Porosity Evolution Model Of Low Cretaceous Bashijiqike Sandstone Reservoir In Kuqa Foreland Thrust Belt,Tarim Basin

Medium to fine lithic feldspar sandstone reservoir and feldspar lithic sandstone reservoir of delta plain facies are developed in the Cretaceous Bashijiqike Formation in Kuqa foreland thrust belt,Tarim Basin.As low-porosity,low-permeability and tight reservoir,the reservoir porosity is between 6%and 12%,and its permeability is between 0.01×10^-3?m²and 1×10^-3?m².The achievements of the research are shown in the following aspects.1.The Bashijiqike sandstone reservoirs have gone through early burial diagenesis and late tectonic diagenesis.The early burial diagenesis can be divided into three stages including the shallow burial?1000m±?diagenesis of weak mechanical compaction,weak siliceous cementation and strong calcite cementation in early Cretaceous;early calcite cementation and epidiagenetic dissolution of feldspar as a result of tectonic uplift in late Cretaceous,reburial,weak gypsum cementation and medium to weak calcite cementation during Paleogene and the end of Kangcun period of Neogene,deep burial and strong compression of tectonic diagenesis after Kuqa period.2.The sandstone reservoirs have gone through the reduction process of both porosity and permeability due to the early burial diagenesis.However,the late tectonic diagenesis make them undergo from pore reduction,minor permeability drop,to permeability rise due to fractures and dissolution pores increase,to the final pore and permeability reduction.Hence,the corresponding reservoirs are formed,one is relatively high porosity and permeability reservoir controlled by normal burial compaction and cementation,the other is low porosity and permeability reservo ir controlled by strong compression of tectonic diagenesis.The sandstone reservoir space include eight kinds of pores?residual intergranular pore,residual intergranular dissolution pore,late enhanced intergranular dissolved pore,late intergranular dissolution pore,dissolved pore in grains,matrix micropore,cement micropore and micropore in grains?and three kinds of fractures?tension crack,shear reticular suture and control fraction?.3.There is great difference in the reservoir evolution among the different structural positions.From north to south,from Kela to Keshen,the tectonic compression increases gradually,the reservoir physical properties degrade gradually,however,the reservoir physical properties are changing for the better cross the strong compressional zone.Fom east to west,the compressional strength is higher in Dabei areas than in Keshen areas,and the cracks are more developed,and the porosity and permeability have greater changes.Vertically,the pores are preserved well and tension fissures are developed above neutral plane where shows more dissolution and better reservoir physical properties.The pores are poor and shear fissures are developed under neutral plane where shows weak dissolution and poor reservoir physical properties.4.Due to the differences of the rock fabric and original porosity,the sandstone diagenesis and porosity evolution are different,the reservoir physical properties finally vary.Due to good sorting,high primary porosity of the fine sandstones,the sandstones have weak early compaction,strong cementation,good late dissolution,and the dissolved pores and the residual intergranular pores are developed,hence,the porosity and permeability are good.The siltstones have fine granularity,high clay matrix,strong carbonate cementation and the worst reservoir physical properties.The inequigranular sandstones have poor sorting,low primary porosity,and medium permeability.5.The sandstone evolution of foreland thrust belt is divided into different stages in time,and various belts in the north south direction,and different segment in the east west direction,and multiple layers in the vertical direction,and different lithologic properties in genetic type.The reservoir quality is controlled by various factors,including depositional setting,paleoclimate,the burial process,low palaeogeothermal gradient,tectonic compression and so on.The reservoir property is eventually controlled by the structural position and the strength of tectonic compression.The reservoir quality is the best in hanging wall of Kelasu fault,and second next to hanging wall of thrust belt,the reservoir quality is worst next to footwall of thrust belt.