Sedimentary And Reservoir Characteristics Of Jurassic And Paleogene In Yuqia-Nanbaxian Area,Northern Margin Of Qaidam Basin

Qaidam Basin is located in the northeastern part of the Qinghai-Tibet Plateau.The Mesozoic-Cenozoic is a typical continental sedimentary basin.Exploration shows that oil and gas in the northern margin of Qaidam Basin mainly come from Jurassic coal-bearing strata and migrate to Paleogene accumulation through regional unconformities and faults.Yuqia-Nanbaxian area is the main oil and gas accumulation zone in the northern margin of Qaidam Basin.At present,the distribution of Jurassic,the development of sedimentary centers and provenance characteristics in this area are not clear,and the research on the development of Paleogene reservoirs is relatively weak.Based on the analysis of drilling,outcrop and seismic profiles,the distribution and evolution of Jurassic and Paleogene sedimentary facies are further described,the provenances of Jurassic and Paleogene and the characteristics of Paleogene reservoirs are analyzed,and the distribution of favorable reservoirs is speculated.The analysis of drilling and seismic profiles shows that the Middle Jurassic is mainly distributed in Yunwushan-Yuqia colliery area.The Paleogene Lulehe Formation and Lower Ganchaigou Formation are mainly distributed in Mahai-Nanbaxian area.According to the characteristics of lithologic association,cycles,marker beds and logging responses,the Dameigou Formation of the Jurassic in the study area is divided into four sections.Comprehensive analysis of paleocurrent,gravel,light and heavy minerals and specific provenance of sandy land shows that the source direction of Jurassic in Yuka area is mainly South Qilian,followed by the provenance from Luliang Mountain.Combining with previous data and seismic profiles,we can see that there are paleouplifts in the South and speculate that there are provenances.The main provenance direction of Paleogene in Hemahai-Nanbaxian area is the South Qilian to the north-east,followed by the Luliang Mountain.Sedimentary analysis based on drilling and adjacent outcrop shows that alluvial fan,braided river,Braided River Delta and lake are the main facies types in Jurassic,and the sedimentary center is mainly in the middle of Yuka area.Segmental sedimentary facies mapping shows that braided river delta plain deposits are mainly developed in the fifth member of Dameigou Formation,while water body deepens during the seventh member of Dameigou Formation,and semi-deep lake and delta front subfacies deposits are developed.Braided River and braided river delta facies are mainly developed in Paleogene of the study area,and the sedimentary center is mainly in the southwest of Maxian area.Paleogene reservoirs are mainly sandstones with porosity ranging from 12% to 32%.The permeability varies greatly.The main distribution ranges from 0.5 *10-3? m2 to 3000.0 *10-3? m2.The physical properties of the reservoirs are generally good.The pore type is mainly primary intergranular pore.The pore throat characteristics are good as a whole.Relatively speaking,the physical properties of the upper part of the Lower Ganchaigou Formation are better than those of the lower part.The analysis of sedimentary facies and provenance characteristics shows that the eastern part of Nanbaxian structure has coarse lithology,good reservoir physical properties and favorable reservoir-cap assemblages.