Study On The Architecture Of Tight Sandstone Reservoir

The Xujiahe Formation in Sichuan Basin,as the main horizon for tight sandstone gas exploration,is characterized by large sedimentary thickness,tight lithology and poor continuity.In order to finely dissect thick sand bodies,define the development characteristics and stacking patterns of tight sandstone reservoir configuration units,and analyze the causes of heterogeneity of tight sandstone reservoirs in Xujiahe Formation,this paper selects the fluvial facies outcrop of the 2nd member of Xujiahe Formation in Hu Xing Village,Gongnong Town,Guangyuan City as a typical profile for reservoir architecture research,which has important practical significance for effective exploration and development of tight gas reservoirs in Xujiahe Formation in Sichuan Basin.Through observation and description of outcrops in the field,establishment of natural gamma profiles and microscopic casting thin section identification,grain size analysis and physical property analysis,the sedimentary,petrological and physical property characteristics of the fluvial sand body in the second member of Xujiahe Formation of the typical profile are clearly defined.On this basis,sedimentary microfacies types,lithofacies types and reservoir architecture units are identified and divided.Heterogeneity parameters are applied to comprehensively analyze the heterogeneity characteristics of the sand body,and the causes of heterogeneity of the fluvial sand body in the second member of Xujiahe Formation are analyzed in combination with the geological background of the study area.The results show that:(1)According to the division scheme of base-level cycles in continental basins,the section of the second member of the Xujiahe Formation in Hu Xing Village,Guangyuan is divided into sequence stratigraphy,with four ? sequence interfaces(B1?B4)and five short-term cycles(SSC1?SSC5).The outcrop is characterized by river braid-bend transformation,and four sedimentary microfacies are identified in the short-term cycle: channel microfacies,core beach microfacies,edge beach microfacies and crevasse fan.The geological factors that affect the braid-bend transformation of rivers in the study area are topographic slope,short-term base-level cycle,sediment supply,paleoclimate and fault activity.(2)Nine lithofacies types are developed in the fluvial facies section of the second member of the Xujiahe Formation in Hu Xing Village,Guangyuan: trough cross-bedding sandstone facies(St),low-angle undercut plate cross-bedding sandstone facies(Sp?),high-angle undercut plate cross-bedding sandstone facies(Sp?),parallel bedding sandstone facies(Sh),massive bedding sandstone facies(Sm),sand bedding sandstone facies(Sr),horizontal bedding siltstone facies(Fh),wave bedding siltstone facies(Fl)and massive bedding mudstone facies(M).(3)Five fluvial facies reservoir architecture units are identified in the profile: channel(CH),transverse sand dam(TB),longitudinal sand dam(LB),oblique sand dam(DB)and crevasse fan(CS).There are five stacking relationships among reservoir architecture units: the crevasse fan is in isolated stacking(CH-CS)at the top of the channel;In the period of beach microfacies deposition,vertical sand bars are vertically stacked and laterally spliced above horizontal sand bars(TB-LB);During the marginal beach microfacies deposition period,the vertical sand bars are vertically stacked and laterally spliced(DB-LB)above the oblique sand bars.During the braided-curved transition period,due to the rise and fall of base level,there are two stacking modes(CH-CH)between single-phase channels:(1)vertical shear stacking of braided river single-phase channel sand bodies;(2)lateral butt stacking of meandering river single-phase channel sand bodies.(4)Taking average porosity and permeability as evaluation parameters,the physical properties of reservoir architecture units in the profile are: meandering river channel,transverse sand dam,longitudinal sand dam,braided river channel and oblique sand dam from good to bad.In this paper,permeability variation coefficient Vk,permeability breakthrough coefficient Tk,permeability range Jk and homogeneity coefficient Tp are used as evaluation parameters to evaluate the heterogeneity in the architecture unit layer.The results show that the heterogeneity in the longitudinal sand dam and transverse sand dam layers developed in meandering river channel and central bank are weak,the heterogeneity in the oblique sand dam and longitudinal sand dam layers developed in edge bank are strong,and the heterogeneity in braided river channel layer is strongest.From the perspective of sedimentary microfacies unit,combined with the scale of single sand body and the comprehensive analysis of stacking relationship in sedimentary microfacies unit,it is shown that the heterogeneity of central bank is the weakest,followed by meandering river channel,the heterogeneity of edge bank is stronger,and the heterogeneity of braided river channel is the strongest.(5)Based on the geological background of the study area,the influencing factors of the heterogeneity of fluvial sand bodies in the second member of Xujiahe Formation are analyzed.One is that tectonic movement and short-term base level cycle lead to A/S value changes and control the stacking mode of fluvial sand bodies.The second is the influence of diagenesis on the microscopic pore heterogeneity of tight sandstone reservoirs.Compaction and siliceous cementation enhance the reservoir compactness.Dissolution plays a significant role in the reservoir pore connectivity in the study area.