Dolomitization Mechanism And Genesis Of Cambrian Reservoirs In Tarim Basin

In Tarim basin,there are dolomites with huge thickness developing in the Cambrian system,and providing great potential for oil or gas exploration.In a long time,controlled by problem such as its old age,deep burial depth,serious reservoir reformation,complex preservation condition,diverse reservoir types and strong reservoir heterogeneity,therefor,the exploration extent is relatively low and leads to no breakthroughs in petroleum prospecting.Fortunately,in 2012,oil and gas were discovered from dolomite under salt rocks of Cambrian formation in Well Zhongshen 1 of Tazhong uplift,which opened a new stage of the exploration of Cambrian dolomite reservoirs.To solve the problem about dolomitizition mechanism and genesis of dolomite reservoirs in Cambrian system of Tarim basin,we referred to the previous research results and based on theories such as sedimentology,petrology,reservoir geology,well logging geology and relevant industrial standards,firstly studied the characteristics of reservoir petrology,reservoir space,petrophysics,pore structure with thin section,scanning electronic microscope(SEM),cathodoluminescence spectroscopy,and mercury injection.Then the diagenesis of reservoir such as dolomitization and carbonate dissolution were analyzed in emphases based on inclusion thermometry,electron probe,high-pressure and high-temperature dissolution expriment and element geochemical tests such as trace element,rare earth elements,carbon and oxygen isotopes,strontium isotopes,magnesium isotopes.Through the study,the main controlling factors of development and formation mechanism of reservoirs are cleared and definited.And at last the distribution of favorable exploration areas are comprehensively predicted based on analyzing sedimentary facies,fault systems,hydrocarbon source rocks,caprock and favorable dissolution area.Firstly,the study show that the rock types of Cambrian reservoirs in Tarim Basin mainly include crystalline dolomite,(residual)grain dolomite,mud microcrystalline dolomite,algal layer dolomite,breccia dolomite,transitional dolomite,mud microcrystalline limestone and dolomitic limestone.In addition,some Non-reservoir rock types such as salt rock,gypsum rock,mud rock locally develop in Cambrian formation.There are common minerals such as dolomite,calcite,organic matter and clay minerals,quartz,pyrite etc.filling in pores and cavities,moreover,some hydrothermal minerals such as gypsum,celestite,barite,quartz,fluorite,phosphate rock and clay mineral including illite,glauconite locally developing in the pores space.Secondly the porosity and permeability of Cambrian reservoir in Tarim Basin vary greatly.The porosity values are among 0.1%and 23.9%,and the permeability values changes from 0.003×10-3?m2 to 11200×10-3?m2.The(residual)particles fine crystalline dolomite and silty crystalline dolomite have the hightest average permeability and porosity value.Affected by fracture,the permeability are significantly increased.The reservoir space of Cambrian carbonate reservoirs is mainly composed of dissolution pores and intergranular dissolved pores.the secondly pore spaces are fractures,intergranular pores,intergranular dissolved pores,intragranular dissolved pores and intergranular dissolved pores.There are 5 periods fracture in the Cambrian strata,and the distribution of fracture is strong in the strata and regions,and its parameters are obviously controlled by the characteristics of faults.The pore structure of the reservoir can be divided into four types including coarse throat-big pores type,fine throat-medium pores type,fine throat-small pores type and micro-throat-micro pores type.The results of three-dimensional pore structure reconstruction with CT scanning imaging show that the residual grain fine crystalline dolomite has better physical properties and pore structure.K-means clustering analysis based on multifractal analysis can be used as an effective method for quantitative classification and evaluation on pore structure of dolomite.Thirdly,the main diagenesis of dolomites in Carmbrian system includes marine micritization,compaction,pressolution,cementation,dolomitization,recrystallization,over-dolomitization,silicification,filling,de-dolomitization,dissolution and fracturing.And dolomitization,dissolution and fracturing of reservoirs are of great significance to the reservoir.The fractures formed by structural fracturing provides the seepage channel for the diagenetic fluid.The diagenetic stage can be divided into syndiagenetic stage,the early diagenetic stage,middle diagenetic stage and late diagenetic stage.The diagenetic sequence and pore evolution model of residual grain dolomite and crystalline dolomite are established in combination with the history of tectonic and strata burial history.Fourthly,according to the characteristics of petrology and geochemistry,the dolomitization of Cambrian in Tarim Basin is mainly composed of evaporative pumping dolomitization,seepage reflux dolomitization,moderately and deeply burial dolomitization and structure controlled hydrothermal dolomitization.According to the Mg isotope,Sr isotope and REE geochemical characteristics,the dolomitization fluid of the Cambrian strata in Tarim Basin is mainly marine fluid.And the source of the hydrothermal solution includes the brine and the mantle derived fluid in the high temperature formation.The corresponding dolomitization model is established,and the buried dolomitization is mainly the mechanism of thermal convection in the formation of the brine.In Bachu area the dolomites of Xiaoerbulake formation are generally formed from burial dolomitization and with modification by hydrothermal fluid;and while the dolomites of Wusonggeer formation and Awatage formation are formed from evaporation pump dolomitization.The dolomites of Shayilike formation mainly generates with evaporation pump dolomitization and burial dolomitization.The dolomites of Lower Qiulitage formation in Tabei area are formed in seepage reflux dolomitization and burial dolomitization,and with hydrothermal modification in a certain degree while it with burial dolomitiazation in Bachu.The hydrothermal dolomitization is widely developed in the Tabei area and Gucheng area and the hydrothermal action is of great significance to the reformation of reservoir in these areas.Under the control of the magma types,the intensity of hydrothermal action in Bachu area is feebler than that of Tabei area and Gucheng area.Fifthly,the reservoir dissolution of the Cambrian in the Tarim Basin are in variety,including the fresh water dissolution,the buried dissolution,the epigenetic karstification and the greatly deep fault cotrolled fresh water dissolution.The results of high-temperature and high-pressure dissolution simulation experiments on buried circumstances show that the calcite is more soluble than dolomite under any temperature and pressure conditions.Both chemical thermodynamics calculation and practice experiments show that the dissolution of carbonate rocks has the optimum dissolution depth range corresponding to the shallow to middle burial stage.The dissolution can improve petrophysics of reservoir effectively,especially on penneability.Futhermore,the dissolution of carbonate rocks is influenced by the pore structure,mineral composition and fracture which provides the dominant channel to dissolve the easily-soluble component.Sixth,The formation,evolution,development,distribution and the quality of Cambrian reservoirs in Tarim Basin are controlled by sedimentary,diagenesis and tectonics.Based on the characteristics of sedimentary,diagenesis and tectonic action,the main control factors and spatial distribution of reservoir in the study area are clarified,and the genetic models of granular dolomite and crystalline dolomite reservoirs in the Tabei and Bachu areas are established.The deposition and its products are the basis of reservoir while diagenesis and tectonic action are the key influences for the evolution and transformation of reservoirs.The macro distribution of the reservoir is controlled by the favorable sedimentary facies belts such as intra-platform shoal,the platform edge reef beach and the gypsum-dolomite flat.And high frequency sequences control the transformation of early atmospheric water dissolution on reservoir space in high energy phase zone and gypsum-dolomite flat.The faults and fracture dense zone formed by the tectonic action provide the channel for the dissolution of later organic acids,CO2,H2S and other dissolution fluids to the reservoir.In the early stage,dissolved pore in particles dolomite reservoirs formed by atmospheric fresh water dissolution are the basis for pore evolution.Meanwhile the organic acid,CO2,hydrothermal fluid and TSR related fluids play an important role in reconstruction of reservoir space.Based on the distribution of hydrocarbon source rocks,caprocks,sedimentary facies and favorable dissolution facies in Tarim Basin,the comprehensive prediction of favorable reservoirs for high quality reservoirs in Cambrian are accomplished.In lower Cambrian formation,the most favorable reservoirs distribute in the eastern margin of Hetian river gas field,No.1 fault of Tazhong area and Tabei uplift;the level ? favorable area mainly distribute in southern and eastern margin of the Tabei uplift and along Mazatage fault;The level ?favorable areas are mainly in intra-platform shoal and gypsum-dolomite flat,which mainly distribute in the northern Hetian River gas field,the north Tazhong area and the south Tabei area and along the Tumuxiuke-Aqia-Shajingzi fault zone.In middle Cambrian formation,the most favorable reservoir area distribute in Tazhong area and the north Yaha area;the level ?favorable exploration area mainly distribute in the Gucheng area and southwest Bachu area;the level ? favorable exploration area mainly distribute in the eastern margin of the Tabei uplift and the southern margin of the Tazhong uplift.In upper Cambrian,the most favorable exploration area distribute in platform edge of Yingmai area and Yaha area;level ? favorable exploration area distribute in platform margin from Lunnan area to Gucheng area.The level III favorable exploration area mainly distribute in Bachu and the northeastern margin of Hetian River gas field,intra-platform shoal,tidal flat depositional area of Tazhong area.