天然气成藏过程中地层水相态变化——以鄂尔多斯盆地上古生界为例

根据千米桥潜山异常高温气藏的生产特点、气井凝析水的采出机理以及采出水的矿化度特征,认为鄂尔多斯盆地上古生界气藏形成于异常高温阶段。异常高温不仅导致了甲烷的生成,还导致了部分地层水的汽化和异常高压的形成。气(汽)相流体在异常高压推动下向上部地层扩散,运移使下部地层中的压力降低,加速地层水汽化,并积聚新的压力和新一轮的运移,如此反复,逐渐将甲烷、蒸汽水及伴生的温度、压力扩散至封存箱内的每个部位,达到封存箱内的区域势平衡,从而形成盆地级的高温高压气藏。抬升剥蚀导致上古生界温度、压力下降,水蒸气的液化使气藏中的蒸汽水密度降低,甲烷气浓度降低,气柱压力降低,从而形成负压气藏。

Phase state changes of formation water in gas accumulation process: a case study from the Upper Paleozoic,Ordos Basin

According to the production characteristics of the abnormal high temperature gas reservoir,the producing me-chanism of condensate water in gas wells,and the salinity characteristics of the produced water in the Qianmiqiao buried hill,we believe that the gas reservoir of the Upper Paleozoic was formed during the abnormal high temperature phase in Ordos Basin.The abnormal high temperature resulted in not only the generation of methane but also the vaporization of some formation water and the formation of abnormal high pressure.The gas (vapor) diffused upward under the driving of the abnormal high pressure.This gas migration led to the decreasing of pressure in the lower strata and the speeding up of formation water vaporization.The vaporization of formation water may in turn increase the pressure of the lower strata,leading to a new round of hydrocarbon migration.This process may repeat again and again,gradually making the me-thane,the vapor,the associated temperature and pressure diffuse to the whole compartment,realizing local balance of potential field in the compartment,and finally resulting in the formation of gas reservoirs of high temperature and pressure in the basin.After that,uplifting and erosion cause the lowering of temperature and pressure,while liquefaction of vapor leads to the decreasing of vapor density,methane concentration and gas column pressure,resulting in the formation of underpressured gas reservoirs.