Origin Of Tightness And Reservoir Heterogeneity Of Chang 6-Chang 7 Sandstones,Southwest Ordos Basin

Upper Triassic tight sandstones in Ordos Basin are typical tight oil reservoirs in China.It is very useful for guiding tight oil exploration and exploitation in the study area to investigate reservoir characteristics,origin of tightness and reservoir heterogeneity.Sandstone core samples in the study area were used to investigate the diagenetic evolution and porosity evolution,origin of tightness for sandstones and pore structure(pore-throat types and distribution)using a combination of thin sections,Xray diffraction(XRD),secondary electron microscope(SE),backscattered electron microscope(BSE),energy spectrum analyses(EDS),mercury injection,core analyses and fluid inclusions.Reservoir heterogeneity was also investigated from sedimentology,diagenesis,pore structure and petrophysical properties.Some conclusions were obtained:Chang 6-Chang 7 sandstones in the study area are mostly of delta front or gravity flow origin.These sandstones have mainly low porosity and permeability,and mechanical compaction and pressure dissolution of quartz grains are intense.Authigenic clay minerals are dominated by illite,chlorite,and there are also less kaolinite and mixed layer I/S.The quartz cement in these sandstones usually occurs as overgrowths or euhedral quartz.Five types of carbonate cements including dolomite,calcite,ferrocalcite,ankerite,and siderite are identified.The dissolution for these sandstones includes meteoric water flushing at shallow depths,replacement and pressure dissolution of quartz and leaching of feldspar,rock fragments and carbonate cements in deep burial sandstones.Diagenesis occurs in a geochemically open system at shallow depths,but in a geochemically closed system at greater depths.At greater depths,four stages of the diagenesis evolution are identified: eogenesis A,eogenesis B,mesogenetic A1,and mesogenetic A2.Five diagenetic types were identified according to their specific diagenetic evolution pathways.Sandstones that were modified by moderate dissolution(such as Type A,B and D sandstones)were commonly preserved to become the best reservoir quality because of their less modification by subsequent cementation.The Type C sandstones often underwent intense compaction and subsequently intense cementation,while the Type E sandstones often underwent intense compaction and limited dissolution.Both of these two sandstone types belong to worse-quality reservoir.Five main types of pores are identified in the study area,and these include intergranular pores,oversized pores,grain-mold pores,intra-constituent pores and fracture pores.Depending on the pore-throat size distributions,three sandstone types with their own specific pore-throat characteristics were identified.Type ? sandstones generally contain a certain amount of macropores.Type ? sandstones usually contain dominant mesopores pores,less important macropores and infrequent micropores.Type ? sandstones are representative by significant mesopores,relatively abundant micropores and rare macropores.Pore throats in larger size make much more contribution to permeability than those in smaller size.The studied sandstones with dissimilar detrital compositions and sedimenraty textures have different original porosity(mainly controlled by sedimenraty textures).They were differently compacted by mechanical compaction(mainly controlled by detrital compositions),which result in the heterogeneous distribution of porosity at the time before cementation.Subsequently,the sandstones were modified by early-stage authigenic clay minerals,which were mainly controlled by depositional factors.Afterwards,dissolution and cementation(mainly controlled by fluid flow)preferentially modified the most porous and permeable sandstones.Therefore,the preferential flow paths would switch with the diagenetic reactions process during burial,and this led to the diagenetic heterogeneity in sandstones and further led to pore throat heterogeneity.Finally,reservoir beds become heterogenous.