储层压力与天然气分布关系——以鄂北塔巴庙地区上古生界储层为例

RELATIONSHIP BETWEEN CHARACTERISTICS OF RESERVOIR PRESSURE AND DISTRIBUTION OF NATURAL GAS:AN EXAMPLE OF TABAMIAO AREA,THE NORTHERN ORDOS BASIN

通过分析塔巴庙地区储层压力的分布特征,建立了研究区泥岩压实曲线,并对影响和控制该区储层压力的主要因素和机理进行了探讨,对“欠压实”剩余压力、生气增压、抬升引起的降压进行了定量计算。分析结果表明,晚三叠—早白垩世末“欠压实”作用和晚侏罗—早白垩世末烃源岩大量生烃作用使塔巴庙地区上古生界形成异常高压;晚白垩系构造抬升降压,使得现今储层为正常—低异常压力。同时证实了“欠压实”对天然气有压力封盖作用,天然气都分布在最大“欠压实”幅度以下的储层当中。烃源岩生气高峰为晚侏罗—早白垩世末,但天然气大规模运移是在晚白垩世,方向以沿过剩压力梯度降低较快的方向(太原—山西组至下石盒子组)垂向运移,通道主要为裂缝,运移的动力主要为异常高压。由于储层致密,聚集成藏能量有限,造成天然气充注不足、气水分异不好、含气饱和度低。

After systematic analysis on the pressure distribution character of the Tabamiao Formation,the compacting line of mudstones was built up.The main factors and mechanism which influenced formation pressure of this area were investigated.Quantitative calculation was used to calculate the descent of pressure which was caused by those factors such as being under compaction,hydrocarbon generation,and structure uplift.The result indicated that being under compaction in Late Triassic-Early Cretaceous and hydrocarbon generation in Late Jurassic-Early Cretaceous were the main factors resulting in abnormal hyper-pressure of Upper Palaeozoic formation.Structure uplifting in Late Cretaceous made present reservoir pressure be normal-abnormal low.A large amount of natural gas was found to be distributed in the reservoir below the maximum value of being under compaction(maximum residual pressure of mudstone).Meanwhile,it also proved that mudstone under compaction became the seal rocks for gas reservoirs.The fastigium of hydrocarbon generation was Late Jurassic-Early Cretaceous,while massive migration time was Late Cretaceous.Migration direction was vertical which was also fast pressure gradient decent direction(from the Taiyuan-Shanxi Formation to the Shihezi Formtion),with the fracture as its main passage,and abnormal hyper-pressure as its driving force.Because of the two reasons,i.e.limited accumulation energy and dense rocks as formation,there was not enough gas entering into the reservoir,water and gas could not be easily detached,and gas-bearing saturation was rather low.