The Sinian System Carbonate Reservoir Development And Its Control Factors Of Tarim Basin

Guided by multiple theories involving carbonate petrology,paleokarst theory,seismic geology,as well as reservoir geology and so on,combined with drill cores,geochemistry and petrophysics data,the paleokarst reservoir characteristics,reservoir control--factors and distribution law are comprehensively studied.On the study basis of petroleum geologic characteristics and pool--forming history,the distribution of quality dolomite reservoir in Sinian system is predicted and potential exploration directions of Sinian dolomites in Tarim basin are suggested.The following major progresses are made.Based on the detailed research of geological outcrops and seismic sections,the unconformity between Sinian and Cambrian system is wide--spread developed within and around Tarim Basin.Two types of unconformities are presented in the study area.One type is parallel unconformity which locates in Kuruktag,Kalpin and the Northern Depression.The other type is angular unconformity which developes near the north ramp of the Central Rise(Tadong Low Uplift--Tazhong Low Uplift--Bachu Fault Uplift).Based on the study of seismic sections in Tarim basin,residual stratum mainly located on the North Depression with two sedimentary centres(Mangal Sag and Awat Sag).The maximum thickness of residual Sinian strata is 1300 m in Mangal Sag and 700 m in Awat Sag.Sinian strata is thinning down gradually from sedimentary centre to uplifts around.The thinning out line is located at the northern limb of Central Rise and Sinian strata lacks in main part of Central Rise and south areas.Three types of paleo--karstification in Sinian dolomites are identified which is respectively syn--sedimentary karstification,supergene karstification and buried karstification.Supergene karstification is the main paleokarstification in Sinian system.Horizone--phreatic zone and deep--slow--flowing zone are the main part of the karst vertical zonation.Larger caves,collapse deposits and honeycomb formation caves which distribute 30 m below the unconformity are the significant evidences of horizone--phreatic zone.Isolated caves,associated with the fractures,which distribute within 60m--80 m below the unconformity are the characteristics of deep--slow--flowing zone.Based on the study of the karst landform of Sinian,Sinian outcrops in Aksu--Kalpin area located in the slope of paleokarst area.Paleokarstification of Sinian was controlled by unconformity and paleo--landform.The paleokarst model is higher--phreatic gentle--slope paleokarst.The rock types of dolomite reservoir in Sinian are cyanobacteria dolomite,granular dolomite,crystal dolomite and breccia dolomite.Cyanobacteria dolomite and granular dolomite composed of the majority of Sinian dolomite.Dolomitization,dissolution and structure disruption are the uppermost diagenesis of Sinian dolomite reservoir.Diagenesis evolution of Sinian dolomite reservoir indicates that karstification which benefits to physical properties is the most important influence factor to form quality reservoirs.The reservoir space types include pores,caves and cracks.Secondary pores(solution pores,solution caves and pores between collapse deposits)are the main reservoir spaces which are caused by corrosion.The most favorable reservoir types are crack--pore reservoir,crack--cave reservoir and crack--pore--cave combined reservoir.The quality reservoirs of Sinian dolomites,formation of which is controlled by diagenesis and tectonic processes,are distributed in the slope margin of the paleo--uplift.So northern slope of Central Rise and southern slope of Tabei Rise are the most favorable areas where quality palaeokarst dolomite reservoirs of Sinian are distributed.Source rocks of Sinian reservoirs are mud shales and carbonates which deposited in continental--rift sedimentary setting in Nanhua--Sinian formation as well as mud shale which deposited in deep--sea basin in Cambrian formation.Hydrocarbon accumulation of Sinian formation is controlled by paleo--uplift,unconformity and distribution of quality reservoirs.Combined with the study of petroleum geologic characteristics and tectonic evolution,three north slope of Tadong low uplift of Sinian dolomites in Tarim basin are suggested.