Fluid Environment And Geological Model Of Ordovician Reservoir In Northern Slope Of Central Tarim Basin

The carbonate reservoir of the lower Ordovician Yingshan formation in Tazhong area(the north slope of the central Tarim Basin)is a favorable oil and gas accumulation zone.It experiences diagenesis such as interlayer karst and buried karst,and controlled by fluids and faults.It is of great significance for oil and gas exploration to study the fluids environments and develop the geological model of karst reservoir in Tazhong area.Petrology and geochemistry are used to analyze the fluids environments of reservoirs.Seismic,logging,drilling and core data are used to analyze reservoir and fractures characteristics and to build geological models in this paper.The calcite cement in the holes,caves and fractures of the Yingshan Formation used is to make stained sections,casting sections,and inclusion sections.The carbon,oxygen,strontium isotopic values of the calcite cement and the homogenization temperature of inclusions in the cement are also measured.The results indicate that there are three phases of fluid activity(phases IV,?,and ?)mainly developed in Tazhong area,which are mixed water(or fresh water)of one phase and hydrothermal fluids of two phases.Phase IV fluid is mixed water(or fresh water),and the calcite cement formed during this phase develops in high-angle fractures.The average salinity of the inclusions is 5.21 %Na Cl.The color of the calcite cement turns dark-orange under cathode rays,and the zonation developed.Those descriptions above show that cement during this phase grew in the burial environment.The salinity of the pahase ?thermal fluid can reach 13.06 %Na Cl,the average homogenization temperature of the inclusions is 202 °C,and the calcite cement does not emit light under the cathode rays.The growth of the cement of pahase ? is related to the Permian volcanic activity.It also develops in high-angle structural fractures and widely distributed.Phase ? fluid is also hydrothermal fluid related to TSR in the study area,with an average salinity of7.94 %Na Cl.The averege homogenization temperature of the inclusions in the calcite cement is 206 °C.Under the cathode ray,the calcite cement emits orange-red light with a medium brightness.This phase fluid developed before the Himalayan tectonic movement.The flow paths are the same as that of the phase V fluid,but its scale is smaller than that of phase V.Then,the reservoir characteristics of Yingshan formation and the relationship between faults and fractures are analysed.The geological models for the interlayer karst and buried karst reservoirs are established.The lithology of Yingshan formation reservoirs is primarily sand-clastic limestone and argillaceous limestone,and fracturedvuggy and vuggy type reservoirs are the main reservoir types.The development of interlayer karst is controlled by paleogeomorphology.In highland,the interlayer karst is well developed.While in low-lying area,the karstification is weak.Most of the fractures and holes developed during the in the course of sedimentary are filled by mud or calcite later,and values of logging porosity and permeability are low.Most of them are invalid reservoirs.However,the pores develope far from the unconformity not filled later,and form effective vuggy-fracture reservoirs with fractures.Two phases of buried karst take the faults-fractures system formed during middle-late Ordovician and Silurian-Devonian tectonic activities as the fluid channels.The first one is the phase V hydrothermal fluid with high salinity related to volcanic activity in Permian,the second one is the phase VI acid hydrothermal fluid related to TSR.Although buried karst has formed many effective pinhole-like dissolution pores,it has played a positive role in the formation of Yingshan formation reservoir.However,calcite cement also precipitates from the hydrothermal fluids in fractures,reducing the reservoir capacity.