Research On Diagenetic Evolution Of Sandstone Reservoirs And Gas Accumulation Of Taiyuan Formation In Shenmu-Shuangshan Area

Ordos is basin of the largest natural gas exploration and development in recent years, In the basin's taiyuan formation clastic reservoirs of the upper Paleozoic was found a large field which has trillion square area of gas in Shenmu-Shuangshan area. This area represents a good prospect. Applying the theories and methods of sedimentology for reservoirs sandstones and diagenesis and combining the researches of the fluid inclusions, reservoir sandstone characteristics, reservoirs sandstone diagenetic evolution and the influence of diagenesis, with the study of the gas filling the temporal relation, reveals the gas accumulation.The observation and identification of rock slices show that the diagenesis of sandstones in reservoirs is complex in the research area. The diagenesis have two kinds:the adverse diagenesis and the beneficial diagenesis. We also found the diagenesis with the hot water deposition. According to the altered minerals relationship of metasomatism, it includes early and late two generations of mineral altered. Early altered mineral represents early diagenesis which stands for the results of acid fluid altered by the coal organic. Late altered mineral represents the results of the alkaline fluid altered with is rich in K+, Na+,Ca2+,Mg2+ plasma.Reservoirs porosity was greatly reduced by the early compaction and dissolution. But this situation does not affect the normal migration of oil and gas. The reservoirs become low-porosity and low-permeability reservoirs eventually by the action of late calcium and siliceous cementation and large altered mineral, which affects the normal migration of oil and gas. The physical properties of reservoirs were improved in the late process of geologic and dissolution of petrophysical property. Then the reservoirs became high porosity and permeability.There are two phases of fluid inclusions in the Shenmu-Shuangshan aera. The first phase of inclusions mainly consist of CO2, N2 and H2O with few hydrocarbons. The temperature main peak is 110℃, which represents the early stage of gas formation. The second phase of inclusions mainly consist of CH4 with saline water and mineral composition which contents are relatively rare. The temperature main peak is 150℃, which represents the mass formation of gas and accumulation. And the accumulation of gas is the most important event in the research area. The salinity of two phase fluid inclusions are higher than 3.5wt% NaCl. That shows the organic fluid in reservoirs is related with hydrocarbon formation in the closed system of the basin. Study by Tt method shows that the time of gas firstly migrated into reservoirs is about 160Ma, which is equivalent to the Late Jurassic. The time of a large-scale natural gas accumulation is about 110Ma, which is equivalent to the late Early Cretaceous.