Research On Fluid Inclusions And Natural Gas Accumulation In The Upper Paleozoic Of Linxing Area In Ordos Basin

Linxing area is located in the north of the eastern margin of Ordos Basin.The Upper Paleozoic Carboniferous-Permian marine-continental coal-bearing strata are widely developed.The Upper Paleozoic strata have frequent interbedded layers of coal,mudstones,and sandstones,and the source and reservoir are in close contact,forming a good hydrocarbon source-reservoir-caprock combination,and formed tight sandstone gas resource with great potential.In this paper,the fluid inclusions are analyzed as the main means.Through the petrographic characteristics,homogenization temperature and salinity of fluid inclusions,combined with the diagenesis of tight reservoirs,and the sedimentary burial-thermal evolution.The accumulation periods of tight sandstone gas was determined,the evolution of reservoir pressure during gas accumulation time was restored,the distribution law of paleo-fluid potential was revealed,the migration direction and accumulation characteristics of natural gas were pointed out,and the accumulation mechanism and model of natural gas were expounded.According to the study of microscopic petrography,fluid inclusions in the study area are mainly divided into 5 categories:brine inclusions,crystal-bearing brine inclusions,hydrocarbon inclusions,hydrocarbon-bearing brine inclusions,and aqueous-carbonic inclusions.According to the occurrence of fluid inclusions,there are mainly 2 phases of fluid inclusions:early and late phases.The former mainly distributed in the early fracture within quartz particles,and the latter mainly distributed in the late fracture that cut across quartz particles and their secondary enlargement edge.Statistical analysis of homogenization temperature of fluid inclusions shows that there are obvious differences in homogenization temperature and salinity of fluid inclusions between different wells,which indicates that the fluid heterogeneity in Linxing area is strong during gas accumulation time due to the influence of the magmatic invasion of the Zijinshan.According to the statistical analysis of homogenization temperature and salinity of fluid inclusions,combined with the reconstruction of sedimentary burial-thermal evolution,the study area is divided into 3 zones:inner-magma baking area,middle-anomal thermal area and outer-normal thermal area.Combining the homogenization temperature of fluid inclusions with the sedimentary burial-thermal evolution,it is revealed that the main accumulation period of natural gas in the Linxing area is from Middle Jurassic to Early Cretaceous.Under the influence of Zijinshan magmatic intrusive tectonic-thermal event,the gas accumulation periods are different in different regions.There was only 1 period of gas accumulation in the magma baking area,and the accumulation time is Middle Jurassic.There were two periods of natural gas accumulation in the abnormal thermal area.The first period occurred from the Middle-Late Jurassic,later than the magma baking area,and the second period occurs in the Early Cretaceous.There were also two periods of gas accumulation in the normal thermal area,the first period of gas accumulation was in the Late Jurassic,and the second period occurred in the Early Cretaceous.The vertical gas accumulation characteristics of natural gas show obvious regional differences.Due to the thermal event of Zijinshan pluton,natural gas accumulation in the magma baking area and the abnormal thermal area was not a slow and gradual process from bottom to top.A large amount of natural gas rapidly generated by the lower source rock rapidly reached the upper formation through faults and cracks generated by the intrusion of the Zijinshan pluton.The time interval between the upper and lower strata was shorter,and the accumulation time was earlier.The normal thermal area was less affected by the thermal event of Zijinshan pluton,and conforms to the rule that the gas accumulated earlier in lower formation than the upper formation.The trapping pressure of fluid inclusions is simulated by PVTsim 20 fluid simulation software,it is shown that the upper Paleozoic reservoirs in Linxing area have obvious overpressure characteristics in the gas accumulation time.The period from Middle Jurassic to Early Cretaceous is the key period for rapid pressure accumulation,which is consistent with the period of natural gas accumulation.The development degree of overpressure in the magma baking area,abnormal thermal area,and normal thermal area decrease in turn,reflecting that thermal baking of Zijinshan pluton controls the thermal evolution and hydrocarbon generation process of source rocks,and further affects the distribution of abnormal reservoir pressure.The study on the distribution and evolution of fluid potential in Linxing area shows that there are obvious differences in fluid potential between vertical formations during gas accumulation time,indicating the potential trend of upward migration of natural gas.The area near the Zijinshan pluton was the high fluid potential area during gas accumulation periods,which indicates that natural gas and other fluids migrated from Zijinshan magmatic pluton to the surrounding area.According to the natural gas accumulation period,reservoir pressure evolution and fluid potential analysis results,combined with hydrocarbon generation and expulsion simulation results,the evolution of transport system and present gas reservoir characteristics,the gas accumulation model controlled by magmatic thermal events were proposed in Linxing area.The accumulation process can be divided into 3 stages.In the first stage,during the early Jurassic,source rocks closed to the Zijinshan magmatic pluton began to generate a small amount of natural gas and enter the adjacent reservoirs.In the second stage,the Middle Jurassic to Early Cretaceous was the key period for gas migration and accumulation.The control mechanism of Zijinshan magmatic pluton was mainly reflected in its thermal baking and reservoir-transport system transformation.In the third stage,from the end of Early Cretaceous to Neogene,Linxing area continued to uplift,and the formed gas reservoirs were further adjusted and destroyed due to a large number of deep faults triggered by Zijinshan magmatic pluton.