鄂尔多斯盆地侏罗系低煤阶煤层气系统演化

Evolution of Jurassic low rank coal CBM system in Ordos basin

低煤阶煤层气作为一种非常规天然气资源，具有良好的勘探开发前景。我国低煤阶煤层气资源丰富，进行低煤阶煤层气系统演化分析，对其富集成藏及开发具有重要的理论意义。鄂尔多斯盆地煤层甲烷的碳同位素δ13C1为-33.1‰～-80.0‰，氢同位素4CHdD 为-235‰～-268‰。该盆地侏罗系煤层气藏主要有次生生物气与热成因气构成的混合型煤层气藏和热成因气藏两种类型。据构造热事件、煤层气组分及成因，结合不同阶段的煤层埋深、变质程度和生气特征等，将鄂尔多斯盆地侏罗系低煤阶煤层气系统演化划分为4个阶段：煤系浅埋-原生生物气阶段﹑煤系深埋-热成因气阶段﹑煤系抬升-吸附气逃逸散失阶段﹑煤系局部沉降-次生生物气补充阶段。其中，煤系深埋-热成因气阶段和局部沉降-次生生物气阶段是低煤阶煤层气资源的主要形成阶段。次生生物气的补充是鄂尔多斯盆地侏罗系低煤阶煤层气成功开发的重要气源。鄂尔多斯盆地侏罗系煤层气藏应属于单斜式富气成藏模式。

coalbed methane of low rank coal as an unconventional gas resource, has good development prospect. There are abundant CBM resources of low rank coal in China. It is of important significance for the study of CBM accumulation and CBM development to analyze the evaluation of CBM system of low rank coal. Generally, CBM is compositionally dominated by methane withδ13C1 values ranging approximately from -33.1‰ ~ -80‰,CH4dDfrom -235‰ ~ -268‰. In Ordos basin, there are mainly two types of CBM reservoir： mixed reservoir of secondary biogenetic gas and reservoir of thermogenetic gas. Based on the analyses of CBM components, tectonic thermal event, sedimentary and burial history of Jurassic coal-bearing strata, combined with metamorphism and gas generation process of coals, Jurassic CBM system in Ordos basin is subdivided into four evolutionary stages as follows： shallowly-buried peat and early biogenic gas stage, deeply buried coal seams and thermogenic gas stage, stage of coal measures lifting and escape as well as loss of adsorbed gas, stage of local subsidence of coal measures and compliment of secondary biogenetic gas. Among them, the stage of deep burial and thermogenetic gas, the stage of local subsidence and secondary biogenetic gas are major CBM-forming stages. Supplement of secondary biogenic gas is an important gas source for successful CBM development of Jurassic low rank coal in Ordos basin. The CBM reservoirs of Jurassic in Ordos basin belongs to the typical monoclinic mode of gas enrichment and reservoir formation.