Evolution of Jurassic low rank coal CBM system in Ordos basin

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 with δ¹³C₁ values ranging approximately from -33.1‰~-80‰, δ_CH4 from -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.