Sequence Stratigraphy And Favorable Petroleum Province Prediction Of Shahezi Formation In The Xujiaweizi Rift Depression

The target layer of this study is Shahezi formation in the Xujiaweizi rift depression.This stratum distribute in north-south direction as a whole and has characteristics of west-faulting and east-overlapping.The distribution of strata and depositional system are generally controlled by this tectonic framework.According to the lithology and well log,and by the sequence stratigraphy theory,the Shahezi formation in Xujiaweizi rift depression can be divided into 6 third-order sequence boundaries(SB1,SB2,SB3,SB4,SB5,SB6),5third-order sequences(SQ1,SQ2,SQ3,SQ4,SQ5)and 5 maximum flooding surfaces.Based on the core description and well log data,combining with seismic data analysis,this paper draw a conclusion that the Shahezi formation in the Xujiaweizi rift depression mainly develops 4 sedimentary facies including lacustrine facies,braided delta facies,fan delta facies and sublacustrine fan facies.The lacustrine facies,braided delta facies and fan delta facies are predominant among them,sublacustrine facies developed in local area.The fan delta facies sedimentation with the characteristics of coarse granularity sediments and chaotic seismic reflection is controlled by the western Xujiaweizi fault belt.The braided delta facies sedimentation with characteristics of coarse granularity sediments and large scale deposits is controlled by the eastern Xujiaweizi gentle slope belt.Under the control of the half-graben like rift depression,the subsidence center which mainly developed fan delta facies is located in the west-central part of the depression.But the depocenter which mainly developed lacustrine facies of fine granularity sediments with sublacustrine fan facies included is located in the central or west-central part of the depression.In the third-order sequences,after the main sediment source direction and channel,sediment controlling effects,relationship with the slope-break zone and incised valley are identified clearly,the distribution of sedimentary facies are concluded by the analyses single well facies,section facies,seismic facies,seismic attributes,various isolines and geochemical data.The planar sedimentary facies graph of third-order sequences are drawn,the sedimentary facies type,spatial distribution and evolution in third-order sequences stratigraphic framework are illustrated.Comprehensive application of core,well log and seismic data,based on tracking of seismic profile and the cross-well section,the sequence stratigraphy,sedimentary system and sedimentary facies in the research area are deeply studied through the seismic and well-log data analysis,then the sequence stratigraphic framework is established,the stratigraphic correlation is determinated,sedimentary systems and sedimentary facies characteristics are illustrated,finally the favorable petroleum province is predicted.The results show that delta front facies and sublacustrine fan facies are the most favourable petroleum accumulating zone;Besides,the area in the fan delta plain facies and braided delta plain facies with well developed trap may also be good gathering place of petroleum.The study will provide reference for prediction of potential area distribution in the next period.