深盆气成藏关键地质问题

Some Remarks on Deep Basin Gas Accumulation

深盆气藏是与源岩紧密相连的气水倒置气藏,具有气水倒置、储层致密、源储相连、无水动力驱动、地层普遍含气、含气面积大、地质储量大、通常出现于盆地的构造较深部位且以含煤地层为主要的气源岩等特征。地层内部天然气能量的最初来源是油气生成过程中由干酪根热化学反应所产生的化学能转换,生气作用过程的持续发生赋予了天然气离开气源岩并进入地层孔隙的基本能量,生烃膨胀力作用导致了气水活塞式的排驱过程和特征,形成了宏观上的气水倒置现象。在地层条件下,气水倒置关系的产生只有两种可能:当天然气最初从源岩排入常规储层时,气水倒置现象产生但具有较小的气柱高度(与浮力作用有关);在深盆气成藏条件达到满足时(如均质性较强、大面积发育且紧邻气源岩的致密储集层发育等),浮力作用失去效力,产生较大气柱高度的气水倒置。与受浮力作用影响(与气柱高度有关)的典型常规气藏相比,典型深盆气成藏与埋藏深度有关。常规气藏表现为原生的高异常地层压力,但深盆气藏具有较大的异常压力变化幅度,两者均可在成藏动力条件达到平衡时具有相对静止的稳态保存特征。

Defined as gas accumulation in direct contact with source rocks, deep basin gas is characterized by tight reservoir, close connection of reservoir and source rocks, reversion relationships between water and gas, abnormal geo-pressure, etc. It usually occurs within the deeper part of basins with no water driving. Energies of expelled gas initially comes from the chemical conversion from kerogen during the hydrocarbon generation, which gives the gas energies to leave source rocks and migrate in tight reservoirs. The expanding forces derived from gas generation that results in the piston-modeled driving of formation waters by continuous gas supply bring about the symbolic reversion contact of formation water and gas. There are only two possible ways leading to the reversion contact within the porous rock media. One is a shorter gas column when gas is initially expelled into the normal reservoirs, which is affected by the buoyancy of gas. The other is a greater gas column when the conditions (such as the tight properties, homogeneity and scaled distribution of reservoir and so forth) are met to accumulate deep basin gases, which are affected by the pressures of water columns but with no buoyancy of gas. Compared with the normally trapped gas, the accumulation of typical deep basin gas is dominantly controlled by the capillary pressure and burying depth of tight sands. The original geo-pressures of normally trapped gas can only be of the super-pressures whereas the original geo-pressures of deep basin gas accumulations are widely varied. As regards to the preservation states, both the normally trapped and deep basin gas accumulations can steadily exist when their dynamic powers become equipoise.