The Paleopressure Evolution In Upper Paleozoic And The Influence To Accumulation Of Natural Gas In Lower Paleozoic In The Southeast Of Ordos Basin

There is rich in natural gas resources in lower Palaeozoic of southeast Ordos Basin, since Jingbian gas field was found, the Ordovician weathering crust became important exploration targets. Source analysis shows that the natural gas of lower Paleozoic gas reservoir mainly comes from carboniferous-Permian coal formation. So the research through combining the hydrodynamic evolution history with sand body and unconformity conductor analysis about the gas’migration process and characteristics from upper Palaeozoic to lower Paleozoic has important theoretical and practical significance.Analyzing abnormal pressure causes and calculating the excess pressure in max buried depth period through the equivalent depth method. Concluding purpose formation’s fluid pressure distribution characteristics and evolution process by basin numerical simulation based on mass data. The research about transmission conductor including its distribution, conducting ability, space configuration and the distribution of effective conductor is based on abundant works about the sedimentary facie distribution, physical characteristics, tectonic evolution. Natural gas composition and isotope characteristics could trace gas migration direction. Accumulation periods are divided using homogenization temperature of inclusion and other elements. Combined accumulation factors above analysis the process of gas migration and accumulation mechanism.Studies shows that the lower Paleozoic gas reservoir in this are accumulated in Jurassic and early cretaceous. After generation, upper Palaeozoic gas migrated along the sand body-fracture conductor under the action of excess pressure and gas potential. In the limit of effective sand body conductor’s distribution, the major gas migration is vertical migration at south, west and a small scale of east side in the area. When gas reaches the bottom of the Benxi group, it get into Majiagou group from the southwest of the area lacking of bauxite and migrate along the unconformity surface. Abnormal fluid pressure in Majiagou group carbonate rock isn’t development, gas easily migrate to uplift in short distance at the area of better physical property. Zhidan, Yanan and Fuxian were inheritance uplift during accumulation period, where were possible accumulated area.