Generation stages and genetic types of coal-rock gas in China

Abstract

Based on the molecular structure transitions, hydrocarbon composition, and reservoir characteristics changes during coal evolution, combined with the production characteristics of coal-rock gas, the generation stages and genetic types of coal-rock gas in China are investigated. The generation of coal-rock gas can be divided into five stages: low-coal-rank biogenic gas generation stage (R_o < 0.5%), mid-coal-rank transitional gas generation stage (0.5% ≤ R_o < 0.8%), mid-coal-rank mature gas generation stage (0.8% ≤ R_o < 1.3%), mid-coal-rank high-maturity gas generation stage (1.3% ≤ R_o < 2.0%), and high-coal-rank overmature gas generation stage (R_o ≤ 2.0%). Based on the burial depth and gas origin, the gas reservoirs are divided into three types: shallow coalbed methane, deep coal-rock gas and exogenous coal-rock gas. According to the hydrocarbon generation stage of coal, deep coal-rock gas is further classified into: mid-coal-rank transitional coal-rock gas, mid-coal-rank mature coal-rock gas, mid-coal-rank high-maturity coal-rock gas, and high-coal-rank overmature coal-rock gas. During the dynamic evolution of coal from shallow to deep depths, the coal has experienced a hydrocarbon generation evolution sequence of “biogenic gas→transitional gas→wet gas→dry gas”, and a process of “primary pores→cleat development→peak organic matter pores→densification and fracturing + fracture opening” of reservoirs formation . The occurrence state gradually shifts from “dominance of adsorbed gas” to “continuous increase in free gas proportion”, and the development modes also transform from “long-term drainage and depressurization for desorption” to “high gas production upon well opening”. In addition, there is another type of coal-rock gas which is externally sourced, with the natural gas originating from underlying strata. This type of coal-rock gas corresponds to low-rank coals with reservoir development, where gas was accumulated under the control of tectonics, with high-proportion free gas and high initial production.