Genetic Mechanism Of Low Permeability Reservoir And Favorable Reservoir Prediction Of Liusan Member In Weizhou 11-7 Oilfield

Weizhou 11-7 oil field is located at the Weixinan depression that is in the center of beibuwan basin,grow in Mesozoic and Cenozoic,including three parts:weizhou11-4N district, 11-7district and 11-8 district.The main oil bearing member is liuyi and liusan member.The strata thickness of liusan member is 180-280m,reservoir thickness is 140-200m,depth is 2600-2900m, it can be divided four groups.Liusan member is Eocene water incursion sediment of lake basin dilatation,main sedimentary system is fan delta and lake,the provenance orientation is located at east of Weixinan low heave,but it has variable provenance and different sedimentary facies and strong heterogeneity.The reservoir quality is bad,The petrophysical of sandstone reservoir show strong heterogeneity from vertical and horizontal.Based on logging,core,thin section,granularity, mercury curve experiment,educe that the lithology is many,the coarse lithology is familiar,based on logging,core,thin section,clay mineral,embedding,temperature and pressure,analyzed the genetic mechanism of low permeability reservoir,through provenance,sedimentary facies, diagenesis,rock composition,sorting coefficient is moderate,to bad,rounding is between sub-edge and sub-rounding,it reflect the fast sediment and near provenance.The permeability of liusan member is main in the range of 10-0.1 md,90 percent of samples are less than 10md,belong to special low permeability and super low permeability reservoir,the rock facies has far more influence to permeability than porosity.moreever,fine sandstone permeability is highest.The pore throat R₅₀ of liusan member is between 0.056-1.72μm,average 0.51μm,is fine throat.According to mercury curve and pore throat R₅₀,we divided the pore throat into four types,the petrophysical characteristics is corresponding with the four flow unit in this paper.The reservoir is much affected by diagenesis and has much complex the genetic mechanism.Diagenesis of this district is complex and diagenesis phenomenon is much,the main diagenesis is compaction,cementation,metasomatism,solution.we divided the reservoir to two diagenesis period:early diagenesis period,late diagenesis period,early diagenesis period can be divided into A,B diagenesis sub-period;late diagenesis period can be divided into A1,A2,B diagenesis sub-period,reservoir is embedded in deep strata,generally,the RO exceed 1.30%, organic matter had mature.Compaction is much,elastic grain is roughness contact,I/S layer transformed into illite much.Based on the burial history,using scherer formula and calculating of cement and solution porosity of different time,build the pore evolution mode.From sedimentary time to 40Ma,the main diagenesis is compaction,porosity decreased obviously along with depth;Between 40Ma and 0Ma,the porosity reduce slowly,take on different variational trend.Prior to 42.5Ma,liusan member is in the stage of fast embedding,the porosity descended from 30%to about 22%.During 42.5Ma and 36Ma,porosity increase by 3.2%.After 20 Ma,liusan member is in the stage of slow embedding and late diagenesis period,all kinds of cementation continued to increase,reservoir porosity continued to decrease.Combined the pore evolution analysis cognition,based on logging,core,thin section,clay mineral,embedding,temperature and pressure,analyzed the genetic mechanism of low permeability reservoir,through provenance,sedimentary facies,diagenesis,rock composition and composing,pore fluid,embedding and temperature and pressure.Liusan member belong to fan delta in gradual slope of near provenance,there are many coarse lithology,the main rock type is Lithic sandstone,Feldspar lithic sandstone and some Quartz sand debris,the contenet of Interstitial material is high,reach 18.94%,Miscellaneous sandstone can reach 16.2%,Compositional maturity is low,Plastic debris decrease the Resistance of pressure,and the multi-quartz inhibit the compaction.The rock grain has Poor sorting,degree of rounding is sub-Prism and sub-rounding.The porosity of sendimentary time is only 28.98%,compared to rock of high structure and compositional maturity,there are about 10%difference,calculated byφo=20.91+(22.9/ So formula,the innate conditions are bad.From the sendimentary time to now,the reservoir was influenced by strong genesis,is the postnatal factors.The main genetic factor of low permeability reservoir is compaction.during 39-46Ma,sedimentation rate of the strata is high,the pressure of increase quickly,the porosity of reservoir descend by 8.8%for mechanical compaction;From the sendimentary time to now,the destructive porosity of compaction can reach 85%,the cementation about 12%,the sample of reservoir belong to medium-strong compaction and weak cementation.and the geothermal gradient can reach to 4.12℃/100m,heat compaction effect is obvious and the organic-inorganic diagenesisi reaction intensify the reservoir heterogeneity.and the embedding environment is block out,the range of fluid-rock reaction is limited.The short provenance distance and low water engery make the rock components and composing bad,caused the rough low permeability reservoir.On the basis of reservoir characteristics and genetic mechanism,interpreted the flow unit by Elman neural net and confirmed A,B,C,D four types flow unit,Main flow unit types of liusan member is C and D.Integrated the well production test,we studied the relationship between the flow unit and well production,different flow unit has different lithology,seepage flow characteristics.The more composition of A and B,the higher of well production,vice versa. Reservoir production has close relationship with flow unit.Through sedimentary facies,thickness of sandbody,flow unit and oil bearing properties of main sandbody properties,predicted the favorable reservoir of Liusan member.as follows:liusan memberâ… reservoir relative high production district is WZ11-7-2,WZ11-8-2,WZ11-8-1 well belt;liusan memberâ…¡reservoir relative high production district is WZ11-7-2,WZ11-8-2, WZ11-8-1 well belt1 liusan memberâ…¢reservoir relative high production district is WZ11-4N-1, WZ11-7-2,WZ11-8-1 well belt;liusan memberâ…£reservoir relative high production district is WZ11-7-1,WZ11-8-2,WZ11-8-1 well belt.Liusan memberâ…¡andâ…£reservoir relative high production district grow well.