SEDIMENTARY FACIES OF THE UPPER PART OF THE FOURTH MEMBER OF THE SHAHEJIE FORMATION IN THE BONANLUOJIA AREA OF THE ZHANHUA SAG

Fault-bounded continental basins are characterized by complex sources of sediment supplies and complex sedimentary facies distribution,due to local topographic variations.In this study,we analyzed the sedimentary facies distribution and evolution as recorded in the upper part of the fourth member of the Shahejie Formation in the Bonan-Luojia area in the Zhanhua sag,on the basis of integrated seismic,well-log,drill core,and particle size data.Particle size data,composition of lithic fragments,and heavy minerals were used to determine sediment sources.Sedimentary microfacies analysis was based on petrographic analysis of drill cores and analysis of well logs.A sedimentary model was established by analyzing east-west and north-south transects of drill core and well log data.Isopachous maps of stratigraphic thicknesses and isopleth maps of sandstone content were constructed to analyze the spatial distribution of depositional elements and stratigraphic units.Fluctuations of lake-level and evolution of sedimentary sequences were inferred on the basis of integrated sedimentary analysis.The upper part of the fourth member of the Shahejie Formation is interpreted as a third-order depositional sequence which includes a lowstand system tract,a transgressive system tract,and a highstand system tract.The lake shore was initially near the Bonan fault,but subsequently migrated southward toward the Chenjiazhuang uplift as lake-level rose.Consequently,the distribution of the lowstand system tract is restricted to the north of the Bonan fault,and it onlaps southward over older strata as fan delta front deposits were formed with sediments sourced from the north and northeast.The transgressive and highstand system tracts are more widely distributed in the study area.The transgressive system tract is represented by steep fan delta front deposits to the north and northeast,gentle fan delta front deposits to the south and southeast,and beach bar deposits to the midwest.The highstand system tract is represented by thick lacustrine evaporates in the central part of the basin,and gentle fan delta front deposits in the southern part of the basin due to long sediment transportation distance and relatively gentle topography.