Aiming at the scientific problem that only part of dolomite acts as dolomite reservoir,this paper takes the multiple dolomite-bearing formations in the Tarim and Ordos basins,NW China and Sichuan Basin,SW China as the...Aiming at the scientific problem that only part of dolomite acts as dolomite reservoir,this paper takes the multiple dolomite-bearing formations in the Tarim and Ordos basins,NW China and Sichuan Basin,SW China as the study object,by means of mineral petrological analysis and geochemical methods including carbonate clumped isotope,U-Pb isotopic dating,etc.,to rebuild the dolomitization pathway and evaluate its effects on reservoir formation.On the basis of detailed rock thin section observation,five dolomitic structural components are identified,including original fabric-retained dolomite(microbial and/or micrite structure),buried metasomatic dolomite I(subhedral-euhedral fine,medium and coarse crystalline structure),buried metasomatic dolomite II(allotriomorphic-subhedral fine,medium and coarse crystalline structure),buried precipitation dolomite and coarse crystalline saddle dolomite.Among them,the first three exist in the form of rocks,the latter two occur as dolomite minerals filling in pores and fractures.The corresponding petrological and geochemical identification templates for them are established.Based on the identification of the five dolomitic structural components,six dolomitization pathways for three types of reservoirs(preserved dolomite,reworked dolomite and limestone buried dolomitization)are distinguished.The initial porosity of the original rock before dolomitization and the dolomitization pathway are the main factors controlling the development of dolomite reservoirs.The preserved dolomite and reworked dolomite types have the most favorable dolomitization pathway for reservoir formation,and are large scale and controlled by sedimentary facies in development and distribution,making them the first choices for oil and gas exploration in deep carbonate formations.展开更多
Ancient marine carbonates experienced complex modifications,making it difficult to identify reservoir genesis and effective porosity before hydrocarbon migration.To solve these issues,we used element mapping and carbo...Ancient marine carbonates experienced complex modifications,making it difficult to identify reservoir genesis and effective porosity before hydrocarbon migration.To solve these issues,we used element mapping and carbonate mineral laser U-Pb radiometric dating techniques to study the diagenetic environments based on geochemistry and diagenesis-porosity evolution based on geochronology of the dolomite reservoir of the Sinian Qigebrak Formation,northwest Tarim Basin.Two major understandings were obtained as follows:(1)Supported by petrographic observations,the element mapping,stable isotopes,strontium isotope,and cathodoluminescence tests were performed on different phases of dolomite cements precipitated in vugs and dissolved fissures.The results show that the dolomite reservoirs of the Qigebrak Formation went through freshwater,marine,extremely shallow burial,burial and hydrothermal diagenetic environments after synsedimentary dolomitization;the reservoir spaces were mainly formed in the synsedimentary period(primary pores)and freshwater environment(supergene dissolution pores)before burial;whereas the marine,burial and hydrothermal environments caused the gradual filling of reservoir space by dolomite cements.(2)Based on the above understandings,each phase of dolomite cement precipitated in the reservoir space was dated by the U-Pb radiometric dating technique,and the diagenesis-porosity evolution curves constrained by geochronology were established.The loss of reservoir porosity mainly occurred in the early Caledonian,and during the peak period of hydrocarbon generation of Yuertusi Formation source rock,the reservoirs still maintained at a porosity of 6%?10%.The above understandings provide a certain basis for the evaluation of accumulation effectiveness of the Sinian Qigebrak Formation,northwestern Tarim Basin,and provide a case for the application of mapping and dating techniques in the study of ancient carbonate reservoirs.展开更多
基金Supported by the China National Science and Technology Major Project(2016ZX05004-002)PetroChina Science and Technology Project(2019-5009-16).
文摘Aiming at the scientific problem that only part of dolomite acts as dolomite reservoir,this paper takes the multiple dolomite-bearing formations in the Tarim and Ordos basins,NW China and Sichuan Basin,SW China as the study object,by means of mineral petrological analysis and geochemical methods including carbonate clumped isotope,U-Pb isotopic dating,etc.,to rebuild the dolomitization pathway and evaluate its effects on reservoir formation.On the basis of detailed rock thin section observation,five dolomitic structural components are identified,including original fabric-retained dolomite(microbial and/or micrite structure),buried metasomatic dolomite I(subhedral-euhedral fine,medium and coarse crystalline structure),buried metasomatic dolomite II(allotriomorphic-subhedral fine,medium and coarse crystalline structure),buried precipitation dolomite and coarse crystalline saddle dolomite.Among them,the first three exist in the form of rocks,the latter two occur as dolomite minerals filling in pores and fractures.The corresponding petrological and geochemical identification templates for them are established.Based on the identification of the five dolomitic structural components,six dolomitization pathways for three types of reservoirs(preserved dolomite,reworked dolomite and limestone buried dolomitization)are distinguished.The initial porosity of the original rock before dolomitization and the dolomitization pathway are the main factors controlling the development of dolomite reservoirs.The preserved dolomite and reworked dolomite types have the most favorable dolomitization pathway for reservoir formation,and are large scale and controlled by sedimentary facies in development and distribution,making them the first choices for oil and gas exploration in deep carbonate formations.
基金Supported by the China National Science and Technology Major Project(2016ZX05004-002)the PetroChina Science and Technology Major Project(2018A-0103)
文摘Ancient marine carbonates experienced complex modifications,making it difficult to identify reservoir genesis and effective porosity before hydrocarbon migration.To solve these issues,we used element mapping and carbonate mineral laser U-Pb radiometric dating techniques to study the diagenetic environments based on geochemistry and diagenesis-porosity evolution based on geochronology of the dolomite reservoir of the Sinian Qigebrak Formation,northwest Tarim Basin.Two major understandings were obtained as follows:(1)Supported by petrographic observations,the element mapping,stable isotopes,strontium isotope,and cathodoluminescence tests were performed on different phases of dolomite cements precipitated in vugs and dissolved fissures.The results show that the dolomite reservoirs of the Qigebrak Formation went through freshwater,marine,extremely shallow burial,burial and hydrothermal diagenetic environments after synsedimentary dolomitization;the reservoir spaces were mainly formed in the synsedimentary period(primary pores)and freshwater environment(supergene dissolution pores)before burial;whereas the marine,burial and hydrothermal environments caused the gradual filling of reservoir space by dolomite cements.(2)Based on the above understandings,each phase of dolomite cement precipitated in the reservoir space was dated by the U-Pb radiometric dating technique,and the diagenesis-porosity evolution curves constrained by geochronology were established.The loss of reservoir porosity mainly occurred in the early Caledonian,and during the peak period of hydrocarbon generation of Yuertusi Formation source rock,the reservoirs still maintained at a porosity of 6%?10%.The above understandings provide a certain basis for the evaluation of accumulation effectiveness of the Sinian Qigebrak Formation,northwestern Tarim Basin,and provide a case for the application of mapping and dating techniques in the study of ancient carbonate reservoirs.
