary coal-rock gas, with the reservoir characteristics such as low reservoir pressure, low gas saturation, adsorbed gas in dominance, and gas production by drainage and depressurization, while deep coal-rock gas is mainly primary coal-rock gas and regenerated coal-rock gas, with the reservoir characteristics such as high reservoir pressure, high gas saturation, abundant free gas, and no or little water. In particular, the primary coal-rock gas is wide in distribution, large in resource quantity, and good in reservoir quality, making it the most favorable type of coal-rock gas for exploration and development.