Characteristics And Controlling Factors Of Carbonate Reservoir In Paleogene Saline Lake In Western Qaidam Basin

Qaidam Basin is a typical plateau Salt Lake Basin.A set of thick lacustrine carbonate strata were deposited in the upper member of the lower Ganchaigou Formation in the western part of Qaidam Basin.Over exploration and evaluation,many high yield oilfiled had founded.In order to further understand the characteristics and controlling factors of lacustrine carbonate reservoirs in this area,this paper takes the upper member of the lower Ganchaigou Formation(E₃²)in Yingxi area of Qaidam Basin as the research object,according to the theory of sedimentology,reservoir geology,petroleum geology and other disciplines as the guidance,according to the core observation description,microscopic thin section identification,SEM observation,mercury injection data analysis and other means.The sedimentary environment characteristics,petrology characteristics,reservoir space types and main control factors of the carbonate reservoirs in the target formation of the study area are systematically studied,and the following main achievements and understandings are obtained:(1)Through the core sampling and experiments,it is found that Yingxi area belongs to the sedimentation center of the basin because of its high paleosalinity and deep water body.Through the interpretation of the seismic section in this area,it is clear that the micro paleogeomorphic in this area is a pattern of the trough among the low uplift phase.(2)The lithology of carbonate reservoir in this study area is complex,with the characteristics of carbonate rock(calcite,dolomite),terrigenous clastic,argillaceous and gypsum salt mixed accumulation.The content of carbonate minerals under salt is higher than that between salt,while the content of terrigenous clastic,argillaceous and gypsum salt under salt is lower than that between salt.(3)Combined with core observation,well logging curve analysis and other means,this paper analyzes the lithologic combination sequence,single well facies,connected well facies and plane facies in this area,combined with the characteristics of paleogeomorphology,establishes the sedimentary model of the underwater low paleo uplift carbonate reservoir in Yingxi area,and the carbonate reservoir with good physical properties can be developed in the high part of the paleo low uplift.(4)In addition to cementation,compaction and pressure dissolution,the reservoir in the study area also developed parasyngenetic atmospheric freshwater eluviation,parasyngenetic evaporation concentration dolomitization and slippage deformation and disruption caused by late tectonic compression.The main reservoir space types are brecciated fracture cave,dissolution pore and dolomite intergranular pore.Among them,brecciated fracture pore and millimeter scale dissolution pore have the best reservoir performance.(6)The sedimentary environment controls by the types of sediments in the lake basin,among which the paleoclimatic environment controls the sedimentary cycle,the salinity of the paleo water controls the composition of the sediments,and the paleogeomorphology controls the distribution of the sedimentary facies belt.The main diagenesis,such as parasyngenetic atmospheric freshwater eluviation,parasyngenetic evaporation concentration dolomitization and slippage deformation,controlled the development of reservoir space.