Study On The Characteristics Of Fluid Inclusions And The Accumulation Period Of The 8th Member Of The Upper Paleozoic In The Eastern Ordos Basin

The study target of this research is the reservoir in Section 8 of Shihezi Formation of Upper Paleozoic in Eastern Ordos Basin.Core observation,analysis of casting thin sections,scanning eletron,SEM,cathode luminescence,Cold and Hot stage,Laser Raman microprobe and other modern testing method were used to study the petrographic characteristics and physical property of the reservoir in Section 8 of Shihezi Formation of Upper Paleozoic in research area.The petrographic characteristics?homogenization temperature?salinity and composition of fluid inclusions in formation were also investigated,combining the buried-thermal evolution history of the reservoir in Section 8 of Shihezi Formation of Upper Paleozoic in research area.The reservoir forming stage,reservoir forming time and the coupling relationship between diagenetic evolution and hydrocarbon accumulation were comprehensively analyzed.The fluid inclusions were divided into six types based on the type and occurrence of the host mineral which was confirmed by the microscope observation of the petrographic characteristics of fluid inclusions.According to the formation order of the host mineral,the formation of fluid inclusions can be divided into three phases.The first term captive fluid inclusions,which formed during the compaction stage,exist in the micro-cracks of quartz grains.The second term captive fluid inclusions,which formed during secondary enlargement of quartz stage,exist in the micro-cracks cut through quartz grains,the micro-cracks in enlarged edge of quartz grains,the micro-cracks cut through the enlarged edges of quartz grains,and the micro-cracks cut through quartz and its enlarged edges.The third term captive fluid inclusions,which formed during iron calcite cementation stage,exist in the iron calcite cement.Regarding the micro thermometry analysis,the homogenization temperature main peak of fluid inclusions distributed in the range of 110?130 ?.The homogenization temperature main peaks of the first term captive fluid inclusions,the second term captive fluid inclusions and the third term captive fluid inclusions distributed in the range of 110?130?,120?140?,and 140?150?,respectively.The salinity of fluid inclusions mainly distributed in the range of 0.35?3.87%.The salinity of fluid inclusions rose with the increase of average temperature,suggesting the successive oil and gas charging process in the reservoir in Section 8 of Shihezi Formation of Upper Paleozoic in research area.In terms of the composition analysis of fluid inclusions,fluid inclusions which contain H2O?CH4?CO2?N2?C and H2S could be divided into brine fluid inclusion,brine fluid inclusion include hydrocarbon,gaseous hydrocarbon fluid inclusion and CO2 fluid inclusion.According to the difference of gas compositions,Hydrocarbon inclusions could be divided into hydrocarbon inclusions rich in CH4 and CO2 which usually exists in the first phase captive fluid inclusions and hydrocarbon inclusions rich in CH4 which usually exists in the second phase and third phase captive fluid inclusions,indicating the evolution of the oil and gas in the reservoir in Section 8 of Shihezi Formation of Upper Paleozoic in research area from low-maturity to high-maturity.According to the fluid inclusions compositions,salinity and average temperature characteristics,coupled with the analysis of buried-thermal evolution history in the study area,it can be concluded that there was one term of nature gas charging in Middle Jurassic,from 185 to 155Ma.Based on the study of reservoir compaction process,it could be known that tight reservoir formed in 175?155Ma.Therefore,the hydrocarbon accumulation mode of the gas reservoir in Section 8 of Shihezi Formation of Upper Paleozoic in research area was accumulation was done before compaction during 185?175Ma,compaction and accumulation simultaneously during 175?155Ma.