Dolomite Genesis Of Miocene Meishan Formation In The Well Xike-1,Xisha Area,South China Sea

The South China Sea has a special tectonic background and complex geological evolution characteristics.Because of its special geographical location and national energy strategy needs,it has always been a research hotspot.Based on the previous research,this study analyzes the rock petrological characteristics by means of collecting rock samples and description of core,rock slice identification,scanning electron microscopy,cathodoluminescence,etc.By the geochemical analysis of major and trace elements,carbon,oxygen isotopes,rare earth elements and hydrogen isotopes,the origin of dolomite in Miocene Meishan formation of Xike-1 well,Xisha Area,South China Sea(SCS)is revealed.The porosity,permeability and mercury data of the Meishan Formation can be used to judge whether the dolomitization of Meishan Formation is conducive to good reservoirs.It provides an example for the later development of Xisha area.The dolostones of Miocene Meishan Formation in the Well Xike-1 is generally not completely dolomitized.From the observation of cores in the Well Xike-1,it is found that the consolidation degree of dolomite is better than that of limestone,and the dolostones are mostly near the exposed boundary.In this paper,the layers with dolomite average value of more than 75% are commonly thought as dolostone.Four dolomite layers are identified in the Meishan Formation.In the middle of the second member of Meishan Formation(896.7m～910.72m),a set of dolomitic limestones and limy dolostone containing a very thin layer of dolomite is found.The average content of dolomite is between 50% and 75%,which is considered as a special dolostone layer in this study.The dolostones are mainly composed of the silty-fine crystalline dolomites,semi-automorphic crystal and flat crystal margins.There are two types of dolomites recognized in the dolomite of Meishan Formation,one is the generally dirty powder fine crystal dolomites with secondary enlarged edge and fog center bright edge structure,the other is the cement dolomites with clean,bright crystal,which is usually distributed along the pores.In cathodoluminescence,the secondary enlarged edge shows the same optical properties as the bright dolomite.Under scanning electron microscope,dissolution phenomenon and crystallographic structure are developed in the surfaces of some dolomites.The dolostones in Well Xike-1 are characterized by lower Sr,Fe,Mn,δCe and δEu values comparing to normal sea water,indicating that it is from an oxidizing seawater environment.The oxygen isotopes of dolomite tend to be positive,which is mainly caused by the environment of slightly evaporation and oxidation.The value of element Sr is low,which may be caused by the dolomitization process.The trend of carbon isotopes in dolomite indicates the inheritance of carbon in the original limestone during dolomitization.The carbon and oxygen isotopes are both positive,suggesting that the dolomite-forming fluid is mainly by the slightly evaporated seawater.But it is not ruled out that the normal sea water is involved.Combined with the structural characteristics,carbon and oxygen isotopic values,paleosalinity(z = 130.19)and the analysis of major and trace elements of dolomite in the Meishan Formation,it is considered that the dolomite in Meishan formation is dominated by the infiltration and circumfluence dolomitization in the quasi syngenetic period.The high content of red algae in interbedded dolomite makes dolomitization easier or earlier to occur and form more pure dolomites.The supersaturated pore water of dolomite during the burial period is the main reason for the cement dolomite in Meishan Formation.The dolostone reservoir of Meishan Formation has the characteristics of high porosity,high permeability and large pore throat.The dolomite layers have better reservoir performance than limestone.As a whole,dolomitization is conducive to the formation of good reservoirs in the carbonate rocks of the Meishan Formation.