Coalbed Gas Reservoir Formation Mechanism In Dongpu Depression

Dongpu depression is the one of the hydrocarbon-prolific terrestrial sub-basins ineast China with Wen23 S4 interval as the representative coalbed reservoir. This articleconsiders coalbed gas reservoir formation condition and has carried out a study focusingon hydrocarbon formation function, gas reservoir geochemistry, coalbed gas migrationforce and phase state, migration transport system, migration stages and process and thendiscussed coalbed gas reservoir formation model and its control factors.Organic geochemical study of source rock shows Taiyuan and Sanxi formationsvertically developed source rocks, among which dark mudstone, carbonaceous mudstoneand coalbed are principle.3 periods of thermal evolution have occurred since upper Palaeozoic deposition.Indo-Chinese epoch: homogeneous temperature<100 ℃ , palaeogeothermgradient=2.6~3.0 ℃ /100m;Early Tertiary-Late Dongying: maximum temperatur,palaeogeotherm gradient=3.1~3.9℃/100m;Since Late Tertiary: higher or close to currentpalaeogeotherm gradient.3 types of natural gases exist in Dongpu depression consisting of oilbed gas, mixedgas and coalbed gas. Source rock of coalbed gas is Carboniferous –Permiancarbonaceous mudstone, mudstone and coal;while source rock of well Wengu2 and Bu11is dark mudstone of lower Tertiary with lacustrine facies.Coalbed gas of Dongpu depression derives directly from source rock kerogencracking. Abnormal temperature developed in source rock is the main force foraccumulation and migration, which moves in a dissociated phase state. GOI analysisshows gas dissipation occurred after Wen23 gas reservoir development. Association ofwell Wengu2 and Bu11 coalbed gas with overmatured crude of lower Tertiary indicateslate hydrocarbon infilling, which also means coalbed gas reservoir formation is ahomeostasis process with accumulation and dissipation.So we think there are mainly 4 factors controlling reservoir formation comprising ofeffective source rock as the base, transport system as the key for its formation, inheritedstructure as the favorable space and sealing condition leading to its abundance.Based on coalbed gas reservoir formation factors and its mechanism, rupturecontrolled reservoir model is built. Secondary hydrocarbon generation window beganactive during late deposition of interval S2 and gas generation reached its peak in lateDongying. Affected by abnormal high pressure, coalbed gas migrated along the faulteffectively and finally accumulated to form reservoir under the local cap rock. Faultmovement, periodic and impulse infilling of hydrocarbon, adjustment destroy and mixedreservoir of structural slope caused the coalbed gas reservoir big to small and early to lateup to downward, according to which coalbed gas in Dongpu depression is divided intotwo migration and accumulation systems.