CHARACTERISTICS AND FORMING CONDITIONS OF LAKE-FACIES CARBONATE-ROCK OIL AND GAS POOLS IN THE NANYISHAN STRUCTURE OF THE QAIDAM BASIN

As a typical example of lake-facies carbonate-rock oil and gas pools consisted by shallow oil pools and medium-deep gas condensate pools,Nanyishan oil and gas field in the Qaidam Basin has altogether four sets of hydrocarbon source rocks. The whole stratigraphic profile is dominated by argillite,micritic carbonate rock comes second. The reservoirs of hydrocarbon are carbonate rocks,including argillaceous micritic dolostone,argillaceous micritic limestone,micritic limestone,algal limestone,etc. The storage spaces are mainly intercrystal pores,intergranular pores,solution openings,fissures and karst caves. The reservoirs,which have the thickness of 2-5m in general,have rather stable plane distribution. Tectonic fractures are all well-developed from surface to underground. The clear reducing of seismic-wave velocity and abnormal high-pressure are observed in the medium-deep layers of hydrocarbon-distributive areas. The fission-track testing of fracture-infilling calcite revealed that fractures occurred before the generation and migration of hydrocarbon. The analysis of biomarkers showed that hydrocarbon come from source rocks deeper than 4 300m.