The Study On Stratigraphic Sequence And Sedimentary Microfacies Of The Shahezi Formation Based On The Long Cores Of The ICDP Scientific Drilling Borehole In The Songliao Basin(SK-2)

SK2 is the deepest scientific drilling well funded by ICDP so far,is also the deepest scientific drilling well in China.The goal of the SK2 drilling project is to obtain Cretaceous continental continuous and complete sedimentary record by developing full coring drilling for Deep Lower Cretaceous strata.Up to now,SK2 borehole has been drilling to 7096.02 m with footage of 4340.09 m,and get 4179.01 m long cores with recovery of 96.29%.In this paper,based on SK2's long continuous Shahezi Formation cores with logging response characteristics and seismic facies characteristics,we established the high precision lithological sequence and stratigraphic sequence of Shahezi Formation and have conducted sedimentary facies analysis and preliminarily explained the change of sedimentary environment of Shahezi Formation,thus the following understandings were obtained:(1)Shahezi Formation is 2214.66 m long continuous cores in SK2 borehole,6 categories and 12 kinds of lithology were identified,mainly sedimentary rocks,in accordance with the thickness ratio from high to low are mudstone,grit stones,siltstone,conglomerate,fine sandstone,medium sandstone and lithology containing volcano material,including tuffaceous sandstone,sedimentary breccia tuff,tuffaceous siltstone,sedimentary tuff,tuff and tuffaceous mudstone.(2)In the Shahezi Formation period,the volcanic activity in the Songliao basin is dormant,the fracture activity is frequent,and sedimentary process is dominant,sedimentary rocks of fan delta facies are developed,with 3 subfacies: fan delta plain,fan delta front,profane delta,6 microfacies: braided channel,subaqueous distributary channel,inter subaqueous distributary channel,subaqueous mouth bar,sand sheet,mudstone of still water,showing a fining-upwards sequence.(3)According to the lithology combination,the Shahezi Formation was divided into 6 cycles which all of them are fining-upwards sequence.However,according to the sediment granularity,the particle size of cycle ?with conglomerate mainly developed is maximum,that of cycle ? with pebbly grit stone widely developed is second,and that of cycle ? with mudstone and silty mudstone mainly developed is minimum.(4)From aspects of the lithology,logging response characteristics and seismic facies,the unconformities developed in the Shahezi Formation was fully analyzed,we concluded that the top interface of the Shahezi Formation(overlying the Yingcheng Formation)is located in 3335.99 m,the bottom interface(underlying the Huoshiling Formation)is located in 5613.41 m.The Shahezi Formation is divided into the upper and lower part with an inner unconformity in the depth of 4899.23m.