Study On Genesis And Distribution Of Cretaceous Tight Sandstone Reservoir In Bozi Area Of Kuqa Depression

Based on field outcrops,core thin sections,physical properties data,drilling and logging data,using the theories and methods of sedimentary petrology and reservoir geology,this paper systematically studies the petrological characteristics,physical properties and main controlling factors of the tight sandstone reservoirs of the Lower Cretaceous Bashijiqike Formation in the Bozi area of the western Kelasu tectonic belt in the Kuqa Depression.On this basis,the distribution of favorable reservoirs is predicted,and the following main conclusions and understandings are drawn :(1)The reservoir of the second member of the Bashijiqike Formation in the Bozi area was formed in the braided river delta front environment,and the reservoir of the third member of the Bashijiqike Formation was formed in the fan delta front environment.The reservoir rock types are mainly feldspar lithic sandstone,lithic feldspar sandstone and a small amount of siltstone and lithic sandstone.The content of quartz in the rock component is between 18 % and 60 %,the content of cuttings is between 14 % and 68 %,and the content of feldspar is between 10 % and 45 %.Among them,feldspar is mainly potassium feldspar,followed by plagioclase,debris is mainly metamorphic debris such as phyllite,followed by magmatic rock debris;particle sorting is medium-good,mainly subangular.(2)The Cretaceous sandstone in Bozi area has experienced strong compaction during burial.The particles are in point-line contact,and concave-convex contact can be seen locally.The primary pores are greatly lost due to strong compaction,and the remaining intergranular pores are compactly cemented by calcite and siliceous and clay minerals.At present,the pores are mainly primary intergranular pores,followed by secondary dissolution pores produced by dissolution,but the amount is limited.The reservoir physical properties of the Bashijiqike Formation are generally good,with an average porosity of 7 % and an average permeability of 0.844 m D.(3)The reservoir petrological characteristics of Bozi area are controlled by sedimentary facies.From north to south,the reservoir physical properties of Bashijiqike Formation become better.Under the same provenance system,the reservoir rock composition and structure maturity of the near end of braided river delta front are low,and the reservoir rock composition and structure maturity of the far end of braided river delta front are high.From west to east,under the same sedimentary facies,the sand ratio in the middle of the lobe is higher than that in the edge of the lobe,and the single-layer sandstone is thick and the degree of sand-mud interbed is high.(4)The physical properties of Cretaceous sandstone reservoirs in Bozi area are controlled by sedimentary conditions,diagenesis and tectonic factors.The sedimentary environment is the material basis for the development of Cretaceous high-quality reservoirs in Bozi area.Diagenesis is the key,and tectonic compression is the main cause of reservoir densification.The closer to the main channel,the stronger hydrodynamic conditions,coarse particles,high quartz content,less matrix,large sand body thickness,weak compaction and relatively high porosity.The edge area far away from the main river channel has weak hydrodynamic conditions,fine grain size,more matrix,more plastic debris,smaller sand body thickness,finer grain size,higher matrix content and poor physical properties.Tectonic action is the main reason for reservoir densification.Due to the strong compaction in the study area,the sandstone reservoir of Bashijiqike Formation has the characteristics of low porosity and low permeability.The tectonic action is divided into three zones from north to south,and the horizontal stress extrusion effect is weak,strongest and weak in turn.The development of reservoirs in weak tectonic compression zone is mainly controlled by petrological conditions.The sedimentary materials from the South Tianshan Pass have short transport distance,coarse grain size,poor sorting,and poor petrological characteristics of reservoirs.The development of reservoirs in strong tectonic compression zone and weak tectonic compression zone is mainly controlled by tectonic action.Horizontal extrusion directly affects reservoir physical properties.The tectonic compression in the south of the study area is weakened and the reservoir physical properties are good.The fractures produced by tectonic action only improve the reservoir in some areas.(5)According to the standard of reservoir porosity > 6 %for class I,3.5 %-6 % for class II,< 3.5 % for class III in Bozi area,the reservoir of Bashijiqike formation in the study area is classified.Class I and class II are effective reservoirs,and class III is non-reservoir.Type I reservoir has good petrological characteristics,weak compaction and cementation,and weak tectonic compression.Although the tectonic compression of type II reservoir is strong,it is located in the main channel of delta front lobe,with good petrological characteristics,developed fractures and improved permeability.Although the structural compression of Class III reservoir is weak,its petrological characteristics are poor,the transportation distance of rock particles is short,and the reservoir effectiveness is poor.The reservoir of Bashijiqike formation in the study area is mainly class III,followed by class I,and class II is less.