Burial,Diagenetic And Porosity Evolution Process And Fluid Activity Characteristics Of He-8 Member Of The Upper Paleozoic In Southwestern Ordos Basin

Since 2004,three main gas-bearing sandstone belts have been discovered in Permian strata by geological exploration in southwestern Ordos Basin.At present,they have been one of the reserve replacement areas for natural gas resources in tight sandstone reservoirs of Upper Paleozoic in Ordos Basin.Natural gas migration channels and storage space conditions are the main controlling factors for the quality of tight gas reservoir in He 8 Member of Upper Paleozoic in southwestern Ordos Basin.The original porosity and permeability of clastic reservoir are mainly related to the original components of sandstone.And diagenesis will further enhance,maintain or destroy reservoir property.Previous studies on Upper Paleozoic He 8 Member sandstone reservoir in southwestern Ordos Basin seldom pay attention to the control of provenance and sedimentary system on the composition of clastic rocks,and the research on the diagenetic response of fluid activity is weak.In this study,the outcrop section in southwestern margin of Ordos Basin is taken as the breakthrough point to intuitively analyze the distribution of oil and gas,interlayer and mudstone intercalation in the transport layer,as well as the diagenetic evolution characteristics of each structural unit,by means of field survey,core observation,identification and statistics under microscope,fluid inclusion homogenization temperature and laser Raman component analysis,carbon and oxygen isotope testing of carbonate cements in micro-area by laser microsampling,etc.Referring to the outcrop section research work in the southwestern margin of Ordos Basin,the study on reservoir configuration characteristics,composition law and variation of clastic rocks of clastic rock,diagenetic evolution path and fluid activity characteristics of the key structural units of drilling cores transport layer in the basin was carried out.Combined with regional burial thermal history,the burial-diagenetic and hydrocarbon charging progress of the key structural units of He 8member is constructed,and the quantitative recovery of porosity evolution of structural units is carried out on the scale of diagenetic facies.Research results show that the outcrop section experienced different hydrocarbon charging processes from the drilling well.Compared with drilling cores,the initial time of hydrocarbon charging recorded by the outcrop sandstone inclusions of He 8 member is slightly earlier(about 80 ?),and the large-scale carbonate cement of organic carbon source origin is formed in the early diagenetic stage B.The large-scale hydrocarbon charging of drilling cores in the basin occurred in the middle diagenetic stage A(about 120 ?)after the maturity of source rocks,and the carbonate cements of organic carbon source origin are mainly formed in the middle diagenetic stage A.Structural heterogeneity of He 8 member transport layer is significant,and there are mainly five structural units: massive bedding medium-coarse sandstone with gravel,graded bedding medium-coarse sandstone,parallel bedding medium-fine sandstone,and Parallel-plate bedding medium-fine sandstone facies and mudstone.Based on the comparative analysis of fluid activity and diagenetic response law between outcrop and drilling,the diagenetic evolution process of different structural units in transport layer is sorts out,and four diagenetic facies are classified: lithic deformation-fragmentation dense facies,calcareous cemented dense facies,clay minerals-matrix filling facies,and dissolved diagenetic facies.There is a good correspondence between the structural units of transport layer and diagenetic facies.The diagenetic facies in the fine sandstone structural units of parallel bedding and parallel-plate bedding are mainly composed of lithic deformation-fragmentation dense facies,which were densified in early stage of diagenetic-porosity evolution and became lowpermeability interlayers.A small amount of clay minerals-matrix filling facies and dissolved diagenetic facies did not significantly improve the transport capacity of such kinds of structural units.While,in graded bedding medium-coarse sandstone and massive bedding medium-coarse sandstone with gravel structural units,clay minerals-matrix filling facies and dissolved diagenetic facies account for an absolute proportion,they always keep good porosity and permeability in the process of burial-diagenesis evolution.Such kinds of structural units belong to the structural units of effective transport layer.On the plane,the effective transport framework of Section He 8 is located in the center of braided river delta distributary channel and underwater distributary channel controlled by southwest provenance,and the center of braided river delta front subfacies underwater distributary channel at the intersection of south and north provenance.