| The reservoirs of Liushagang formation on the gentle slope belt in Wushi depression,Beibuwan Basin and the Zhuhai formation on step-fault zone inWenchang A depression in Zhujiangkou basin were investigated and analyzed in this study.These two areas belong to typical low porosity and permeability reservoirs,which share the similar regional tectonic background,depositional system type and formation age.However,the reservoir physical parameters tell the totally different story,suggests the genesis characteristics and pore evolutionary processes theyundergone are different.It is of great significance to the exploration and development of such reservoirs in Beibuwan Basin and Zhujiangkou Basin or even in the wholeWestern-South China Sea to improve reservoir evaluation system by analyzing the evolution and confirming the reservoir genetic mechanism.This thesis is based on the previous research and using reservoir sedimentology,reservoir petrology,petroleum geology and other related theoretical knowledge.The samples were screened via methods of cores examination,thin section,scanning electron microscopy,mercury injection capillary pressure test,porosity and permeability test,carbon-oxygen isotope,etc.The relationship among tectonic evolution,reservoir sedimentary,reservoir petrologic and diagenesis was systematically analyzed,while factors of structure evolution,compaction,burial time,authigenic mineral and precipitation mechanism were evaluated to set up an evolution model of sandstone reservoirs porosity at different structures in asymmetric dustpan-shaped fault basin.The results and achievements are organized as follow:?1?For the third member of Liushagang formation in Wushi depression,provenance mainly comes from eastern part of Qixi uplift,sedimentary system can be categorized as fan delta-lacustrine deposit system with stable lake background.In this area,sandstone corresponds to fan delta-front subaqueous distributary channel and sand sheet,commonly with vein bedding,lenticular bedding and wave bedding.There are glutenite sandstone,pebbly sandstone,coarse sandstone,medium sandstone and fine sandstone.Based on the analysis of the thin section,the detrital fraction of sandstone is dominated by quartz and metamorphic lithic,followed by lots of matrix and less feldspar,therefore,the sandstone is mainly lithic sandstone,less feldspar lithic sandstone.Sandstones in third member of Liushagang formation are characterized as low compositional maturity and low textural maturity,abundant debris,high matrix contents,the grains mainly have point contact,particle supported,contact typed cementation,show characters of near provenance,short transportation and little late reconstruction,which named“Original Accumulation”depositional feature.Analysis on reservoir sedimentology suggests that the third member of Liushagang formation locates on gentle slope belt of Wushi depression,and formed by slow continuity bury in medium depth?average depth is about 2500m?,the reservoir is now at phase A of mid diagenetic stage.With strong-moderate compaction,weak diagenesis,main cements in third member of Liushagang formation are kaolinite,illite,a few quartz cements and siderite cements,other cements are rarely found.Feldspar dissolution develops intensely,the primary pores well developed,and organic matter was fast transformed to reservoir with high organic acid,which brings about dissolution of feldspar with strong acid fluids in open system.Four diagenetic facies are identified in third member of Liushagang formation,based on the sedimentary facies and diagenetic processes.The four diagenetic facies are weak compaction-weak cementation diagenetic facies,medium compaction-strong feldspar dissolution diagenetic facies,strong compaction-medium feldspar dissolution diagenetic facies and compaction-filling diagenetic facies,the former three are positive to high-quality reservoir evolution.?2?The third member of Liushagang formation reservoir belongs to middle porosity-low permeability reservoir,porosity mainly distributes in 10%-23%,with an average of 16.20%,and permeability mainly distributes in 0.3×10-3?m2-80×10-3?m2,with an average of 40.92×10-3?m2.Through the analysis of cast thin sections and scanning electron microscopy,the present sandstone pores are mainly intergranular dissolution pores,intragranular pores,residual primary intergranular pores and partly developed kaolinite intergranular pores.Capillary pressure curve are with low displacement pressure,high mercury saturation and mercury removal efficiency,suggest mesopore throat and good pore-throat connectivity.Reservoir classification valuation shows that type?and?are most common in third member of Liushagang formation reservoir and type?is less common.?3?For the Zhuhai formation in Wenchang A depression,provenance mainly comes from middle part of Shenhu uplift,sedimentary system can be categorized as fan delta-lacustrine deposit system with littoral-neritic sea background.In this area,sandstone corresponds to fan delta-front subaqueous distributary channel,sand sheet and shore sand platform,commonly with parallel bedding,wave bedding,lenticular bedding and herringbone cross bedding.There are coarse sandstone,medium sandstone,fine sandstone and siltstone,based on the analysis of thin section,the detrital fraction of sandstone is dominated by quartz and igneous lithic,followed by some feldspar and less matrix,therefore,the sandstone is mainly lithic sandstone and feldspar lithic sandstone.Sandstones in Zhuhai formation are characterized as low-middle compositional maturity and textural maturity,low matrix contents,the grains mainly have line-contact to concavo-convex contact,particle supported,pore-contact typed cementation,show characters of near provenance,short transportation and tidal-wave reworking,which named“Tidal Modification”depositional feature.Analysis on reservoir sedimentology suggests that the Zhuhai formation locates on step-fault zone of Wenchang A depression,and formed by fast continuity bury in deep depth(average depth is about 3800m,the reservoir is now at phase B of mid diagenetic stage.With strong compaction and diagenesis,main cements in Zhuhai formation are carbonate cement such as ferrocalcite and ferrodolomite,siliceous cement such as quartz overgrowth,a little chlorite and illite,and no kaolinite found in deep buried layer.The primary pores are almost exhausted due to the strong compaction-densification in early stage,organic acid was hard to transform into reservoir,and feldspar dissolution develops weakly in Zhuhai formation.Five diagenetic facies are identified in Zhuhai formation,based on the sedimentary facies and diagenetic processes.The five diagenetic facies are medium compaction-strong feldspar dissolution diagenetic facies,strong compaction-medium feldspar dissolution diagenetic facies,quartz cemented diagenetic facies,carbonate cemented diagenetic facies and compaction-filling diagenetic facies,the former three are positive to high-quality reservoir evolution.?4?Zhuhai formation reservoir belongs to low porosity-low ultra-low permeability reservoir,porosity mainly distributes in 4.9%-17.4%,with an average of10.39%,and permeability mainly distributes in 0.1×10-3?m2-15.9×10-3?m2,with an average of 5.20×10-3?m2.Through the analysis of cast thin sections and scanning electron microscopy,the present sandstone pores are mainly residual primary intergranular pores,intergranular dissolution pores,illite intergranular pores,and partly developed intragranular pores.Capillary pressure curve are with medium-high displacement pressure and median pressure,high-medium mercury saturation and low mercury removal efficiency,indicate the pore structure is with fine throat and poor pore-throat connectivity.Reservoir classification valuation shows that type?and?are most common in Zhuhai formation reservoir,type?is less common,and type?is partly developed.?5?The differential evolution characteristics of primary pore are characterized in third member of Liushagang formation reservoir and Zhuhai formation reservoir,based on depositional environment,detrital composition,mechanical compaction,burial time,type and content of cements.The former belongs to original accumulation with slow continuity burial history depositional system,rigid particle and plastic particle content is about 50%:50%,respectively.High content flexible grains and matrix reduced compression resistance of reservoir,caused the low porosity and permeability.The latter belongs to tidal modification with fast continuity burial history depositional system,rigid particle and plastic particle content is about 60%:40%,respectively,indicates strong compression resistance and preserved several primary pores.The strong compaction resulted from a deep burial depth and strong cementation at early stage.In a word,the factors of primary pores reduction are totally different between third member of Liushagang formation reservoir and Zhuhai formation reservoir.?6?The evolution characteristics of secondary pores are characterized based on pore development mechanism study.Third member of Liushagang formation reservoir underwent a long time early diagenetic stage,weak cementation result in high residual primary porosity.Open diagenetic system was developed in third member of Liushagang formation,organic acids entered the reservoir in late stage,played the leading role in feldspar dissolution,and the total increase of secondary porosity was about 4.87%.Zhuhai formation reservoir fast buried to deep depth in early stage and early cementation was highly developed,remaining primary porosity was less than 10%.Closed diagenetic system with low acid was developed in Zhuhai formation,kaolinite transformation slightly promoted feldspar dissolution,and the total increase of secondary porosity was about 1.99%.?7?In the current work,the pore evolution models of“original accumulation with slow continuity burial history depositional system”and“tidal modification with fast continuity burial history depositional system”in low porosity low permeability sandstone reservoir was proposed with factors of mechanical compaction,burial time,precipitation of minerals,developmental mechanism of secondary pore,etc.The model of third member of Liushagang formation reservoir can be categorized into the former type,in this model,porosity loss caused by mechanical compaction was about 12%,caused by burial time was about 0.99%,caused by matrix filling was about 5.73%,caused by authigenic clay minerals was about 6.18%,other cementation about 1.37%,while total increase of secondary porosity was about 4.87%,makes porosity near16.21%.The model of Zhuahai formation reservoir can be categorized into the latter type,in this model,porosity loss caused by mechanical compaction was about 17.19%,caused by burial time was about 0.504%,caused by matrix filling was about 1.64%,caused by authigenic clay minerals was about 2.26%,carbonate and siliceous cementation was about 8.43%,while total increase of secondary porosity was about1.99%,makes porosity near 11.44%. |
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