Study On The Sedimentation And Reservoir Of Jurassic Qigu Formation In Yongjin Area, The Junggar Basin

Yongjin area is located in the northeast of Shawan Sag,the central depression in the central part of the Junggar Basin.Jurassic Qigu Formation is an important oilbearing series,showing the characteristics of oil-bearing continuity.It is a realistic position to build a large oil and gas field.However,the lithology,thickness,sedimentary cycle sequence and physical properties of the Qigu Formation change rapidly in time and space,resulting in great differences in single well productivity,which affects the exploration and development process of Yongjin Oilfield.This article comprehensively analyzes the core data,logging data,threedimensional seismic data,and experimental analysis of the Jurassic Qigu Formation in the Yongjin area.Combining modern sedimentology,seismic sedimentology,and reservoir geology,this paper identifies sequence boundaries such as lithology,lithofacies transition surfaces,and lake flooding surfaces,clarifies the sequence stratigraphic division plan of the Qigu Formation,and divides it into a third order sequence SQ1,Internally,it is divided into the Low System Tract(LST)and the Transgressive System Tract(TST),where the Low System Tract corresponds to the 1,2,and 3 sand formations,and the Transgressive System Tract corresponds to the 4 sand formations.Based on the establishment of sequence isochronous stratigraphic framework,the division scheme of sedimentary facies of the Qigu Formation is established by using rock color,sedimentary structure characteristics,sedimentary structure,logging facies,geochemistry and other indicators.It is considered that shallow braided river delta front subfacies are developed in Qigu Formation,and the microfacies are mainly underwater distributary channel and interdistributary bay.During the sedimentation period of the 1st to 3rd sand formations of the Qigu Formation,the lake level decreased,and the delta front advanced towards the center of the lake basin.The outer front was affected by wave transformation,and the underwater distributary channel migrated eastward with poor continuity.During the sedimentation of the 4th sand formation,the lake level rose and the delta front shrank.Based on the planar distribution of sedimentary facies,the reservoirs of the Qigu Formation are divided into four types,and it is believed that the quality of the underwater distributary channel sand reservoirs in the delta front is good.