Diagenetic Evolution Of Reef Carbonate Rocks And Dolomite Diagenesis In The Xisha Islands

Reef oil and gas fields are widely developed around the world.Because of its good storage performance and high recovery rate,it has received wide attention from petroleum geologists.The South China Sea,as the largest marginal sea in our country,is naturally rich in oil and gas resources.Drilling revealed that there are huge dolomite formations in the Paracel Islands in the South China Sea.Therefore,studying the genetic model of the dolomite strata has important theoretical and practical value for the exploration and development of the reef reservoirs in the South China Sea.This article is based on extensive investigations of previous research data,starting with petrological and mineralogical characteristics.At the same time,it combines geology and geochemistry,diagenesis and other related theories,through the combination of geology and geochemistry.In addition,with the help of core observation,thin section identification,analysis of trace elements and rare earth elements,isotope analysis,fluid inclusion reduction temperature,salinity and other analytical testing methods,the only full-core well in the Xisha Islands that drilled through carbonate formations Xike 1 Well conducted research on diagenesis and the genetic mechanism of dolomite.The following results have been achieved:The dolomites in Xike 1 Well are mainly distributed in the upper part of the lower submember of the Pliocene Yinggehai Formation,the upper Miocene Huangliu Formation,the lower part of the Meishan Formation,and the upper submember of the Lower Miocene Sanya Formation.The degree of crystallization gradually increases from top to bottom,and at the same time,it is mainly transformed from micrite dolomite to grain dolomite,and saddle-shaped medium-coarse crystalline dolomite is occasionally seen in the deep part.In addition to dolomite and calcite,dolomite also contains minerals such as pyrite,limonite,pyrolusite,and gypsum.Moreover,in the Sanya Formation and the lower submember of Meishan Formation,metallic minerals often develop along fractures.The diagenesis experienced by dolomite mainly includes compaction and pressolution,cementation,recrystallization,dolomitization,superdolomitization,dissolution and corrosion,dedolomitization and other diagenesis types.The test results of carbon and oxygen isotopes show that the correlation between carbon and oxygen isotopes in dolomite samples is low,which may reflect that the dolomitization process has not been affected by atmospheric fresh water and magma source fluids.It also indicates that the dolomite diagenetic fluid is seawater with a higher concentration.From the point of view of the uniform temperature shown by the temperature measurement of the inclusions,the deep dolomite may be affected by the high temperature pore fluid.The rare earth elements show that the samples with different degrees of dolomitization are the same as the limestone samples,and have good similarities with modern seawater.The only difference is that most samples show obvious negative Eu abnormalities.The index of rare earth elements such as ∑REE,Pr/Yb,La/Nd and Y/Ho of the dolomite samples are all within the range of normal seawater and marine sediments.There are mainly two genetic models of dolomites in this area: the first is the dolomitization model of permeation backflow and the second is the dolomitization model of burial.The dolomitization model of seepage reflux,which may be the basis for the formation of most of the dolomite in Xike 1 Well.The burial dolomitization mode driven by compaction under medium-deep burial conditions and thermal convection related to volcanic activity has further reformed the existing dolomites in the mid-deep section.Microbes in some well sections may also have an impact on the formation of dolomite,but it is not strictly a microbial origin dolomite.