The Genesis and Characteristics of CO₂-bearing Condensate Gas Reservoir in Q29 Structure in the Actic Area,Qinnan Depression,Bohai Bay Basin

CO₂-bearing condensate gas reservoir in Q29 Structure in Qinnan depression,is characterized by good physical property,low level of dawsonite,which is different from most of CO₂-bearing reservoir.Through analysis with polarizing microscope,scanning electron microscope,X-ray diffraction and isotope analysis,etc,the genesis of CO₂ and origin of CO₂-bearing high-quality reservoir have been made clear.In the study area,the content of CO₂ ranges from 25% to 59%,the value of δ¹³C_CO2 is from -5.2‰ to -6.4‰(PDB) and the R/Ra is from 3.9 to 4.4,all of which are the obvious features of mantle origin magma.The diagenesis paragenetic association are composed of overgrowth quartz,authigenic quartz and authigenic kaolinite after the incursion of the thermal fluid of CO₂,which increased reservoir dissolution.Therefore,the secondary pores are well developed in CO₂-bearing reservoir.Compared with the reservoir without bearing CO₂ developed at the same diagenetic stage,the relative content of authigenic kaolinite is higher and the content of illite is lower in CO₂-bearing reservoir.The dissolution and redistribution of carbonate cementation made by the thermal fluid of CO₂ are favorable for the improvement of physical properties of main reservoir.For multiperiod acidic fluid invasion,the content of dawsonite cementation is low and poorly crystallization,which has little influence on reservoir.All of above are the main reasons for the higher porosity and high permeability in CO₂-bearing reservoir.