煤层气已成为一种新兴的非常规天然气资源。煤层气是成煤物质在煤化过程中生成并储集于煤层中的气体。按其成因类型分为生物成因气和热成因气。生物成因气有原生和次生两种类型，原生生物成因气一般在低级煤中生成，很难保存下来。次生生物成因气常与后来的煤层含水系统的细菌活动有关。热成因煤层气的生成始于高挥发份烟煤（Ro＝０.5％～0.8％）。与分散的Ⅰ／Ⅱ型或Ⅲ型干酪根生成的气体相比，煤层气的地球化学组成变化较大，反映了控制煤层气组成和成因的因素多而复杂，主要的影响因素包括煤岩组分、煤级、生气过程和埋藏深度及相应的温度压力条件。此外，水动力等地质条件和次生作用等也影响着煤层气的组成。

Only recently has coalbed gas been recognized as a new nonconventional natural gas resource. Coalbed gas is generated and stored in coal by two distinct processes during the coalification process: biogenic and thermogenic. Biogenic gases can be divided into primary biogenic gas and secondary biogenic gas depending on differences of geologic time and source rocks. Primary biogenic gas is gerenated at lower coal rank and is difficult to perserve. Secondary biogenic gas is usually associated with late-stage groundwater system and bacteria activity. Generation of thermogenic coalbed gas begins at Ro= 0.5%~0.8%. Coalbeds are source rocks for liquid hydrocarbons and gases. Compared with gases generated from dispersed types and kerogen or type kerogen, isotope and composition of gases produced from coalbeds are highly variable. The variation indicates many and complex controls on composition and origin of coalbed gas, including maceral composition, coal ranks, gas generation processes and depths of burial(temperature and pressure). In addition, hydrologic setting and secondary processes also influence composition of coalbed gas.