Natural gas accumulation mechanism in Upper Shihezi and Shiqianfeng Formations of Shenmu-Yulin district in northeastern Ordos Basin

According to the development and evolution of fluid dynamics,the present fluid pressure in Shenmu-Yulin district of the northeastern Ordos Basin was analyzed.The ancient fluid dynamic conditions were reconstructed by means of some techniques including research of mudstone compaction,measurement of fluid inclusions in sandstone and basin numerical stimulation.The accumulation conditions,periods and types of natural gas were investigated.The fluid dynamics analysis indicates that the overpressure phenomenon appeared obviously in the geological history,and the present ordinary pressure or negative pressure was formed subsequently.The history of increase and decrease of pressure constituted two cycles of excess pressure.Just in gas accumulation areas of Upper Shihezi Formation and Shiqianfeng Formation,the probability of hydrodynamic seal is relatively weak vertically.Natural gas could migrate into the shallow layers and be accumulated under the actions of buoyancy itself and the difference between high excess pressure downwards and low excess pressure upwards.On the background of this fluid dynamic environment,natural gas accumulation has obvious evolution stages,including the Jurassic period of slow accumulation, the Cretaceous period of rapid accumulation and slow adjustment from the Tertiary period to the Quaternary period.The types of gas accumulation can be concluded into lithologic and structural gas reservoirs in normal reservoir rock condition and gas reservoirs in tight sand-stones.The major reason for natural gas accumulated in Upper Shihezi Formation and Shiqianfeng Formation may be related to gas migration in "chimney" style from Lower Permian series.