Study On Characteristics Of Dolomite In Well Xike 1 Of Xisha Islands

The reef in the Xisha islands have been developed continuously since Miocene.The well Xike 1 reveals that the thickness of the reef formation is 1257.68m.It is a complete core well with a large number of core specimens to study its rock characteristics,reservoir and genetic mechanism.The study area is located in the Xisha Islands reef area.Based on the well Xike 1of the Xisha Islands and the latest findings of 11 layers of dolomite samples,through the specimen of core observation,thin section analysis,X-ray powder diffraction analysis,degree of order analysis,the main methods of trace elements and isotopic geochemical analysis,the well Xike 1 of the Xisha Islands'dolomite petrographic characteristics,geochemical characteristics has carried on the detailed research,and further analyzes of the diagenesis of dolomite,dolomitization fluid sources and formation mechanism.There are 11 layers of dolomite distributed in different depths in well Xike 1.With the increase of depth,the degree of consolidation becomes better.From shallow to deep,the dolomite grains are composed of mud-crystal dolomite-powdered crystal dolomite-fine-grained dolomite.The order value also shows a tendency to become larger,but the overall order value is low.Dolomite has a certain degree of residual structure.It can be seen that the dolomite consists of raw or incomplete dolomitized chips,and the core has more sugar-like grain structure.The proportion of dolomite without residual structure in well Xike 1 is small,accounting for less than 10%.Its dolomitization is remarkable,showing grain dolomite,mainly distributed in the Sanya Formation with large buried depth.The reef carbonate rocks are pure in composition.The main rock-forming oxides are mainly Ca O,Mg O and CO₂,with a total amount of more than 80%,generally more than 90%.Loose deposits in the surface and shallow rock tend to be relatively high,and rapidly decrease as the depth increases.For dolomite,the content of strontium is less.The carbon and oxygen isotope contents of the dolomite layer are generally high.The carbon isotope is within the range of 1.21‰?3.15‰,within an average of 2.38‰;the oxygen isotope is whthin the range of1.78‰?4.77‰,within an average of 2.55‰.The content of Mn and Fe in the dolomite of well Xike 1 is generally lower,which is mainly formed in the closed environment,indicating that the dolomitized fluid has significant seawater characteristics,which the seawater or sea source fluid is its main dolomitized fluid.The carbon isotopic characteristics of ocean carbonate rocks show that the carbon in calcite,aragonite or sea source flow is the main source of carbon in dolostone.The?¹⁸O of dolomite is within the range of 1.78‰?4.77‰,which is suitable for the low temperature genesis?¹⁸O range,that is,the dolomite of well Xike 1 is a low temperature genetic dolomite.To sum up,the dolomite in well Xike 1 was formed in a highly saline lagoon environment,and seawater was the main source of dolomization fluid.Due to the repeated changes in the global sea level,the Xisha sea area has undergone many flooding and exposure processes since its formation,thus developing multiple dolomite layers in the well Xike 1.