Diagenetic Mechanism Of Tight Dolomite In The First Member Of Sanya Formation In Well Xike 1,xisha Islands

The Xisha Sea has developed a thick bio-reef formation since the Miocene,of which the Xike 1 well in the Xisha Islands revealed a bio-reef formation thickness of 1257.52 m.It is a full core well.Based on the drill core of the Xike 1 well,this paper finds that a section of dolomite formation in the Sanya Formation in the study area has low porosity and low permeability characteristics,and consists of two mineralogical end elements,namely peritectic rock and gap-filling material,so it is called dense dolomite.On this basis,the dense dolomite has been studied by microscopy,scanning electron microscopy and energy spectrum analysis,cathodoluminescence,main trace elements,fluid inclusions,isotope geochemistry and in situ micro-area laser exfoliation rare earth element testing,combined with the results of previous X-powder diffraction tests to investigate the lithology,geochemical characteristics and diagenesis,diagenetic environment and dolomitization mechanism of the dolomite.The study shows that three types of dolomite are mainly developed in the study area,mainly crystalline dolomite,with a small amount of residual biotite dolomite,of which the dolomite in the enclosing rocks is mostly authomorphic,semi-authomorphic,with a small amount of other forms,and typical mist-core bright-edge structures are developed,with dolomite crystals showing standard rhombic crystals,with fine crystals-powder crystals(1)predominating,and visible sandy-meal grainy(11)dolomite,mosaic contacts,and cathodoluminescence photographs of both showing dolomite Clear multi-circular light and dark rings are visible,indicating that the dolomite has undergone modification by multiple phases of diagenetic fluids during the burial diagenesis.The dense dolomite fillings are filled in the surrounding rocks,with white stellate distribution,dominated by calcareous microcrystalline-mud crystal(111)dolomite,with algal and gypsum pseudocrysts visible,mostly semi-automorphic-it-shaped,with some rhombic crystals evident,and inter-crystal contacts including mosaic,point,line and surface contacts.The dense dolomite mineral phase type includes dolomite,calcite and iron dolomite,which are not highly ordered,with significant de-dolomitization,and the pores are mostly intergranular,small and separated,with few secondary solution pores.The oxides are mainly Ca O,Mg O and CO2,with a pure composition,followed by Na2 O,Fe2O3,K2 O,Al2O3 and Mn O.The carbon and oxygen isotope,strontium isotope and main trace element results for the dense dolomite indicate that the target section was accompanied by a transient sea level rise and fall,with a gradual downward trend in sea level.It was formed in a relatively open near-surface environment during the early stages of diagenesis,and the dolomitic fluids are mainly brines formed by evaporation and concentration from the overlying tidal flats or lagoons.In addition,the rare earth element enclosures and fillings show typical marine features such as LREE loss,positive La anomalies,negative Ce anomalies and high Y/Ho values.The positive Eu anomaly in the enclosing rocks,combined with the homogeneous temperature analysis of the fluid inclusions,is thought to be related to the influence of magmatic hydrothermal fluids from deep within the formation.The fill shows negative Eu anomalies,indicating a low-temperature environment,and is presumed to be related to the early Miocene series of cold events.The dense dolomite section underwent a variety of diagenesis under confined-semi-confined conditions,and the dolomitic fluids modified the host rock in multiple phases and filled the early pores,which also influenced the reservoir materiality.The four diagenetic mechanisms are: fine-powder crystal dolomite diagenesis,"sandy sugar","wheat grainy" powder crystal dolomite diagenesis,gap-filling calcareous micro-mud crystal dolomite diagenesis and de-dolomitization.The present study can therefore provide a reference for understanding the dolomite genesis of the Xisha and the exploration and development of oil and gas in the South China Sea bioreefs.