Research On Reservior Formation Of Majiatan Area In Western Ordos Basin

Mesozoic oil and gas exploration of Yanchang formation in Western Ordos Basin always attracted people's interest.Tectonics of Western belt has closely relationship with the evolution and reconstruction of Ordos Basin.It is very significant for oil and gas exploration to research on discipline of oil and gas accumulation in Western Ordos Basin.Physical and chemical parameters of oil-gas inclusion of Mesozoic Yanchang formation in majiatan area were analyzed.Fluid inclusions analysis technology and research method were used to study phase,time and conditions of oil-gas accumulation.Features of oil-gas migration and accumulation together with tectonic evolution were discussed.It indicates that there are two kinds of oil-gas inclusion distributes in tectonic cracks formed at different tectonic stage in the area.According to time sequence of inclusions and digenetic mineral generation,it proposed that oil-gas generated and migrated to form reservoir during the time of early Cretaceous and later of early Tertiary.Statistics of oil-gas inclusion abundance illustrates that not only the oil well,but also dry well and water well have the same oil-gas inclusion abundance(GOI) of more than 5%.The study suggests that present water beds with more than 5%of GOI used to be oil beds of reservoirs.Present oil beds of reservoirs are residual oil deposits or destructed oil deposits.Inclusion analysis results show that there are two peaks of homogenization temperature(Th) of fluid inclusion of Chang 8 reservoir in Western shear zone,which are 120℃and 160℃,but 120℃and 180℃in Tianhuan depression.The low temperature peak of Th in Western shear zone is as same as that in Tianhuan depression,but the high temperature peak of Th in Tianhuan depression is higher than that of in western shear zone,which is related to the difference their tectonic evolution.Fluid inclusion salinity distribution also has two peak values,displays asymmetric skewness distribution.With increase of depth,fault quantity decrease,but fluid inclusion salinity increase,which indicates that oil and gas accumulations at early stage were destroyed by faults.Raman spectrum testing results show that composition of gas and liquid of inclusions mainly consist of CO₂,CH₄ and H₂O,in which CO₂ content is over 30%.CH₄ content of gas phase and liquid phase and total hydrocarbon content in early inclusions is obviously more than those in later inclusions,but CO₂ content is opposite.CH₄ content has negative relationship with CO₂ content.The reducing coefficient of early inclusions[the ratio of CH₄/ (CO₂+H₂O)]is higher than that of later inclusions,which indicate that early inclusions formed in enclosed environment,while later inclusions formed in oxidized environment.Total hydrocarbon content of inclusion(∑CH),oil-gas migration index,the ratio of (CH₄+C₂H₆+C₃H₈)/∑CH and the ratio of CH₄/∑CH in Western shear zone are higher than that of in eastern Tianhuan depression,which indicates that oil and gas migrated from east to west and oil and water separated along migration way.Geochemical characteristics of fluid inclusion indicate that paleo-fluid in reservoirs has the characteristics in sealed underground water but present underground water mixed with surface water.Oil-gas reservoir of Western shear zone experienced invisible unwraping during late geology tectonic movement,which resulted in destroy of early oil reservoir。Oil and gas migration path and accumulation trend were analyzed by means of present fluid potential and pale-fluid potential recorded by mineral inclusion.Results show that both ancient and present fluid potentials are low in Majiatan-Maerzhuang areas of western shear zone where there area regional low fluid potentials areas for a long time.Paleo-fluid migrated to the northwest.Two favorable oil-gas migration paths lie in Gufengzhuang district and Yujialiang-Dashuikeng district in the area.Present oil and gas fluids migrate from Tianhuan depression in east of Majiatan fault to western shear zone and accumulated in low potential area.