Genetic Mechanism Of The Middle Permian Dolomite In The Sichuan Basin

Numerous oil and gas prospects around the world are found in structure-controlled hydrothermal dolomite bodies.This type of dolomite reservoirs often contains abundant natural gas resources.Large-scale dolomite bodies have been found in the Middle Per-mian Qixia-Maokou Formation in northwestern,central and southwestern Sichuan Ba-sin,which are of huge exploration potential.Traditionally,the Middle Permian dolo-mites in the Sichuan Basin are often interpreted as hydrothermal dolomite?HTD?given its association with the Emeishan Large Igneous Province?ELIP?.However,there are still controversies on many issues,such as the dolomite formation stages,diagenetic fluids,genetic mechanism,and spatial distribution of various dolomite types,etc.Com-bined with the tectonic background and sedimentary evolution characteristics,this pa-per comprehensively utilizes the observasion of field sections,drilled cores,well log-ging data and geochemical data of various dolomite type samples to investigate the origin and formation process of different types of the Middle Permian dolomites in the Sichuan Basin.The Middle Permian dolomites in the Sichuan Basin are summarized into four types based on its petrological characteristics:three matrix dolomites?Md1,Md2,and Md3?and dolomite cement?Cd?.Md1 is fabric-retentive microcrystallineb dolomite,and the abundance in the Sichuan Basin is relatively low.Md2 is fine-medium crystal-line dolomite with planar-e to planar-s crystals,which commonly has a cloudy rhombic center surrounded by a clear rim,and the original structures are destroyed.It is the most abundant type in the Sichuan Basin.According to the degree of being dolomitized,Md2can be further divided into porphyritic dolomitic limestone Md2-A and crystalline do-lomite Md2-B.The Md3 dolomite is a medium to coarse crystalline dolomite with non-planar-a crystals,turbid crystal faces,undulatory extinction and high abundance.Cd dolomite is a heterogeneous-saddle-shaped dolomite cement,which have large crystal size and clean crystal faces.It mainly develops in the form of pores and crack fillings in central and southwestern Sichuan Basin.In the northwestern Sichuan region,dolomites are distributed in thick layers in the platform margins of the Qixia Formation.Comprehensive analyses suggest that the do-lomites in this area are mainly controlled by thermal convection.The petrological char-acteristics and relatively similar geochemical characteristics(?¹³C values,⁸⁷Sr/⁸⁶Sr ra-tios,REE characteristics and Mn and Sr contents)indicate that Md1 were formed in penecontemporaneous seawater,while Md2,Md3 and Cd were formed in dolomitizing fluids dominated by Permian seawater during the shallow burial period,which were non-hydrothermal fluids.The lower?¹⁸O values and higher fluid inclusion homogeni-zation temperatures suggest that Md2,Md3,and Cd dolomites formed at high temper-atures,indicating that ELIP events provided heat for the dolomitization in the carbonate platform during the Qixia period.The temperature difference between the warm pore water in the platform margin zone and the cold seawater outside the slope created a significant fluid potential contrast,which led to rapid convective exchange of fluids between the platform and the external open deep seawater and promoted the formation of dolomites.Therefore,Md2 dolomites are the results of lateral open thermal convec-tion dolomitization.During the active period of the ELIP events,the temperature would further increase,leading to a higher temperature and more rapid thermal convective cycle in local areas,forming high temperature thermal convection dolomitization dom-inated Md3 dolomite and Cd dolomite cements.In the central Sichuan area,the dolomites are concentrated in the open platforms in the middle of the Maokou Formation around the basement fault zones.It is suggested that the dolomites in this region were caused by thermal convection dolomitization and structure-controlled hydrothermal dolomitization.The similar geochemical character-istics(?¹³C values,⁸⁷Sr/⁸⁶Sr ratios,REE characteristics,and Mn and Sr contents)of Md2 and limestones indicate that Md2 dolomites were formed in diagenetic fluids dom-inated by Permian seawater in early diagenesis.The relatively low?¹⁸O values and high homogenization temperatures show that Md2 dolomites were formed at a higher tem-perature.High ⁸⁷Sr/⁸⁶Sr ratios,Eu positive anomalies,high Mn contents,low?¹⁸O val-ues,and much higher homogenization temperatures and salinities indicate that Md3dolomites were seriously affected by the structure-hydrothermal fluids.The high tem-perature characteristics of Md2 dolomite suggest that its formation was also affected by the ELIP thermal events.The intense tectonic activities may have activated the base-ment faults.With geothermal heating,the warm pore water in the shallow buried plat-form exchanges fluids with the colder seawater at the top through the permeable fault zones,and the cold seawater at the top continuously flushed into the successions through active faults.Therefore,the formation of Md2 was controlled by open thermal convection at the top.During the active period of ELIP,a large number of magmatic hydrothermal fluids surged up,and its thermal effects further promoted the thermal convective circulation of the mixed fluid of hydrothermal fluid and seawater,forming the structure-controlled hydrothermal Md3 dolomites and Cd cements.In the southwestern Sichuan basin,dolomite is widely distributed in the restricted platform environment of the Qixia and Maokou Formation.Based on petrological and geochemical analyses,Md1 dolomites were formed by the penecontemporaneous sea-water seepage-reflux dolomitization.The geochemical characteristics of Md2 and Md3dolomites are similar to those in central Sichuan respectively.Hence,the diagenetic fluids of Md2 dolomite were the heated Permian seawater,and the diagenetic fluids of Md3 dolomites were seriously altered by high temperature hydrothermal fluid.Due to the higher proximity to the inner zone of ELIP,the southwestern Sichuan region expe-rienced higher geothermal heating and stronger hydrothermal effects.In the platform margin belt,the Md2 dolomites were mainly formed by lateral open thermal convection,and the Md2 dolomite in the restricted platform is formed by the top open thermal con-vection.Md3 and Cd dolomites are structure-controlled hydrothermal altered dolomites.The comprehensive comparison of the characteristics and genetic models of dolo-mites in the three regions shows that Md1 dolomites were formed by penecontempora-neous seepage-reflux dolomitization,Md2 dolomites were formed by thermal convec-tion dolomitization,and Md3 dolomite were formed by high temperature thermal con-vection and structure-controlled hydrothermal dolomitization.Meanwhile,various ge-ochemical indicators show that from southwestern Sichuan,central Sichuan to north-western Sichuan,the formation temperatures of dolomites gradually decreased,the hy-drothermal signals gradually weakened.The distribution of dolomites in the platform is mostly affected by the combination of permeable shoalsand basement faults,which further confirms that the Middle Permian dolomites in the Sichuan Basin were formed under the control of the ELIP events.Additionally,thermal convection dolomitization should be widely present in the structure-controlled hydrothermal dolomitization sys-tem related to abnormal thermal events.Thermal convection is the main driving force for the large-scale flow and migration of dolomitizing fluids.However,thermal con-vection is often indiscernible due to the overprint of the late-stage hydrothermal signals.In summary,this study establishes the ELIP-related Middle Permian dolomitization model in the Sichuan Basin,which could provide effective guidance for the formation and the prediction of the abnormal thermal events associated dolomite bodies.