中国煤岩气生成阶段及气藏成因类型OA

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(Ro＜0.5％),mid-coal-rank transitional gas generation stage(0.5％≤ Ro＜0.8％),mid-coal-rank mature gas generation stage(0.8％≤ Ro＜1.3％),mid-coal-rank high-maturity gas generation stage(1.3％≤ Ro＜2.0％),and high-coal-rank overmature gas generation stage(Ro ≥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 rock,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 rock from shallow to deep depths,the coal rock 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.