Phase Identification And Genetic Mechanism Of Condensate Gas Reservoirs In The Bozhong Depression,Bohai Bay Basin

The Bohai Bay Basin is a typical oil-prone basin in eastern China.In recent years,with the extension of exploration to the deeper areas,for the first time,the large-scale condensate gas reservoir in the Bozhong depression have been discovered,in which initially shows great exploration potential.The type and distribution characteristics of hydrocarbon fluid phases in condensate gas reservoir are important basis for oil and gas resource evaluation However,the current understanding of the characteristics,influencing factors and formation mechanisms of the hydrocarbon fluid phases in the condensate gas reservoir of the Bozhong depression is not deep enough.Therefore,in this paper,a method combination of high temperature and high pressure physical property test?PVT?and fluid phase theorical simulation of formation fluid is used.Based on the experimental results of hydrocarbon fluid PVT experiments,the fluid phases theorical calculation methods is used to calculate the physical property parameters and phase envelope of hydrocarbon fluids by means of PVT phase state simulation software.Then,based on the experimental test data and fluid phase simulation results,the types of phases states of hydrocarbon fluids are discriminated,and the fluid phases control factors are comprehensively analyzed.Finally,based on the definition of the oil and gas source and filling period of the BZ19-6 condensate gas reservoir,the genetic mechanism of this condensate gas reservoir was ascertained.Then the BZ19-6 condensate gas reservoir compared with the BZ21-22 gas reservoir in the neighboring area,the oil and gas accumulation process of the BZ19-6 condensate gas reservoir is revealed.The fluid composition obtained from the PVT experiment of hydrocarbon fluids in BZ19-6 condensate gas reservoir:average methane content is 66.8%,C₂^C₆ average content is 14.66%,C₇⁺content is 7.3%,and inorganic CO₂ content is as high as 11.24%.The pressure difference between formation pressure and dew point pressure is only 0.59 MPa,and the average condensate content on the ground is 615.96g/m³,which belongs to the ultra-high condensate condensate gas reservoir with oil ring.The test critical temperature and pressure are less than the formation temperature and pressure,so the hydrocarbon fluid may be supercritical fluid under formation conditions.The fluid phase is mainly controlled by the hydrocarbon fluid component and the temperature and pressure system.On one hand,the fluid composition determines the critical temperature and pressure of the hydrocarbon system and plays a decisive role in the fliud phases.As the content of heavy hydrocarbon components in the hydrocarbon system increases,the critical temperature increases and the phase envelope migrates to the lower right of the phase diagram.On the other hand,the temperature and pressure system are important control conditions for the fluid phases,with the temperature and pressure decreasing the condensate gas reservoir can be converted into a reservoir;the increase in temperature and pressure can cause the crude oil to crack and turn into a condensate gas reservoir or even a pure gas reservoir.According to the comprehensive analysis,the BZ19-6 condensate gas reservoir is a secondary condensate gas reservoir formed by the reverse evaporation caused by the deepening of the burial depth,the CO₂ filling of the deep mantle source and the late natural gas intrusion.The formation process is mainly divided into two stages.In the first stage,large-scale oil accumulation at the end of the lower part of the Minghuazhen Formation?5Ma?,and the second stage,the end of the upper part of the Minghuazhen Formation?2Ma?inorganic CO₂ and late natural gas is filled with oil reservoirs formed earlier.This has led to a significant decrease in the critical temperature of hydrocarbon systems,which has transformed the original reservoir into the current condensate gas reservoir.