Genetic Model And Reservoir Controlling Factors Of Middle Permian Dolomite In SW Sichuan Basin

The dolomitization of the Middle Permian carbonate platform in SW Sichuan basin is extensive,and it has good prospects for exploration.Four types of dolomitization indicated by petrography and geochemistry consist of three matrix replacive dolomites?micrite dolomite,fine-crystalline dolomite,mesocrystalline dolomite?and one cement dolomite?saddle dolomite?.Micrite dolomite are very fine?<50?m?crystals,the lithology indicates that it is low dolomization degree penecontemporaneous dolomite.The high ⁸⁷Sr/⁸⁶Sr ratio and low?¹³C are considered to the trace terrigenous material contamination,but it's rare earth elements and?¹⁸O characteristics are similar to those of precursor limestone,indicating that the genesis of micrite dolomite is similar to limestone.The lack of evidence for the evaporation indicates that the micrite dolomite could have been formed via seepage-reflux dolomitization during penecontemporaneous stage.The fine-crystalline dolomite is characterized by fine to medium?50 to 200?m?and non-planar-a to non-planar-s crystals and does not contain saddle dolomite.Its geochemical signatures include ⁸⁷Sr/⁸⁶Sr ratios from 0.707 085 to 0.708 042 within the range of contemporaneous seawater,negative?¹⁸O,and fluid inclusion homogenization temperatures between 88°C and 120°C.The salinity calculated from the final melting temperature ranges from 7.5 to 11 wt.%NaCl equivalent.These implies that the dolomitizing fluids were derived from evaporated seawater and modified to a limited degree through fluid–rock interaction under the influence of thermal convection.Mesocrystalline dolomite shows non-planar-s to non-planar-a crystal shapes with coarser crystal sizes?200 to 500?m?.Mesocrystalline dolomite has a high porosity,is filled with a large amount of saddle dolomite,and includes hydrobrecciation.Its geochemical characteristics include enrichment in radiogenic strontium,extremely negative?¹⁸O and high fluid inclusion homogenization temperatures?155-232°C?.These suggest that mesocrystalline dolomite formed from hydrothermal fluids;and its slightly negative?¹³C suggests that these dolomitization fluids have been associated with igneous activity.These Middle Permian dolomites provide an excellent example of multiple phases of different dolomitization processes,including dolomite associated with hydrothermal thermal convection during the later emplacement of the large igneous province volcanism;and distinction of thermal-convection dolomitization from the more common burial dolomitization.In the extensive and diverse dolomite strata,the mesocrystalline dolomite in the hydrothermal dolomitization mode is the most promising dolomite reservoir,and the controlling factors of reservoir development are mainly dolomitization,sedimentary facies,diagenesis and tectonic activity.