深盆气藏的压力特征及成因机理

深盆气藏是一种特殊机理形成的具有气水倒置和负压异常等特征的致密气藏.储层致密是导致深盆气藏中气水倒置和负压异常的根本原因.在深盆气藏形成后,气藏内部任意一点的地层压力等于气水界面之上的静水柱压力与气藏内该点至气水界面处的气柱压力之和.由于气体的密度远小于水的密度,因此上述气水两段压力之和小于该两段都为静水柱时的压力之和,所以深盆气藏表现为负压异常.依据深盆气负压的特征及大小,可以判别含气层位的连通性、气柱高度和分布范围.在常规天然气成藏后,气藏内部任一点的地层压力等于气水界面之上的静水柱压力与气藏内该点至气水界面处的气柱压力之差.由于气体密度小于水的密度,气层内的压力比对应深度的静水柱压力大,因而常规气藏表现为超压异常.超压异常是常规气藏的一般特征,在一般地质条件下不能作为深盆气藏的判别标志.在几种特殊情况下,深盆气藏范围内也可以表现超压异常.

PRESSURE FEATURES AND TRAPPING MECHANISM OF DEEP BASIN GAS POOL

Deep basin gas is an abnormal gas accumulation with characters of subnormal pressure and gas_water inversion caused essentially by its tight reservoir.Under formation of deep basin gas pool,the formation pressure at any point in the gas reservoir is equal to the sum of hydrostatic pressure above the gas_water interface and the pressure of gas column from the point to the gas_water interface.Since the density of gas is far lower than that of water,the sum pressure mentioned above is smaller than that of two parts being both water,and the deep basin gas pool behaves the underpressure feature.The connectivity,height of gas pillar and distribution range of gas_bearing formation can be determined according to the feature and scale of subnormal pressure.However,under formation of common gas pool,the formation pressure at any point in the gas reservoir is equal to the difference between the hydrostatic pressure above the gas_water interface and the gas column pressure from the point to the gas_water interface.Since the density of gas is lower than that of water,the sum pressure mentioned above is bigger than that of the hydrostatic pressure above the point,and the common gas pool behaves the superpressure feature.Under general geologic conditions,supernormal pressure is of the general feature of common gas pool,and it couldn't be employed as a distinct symbol of deep basin gas pool,except in several special cases.