Study On The Characteristics Of Tight Dolomite Reservoir In Well Xike 1,Xisha Islands

Based on the drilling core and the test data of porosity and permeability of well Xike 1 in Xisha islands,the intervals 1044.13?1089.42m and 1100.42?1173.62m of the Sanya formation in the study area are determined to be tight dolomite.On this basis,the 118.5m core is tested and analyzed by using rock slice,X-ray powder diffraction,Scanning electron microscope and geochemistry techniques.The study of petromineralogy,diagenetic environment and genetic mechanism,reservoir characteristics and main controlling factors has been completed.There are mainly residual bioclastic dolomite and crystalline dolomite in the study area.The dolostone crystals are relatively standard rhombic polyhedrons with powder-fine crystal.These crystals are mostly euhedral–subhedral with a small amount of anhedral form and are mostly pointly,linearly or concave-convex contacted and tightly mosaicked.The mineral facies are mainly calcite,dolostone and iron dolostone with medium order degree.Intercrystalline pores,intracrystalline dissolved pores,intercrystalline dissolved pores,biological lattice pores,moldic pores and biological cavity pores are the main reservoir space,but the pores are mostly isolated and there is no effective connecting channel.The pore permeability test data also shows to some extent that the target interval of the study area is a low porosity and low permeability layer with a porosity of 1.65%?16.40%and an average porosity of 6.69%,mainly concentrates at 4.00%?8.00%.The permeability is relatively low,and the permeability less than 10×10^-3?m² samples account for more than 80%of the total number of samples.The rock-forming oxides are mainly Ca O,Mg O,Si O₂ and CO₂,and the total amount is above 99.28%,followed by Na₂O,Fe₂O₃,K₂O and P₂O₅,and the content is basically less than 0.10%.In the study area,the upper section 1044.13?1089.42m and the lower section1100.42?1173.62m of the Sanya formation are found,which belong to the turbulent water body accompanied by the short-term eustacy of sea level and the calm water body which is less affected by the external water.The sea level is gradually decreasing.It is mainly formed in the relatively open near-surface environment in the early stage of diagenesis.The formation mechanism is reflux permeation dolomitization,and the dolomitized fluid is derived from the brine concentrated in the tidal flat or lagoon overlying it.Through genetic analysis of tight dolomite,it is believed that the diagenetic environment and diagenesis are the main controlling factors for the formation of tight dolomite.Reef facies carbonate rocks lays the material foundation of the development of high quality reservoir.Early dolomitization has a role in maintaining the original pores.Theoretically,dolomitization will increase the porosity of dolomite,but the dolomite in the target interval have experienced multiple diagenesis in a multi-period under closed environmental conditions.The transitional dolomitization leads to the multi-stage transformation of the parent rock by the contemporaneous dolomitization fluid and the filling of the early formed pores,which has an adverse effect on the reservoir property.Multi-phase dissolution contributes to the development of reservoir space.Compaction also affects reservoir properties to some extent.The microstructure pores of dolomite control the types of pore development significantly.