Geological Conditions Of Shale Gas Generation Of Wufeng-Longmaxi Formation In Western Hunan-Hubei

As the conventional oil and gas resources being exploited constantly in our country,the resource is becoming more and more exhausted that leads to the decrease of oil and gas production.In this case,the exploration and development of unconventional oil and gas resources receive more attention.According to preliminary estimate,the total reverses of Chinese unconventional oil and gas is similar to American,which indicates our country has good potential on unconventional resources development.Strata of lower palaeozoic in Western Hunan-Hubei have good geological conditions to form shale gas,so it becomes one of the most important shale gas research areas of our country.This article based on other researchers'previous research,and combines a lot of data of new sections and wells,and uses the theories of petroleum geology,organic geochemistry and sedimentology as guidance,to do deep research on the geological conditions for shale gas hydrocarbon generation of Wufeng-Longmaxi formation in Western Hunan-Hubei by analyzing facies and parameters of organic geochemistry.The plane distribution rule of geological conditions that mentioned above can be used to predict the facorable districts of shale gas hydrocarbon generation.The strata of Wufeng-Longmaxi formation in Western Hunan-Hubei mainly developed as deep-water shelf and shallow shelf.The main lithologies include mudstone,shale,and siliceous or carbonaceous shale.In this area,organic-rich shale can be found mostly in deep-water shelf environment.Strata thickness of dark shale in this area changes a lot,the larger strata of the dark shale are mainly distributed in the west of Baojing-Cili and the northeast of Yichang,and target strata in these two districts are also high on TOC.According to the sections and wells,strata that high on TOC mainly concentrate in Wufeng formation and the bottom of Longmaxi formation,that's also deep-water shelf environment,on the countary,shale in shallow shelf is low on TOC.And the Ro data shows that over half area in Western Hunan-Hubei is in the stage of high-over mature evolution,a few area have even reached the hydrocarbon generation threshold.Carbon isotope analysis indicates that type of kerogen is mixed,belongs to the II₁ type.The buried depth of Wufeng-Longmaxi formation in this area changes significantly,most area is in 0²000m,in Sangzhi-Lixian-Dangyang area and the south of Wanzhou,the buried depth can be over 3000m,even more than 4000m in some place.The result of thermo-evolution simulation shows that the source rock of Wufeng-Longmaxi formation entered the oil generation threshold in the late Silurian,and reached the oil generation peak in middle Triassic,in the late Triassic gas started to generate,and reached the gas generation peak in early Jurassic,entered the stage of over mature evolution.According to the definition standard of shale gas given by the Ministry of Land and Resource,and uses the plane distribution data of TOC,thermo evolution,thickness,buried depth and other useful information to do multiple geology factors overlay technique.The result shows that favorable districts of shale gas hydrocarbon generation of Wufeng-Longmaxi formation in Western Hunan-Hubei mainly distribute in northeast of Yichang,like Dangyang,and Xuanen-Enshi-Badong area,Baojing-Longshan area.