Variation Of Sedimentary Environment Of Jurassic Buqu Formation In Gas Hydrate Scientific Drilling Hole QK1 In The Southern Qiangtang Basin

Qiangtang Basin is not only an important strategic district for oil and gas resources in China,but also the best reservoir for gas hydrates on land.Carbonate reservoir plays an important part in the production of oil and gas,while carbonate reservoir of Buqu Formation in Qiangtang stratum area occupies a more important role.Previous studies of Buqu Formation are mainly in the northern Qiangtang area,and the studies are relatively weak.This study area is located to the south of the Biluo Cuo,southern Qiangtang.By the study of microfacies of cabonate rocks,sequence stratigraphy,conditions of oil,gas and hydrates reservior of Buqu Formation in hole QK1,as well as the stratigraphic comparation with other part of the area,for the purpose of inversion of ancient geographical process,this study would add some new knowledge to the understanding of oil,gas and hydrates reservior.This paper disscusses about the Buqu Formation in hole QK1,by the method of microfacies of carbonate rocks analysis.7 facies zones are identified,as well as 11 standard microfacies,and the vertical sedimentary facies evolution is recognized:from Outer ramp-Mid-ramp environment to Inner ramp environment.Using the method of sequence stratigraphy,under the ramp-shelf carbonate sequence stratigraphy model,two complete three-levels sequences are identified in Buqu Formation:Sequence 1(Sql)which mainly develops Basin-deep ramp carbonate rocks,ramp-shelf transgressive systems tract(TST),consists of basin,outer ramp environment;ramp-highstand systems tract(HST),consists of outer ramp-inner ramp environment.Sequence 2(Sq2),mainly develops shallow marine carbonate rocks of inner ramp facies.TST consists inner ramp-outer ramp environment;HST consists inner ramp facies.By contrast with deposition model of Shuanghu area,Shuanghu-drilling area lithofacies paleogeography is established.In early time of Buqu Formation Drilling area,from south to north,distributes basin to open marine,within continuous ramp sediments.In middle time of Buqu Formation,from southeastern area of Shuanghu to the southwest,distributes ristricted-platform margin-platform margin slope.and drilling area distributes open marine sediments.Reserviors characteristics of Buqu Formation in hole QK1 is disscussed.242m-450m and 562m-660m is delineated to be the best reservoir,which is dolomite and fracture type reservoir,with integrated information about sedimentary background,environmental control,lithology,porosity,faults,fissures and other controlling factors of reservoir.Segment from 760m to 866m is full of asphalt indication,which suggests good hydrocarbon migration pathways.Although stratum of Buqu Formation are good reservoir,it is not conducive to the formation of hydrates,due to the high geothermal gradient and the lack of gas supply.