Study Of Fluid Inclusions In Carbonate Reservoirs Of The Middle Permian Qixia Formation In Northern Guizhou

The carbonates of the Qixia Formation in northern Guizhou are often studied as hydrocarbon source rocks,but with the development of unconventional oil and gas formation theory and major breakthroughs in drilling wells such as the An Ye 1 well and Daozhen Datang in the region,it is important to conduct fluid inclusion studies on the Qixia Formation carbonates in northern Guizhou as reservoirs for carbonate gas reservoir exploration.In the long geological history of northern Guizhou,the region has experienced multiple phases of tectonic movements,while the thermal evolution of organic matter has continued.The opening of tectonic fractures in the reservoir in each phase is often accompanied by the filling of subsurface fluids,and the formation of fluid inclusions in the vein body as the best sample for the study of paleogeological fluids,not only preserves various geochemical information of the geological environment at that time,but also has a certain resistance to various late modifications,which can effectively recover the temperature and pressure conditions and hydrothermal composition characteristics at the time of fluid action,so it is widely used in delineating oil and gas filling periods,determining reservoir formation time,reservoir formation temperature,and oil and gas transport channels.Petrographic observation,microcalorimetry and laser Raman spectroscopy analysis of fluid inclusion samples from carbonate reservoirs of the Qixia Formation in northern Guizhou combined with burial-geothermal-thermal evolution history can provide theoretical basis for exploration of carbonate gas reservoirs.The results of the study show that:Gas-liquid two-phase brine inclusions,pure gas-phase and pure liquid-phase inclusions are mostly developed in the fractured vein bodies of carbonate reservoirs of the Qixia Formation in northern Guizhou.Microcalorimetric analysis of fluid inclusions shows a wide range of uniform temperature distribution from 140 to 340°C.The salinity（wt%Na Cl）of reservoir fluid varies from 1.91%to 17.43%（wt%Na Cl）,and the density of reservoir fluid ranges from 0.78 to 0.99 g/cm³.As the uniform temperature increases,the salinity of fluid gradually increases and the density gradually decreases.The analysis of the composition of individual fluid inclusions shows that the gas-phase composition of pure gas-phase inclusions and gas-liquid two-phase brine inclusions is mainly CH₄.According to the evidence of fluid inclusions,the formation period of natural gas reservoirs in carbonate reservoirs can be divided into 3 phases:the mean temperature of fluid inclusions in phase 1 ranges from 140 to 220°C,and the corresponding capture time is Late Triassic-Middle Jurassic（219-164 Ma ago）,in the early-middle Yanshan Movement,with a formation pressure coefficient of about 1.99-2.22;the mean temperature of fluid inclusions in phase 2 ranges from 220 to 280°C,and the corresponding capture time is Middle Jurassic-Early Cretaceous（164-121 Ma ago）,with a formation pressure coefficient of about 1.21-2.22;the mean temperature of fluid inclusions in phase 2 The mean temperature of fluid inclusions in stage 2 is between220 and 280°C,and the corresponding capture time is the Middle Jurassic-Early Cretaceous（164-121 Ma ago）,in the middle-late Yanshan Movement,with a stratigraphic pressure coefficient between 1.21 and 1.82;the homogeneous temperature of Phase 3 fluid inclusions is between 280 and 340°C,and the corresponding capture time is Early-Late Cretaceous（121-96 Ma before present）,in the late Yanshan Movement,with a stratigraphic pressure coefficient between 2.03 and 2.45.The paleosol pressure variation range of the Qixia Formation carbonate reservoir in northern Guizhou is 65.29 to 143.07 MPa,and its burial depth is nearly 8 km.