Reservoir Characteristics And Controlling Factors Of Xiayoushashan Formation In The Northwestern Qaidam Basin

The Xiayoushashan Formation in the northwestern part of the Qaidam Basin is one of the emerging fields of oil and gas exploration in the Qaidam Basin and has important research significance.The Xiayoushashan Formation is complex in lithology and has mixed characteristics.At present,there is no unified understanding of the classification of lithology types.In this period,the sedimentary grain size is fine,and more tight reservoirs are developed.The reservoir evolution and the main controlling factors of favorable reservoirs are unclear,which restricts the further development of oil and gas.The study took the Xiayoushashan Formation in the northwestern part of the Qinghai area as an example to do the reservoir type classification,physical properties and diagenetic evolution studies.The study also analyzed the main controlling factors of reservoir development and made predictions of favorable reservoirs.Based on the core observation of 21 wells in the study area,as well as a large number of cast and dyed flakes,X-ray diffraction analysis and X-ray diffraction clay analysis,the lithology types and their characteristics were studied in detail.The research indicates that the rock types of the Xiayoushashan Formation in the northwestern part of the Qinghai area are mainly composed of lacustrine fine sandstone,mudstone,carbonate rock and mixed rock,and further divided into 10 subtypes.Combined with scanning electron microscopy,electron probe and high pressure mercury,the reservoir properties were studied on the basis of lithology classification.The reservoir of this formation is characterized by an ultra-low porosity,low permeability and tight reservoir.The main storage space includes intergranular pores in siltstone,intercrystalline pores in limestone and intergranular enlarged dissolution pores.Pore structure is mainly micropores and microthroats,and the number of special small pores – extra thin throats and smaller ones account for only 16% to 37%.Based on the structural characteristics of the reservoir,the evolution characteristics of clay minerals and a large number of geochemical data,the diagenetic evolution stage was clarified: the Xiayoushashan Formation is in the B stage of the Early Diagenesis-B stage of the Middle Diagenesis.On this basis,the controlling factors and development laws of the reservoir were discussed.It is believed that the sedimentary environment of the upper part of N21(early diagenetic B stage)controls the physical properties of the reservoir,and the sedimentary environment of the lower part of N21(middle diagenetic A stage)and diagenetic evolution degree jointly control the physical properties of the reservoir.Based on this,the reservoir evaluation criteria were further established,and the reservoir evaluation work was carried out to predict the favorable zone of reservoir development.The study expects to provide some reference for the research work of the Xiayoushashan Formation reservoir in the area.