Study On The Main Controlling Factors Of The Coal-Formed Gas Reservoir Forming In Dongpu Depression

In order to discuss the main controlling factors of coal-formed gas accumulation in Dongpu sag,source rock thermal evolution history,coal-formed gas distribution characteristics and reservoir forming process of typical coal-formed gas reservoir were all studied in this work,applying the theories and methods of petroleum geology,organic geochemistry,structural geology and basin modeling analysis synthetically.The C-P source rock and its thermal evolution history in Dongpu depression are studied by thermal simulation experiment and basin simulation technology.The results are as follows: the hydrocarbon potential of C-P coal rocks,carbonaceous mudstone and mudstone decreases in turn,the type of kerogen are mainly III type,and part of II2 type;the gas generation from different coal measures can be divided into 5 stages,which are in a lot of gas generation stage when Ro=2.2%,gas generating capacity of coal is the strongest,the hydrocarbon generating capacity of the secondary hydrocarbon is obviously lower than that of the first one,the drying coefficient is increased,and the coal rock still have strong hydrocarbon generating capacity;The C-P source rocks mainly generated in the late Dongying stage and Minghuazhen-nowadays stage,The main hydrocarbon generating area concentrated in Qianliyuan sag,Gegangji sag,Menggangji sag and Qiaokou-Xuji area.Through the analysis of natural gas composition,carbon isotope,argon isotope of coal-formed gas for identification,the distribution of coal-formed gas is determined.Coal-formed gas is mainly distributed in Wen 23,Hubuzhai,Machang,Baimiao,Fangliji and Huzhuangji areas.The coal-formed gas accumulated in the central uplift belt mainly distributes in Es4 and C-P members.The coal-formed gas accumulated in the downthrown of Lanliao fault and western slope mainly distributes in Es4,Es3,Es2 and C-P members.Integrated the development characters of fault,sand,fracture,and their time and space effectiveness,combining with natural gas migration path tracing,finally determined the coal-derived gas in the study area have ?far from the source with the fault-sand matching transportation system,the transporting ability of gas source faults is mainly influenced by fault activity and the contact area of fault and hydrocarbon source rock,the convex shape of gas source faults indicates the favorable migrating direction.Analysis of fluid inclusions testing technique has been clear about the coal-formed gas of the structural belt reservoir in the study area are mainly accumulated in dongying period.Analysis on gas reservoir preservation condition through the fault-cap rock configuration relations and GOI technique shows that relationship between fault and cap configuration affects the accumulation and preservation of hydrocarbon.The preservation conditions of Wen 23 and Hubuzhai areas are better than Baimiao and Machang areas.Comprehensive study shows that,the main control factors of coal-derived gas accumulation in the study area mainly includes following three aspects: mature source rock – gas source fault-trap effective time and space configuration is a key factor of coal-derived gas accumulation,gas-preservation conditions complementary relationship control the scale of coal-derived gas accumulation,fault-cap combination control of coal-derived gas enrichment;proposing reserves control coefficient(RCC)for quantitative characterization on gas supply and preservation conditions complementary relationship.Combining with the coal-derived gas accumulation characteristics of different structural belt in the study area,two accumulation models were established: "single sourse supply for hydrocarbon–fault-sand transformation-single formation accumulation" and "double source supply for hydrocarbon-branch of transformation-the multi-formation accumulation".