中国东部中、浅层气藏成藏动力学特征——以济阳坳陷中、浅层气藏为例

Dynamics features of gas reservoir forming with shallow?middle depth in the east of China——taking gas reservoirs with shallow?middle depth of Jiyang depression as an example.

中、浅层气藏是我国东部以产油为主的盆地中重要的天然气勘探目标。气藏分布范围主要在埋深 5 0 0～2 0 0 0m。地下温压场控制了气藏的成藏动力学。当地层压力与油藏饱和压力之差低于 3MPa ,天然气会从石油或地层水中游离出来 ,形成气顶或纯气层。中、浅层气藏主要是次生气藏 ,天然气成因多样 ,其中油田伴生气是主要来源 ,其次 ,油藏中石油降解形成的甲烷气也是浅层气藏的气源。盖层控制了气藏的纵向分布 ,气藏的剩余压力低于盖层的突破压力是形成天然气聚集的必要条件 ,一般从埋深 80 0m开始 ,泥岩盖层就开始具备了封盖能力。断层是天然气垂向运移的主要通道。油气藏内的重力分异和沿断层及不整合面的运移分异是中、浅层气藏形成的机制 ,因此 ,中、浅层气藏在盆地中的分布主要受断层控制。在油气源充足及存在一系列沿倾向连通的圈闭时 ,依据圈闭的封闭机制会发生两种类型的差异聚集 ,油气藏的分布亦复杂化。

Gas reservoirs with shallow and middle depth are the important natural gas exploration target among the oil?producing basins in the east of China. Burial depth of gas reservoirs ranges at 500～2000m mainly. Subsurface temperature?pressure field controls dynamics of gas reservoir forming. When the difference between formation pressure and reservoir?saturated pressure is less than 3MPa, natural gas will free from oil or formation water and then form gas cap or clean gas?bearing formation. This kind of gas reservoir belongs to secondary gas pool and natural gas genesis, and has various gas origins in which associated gas is dominant. Moreover, the methane gas derived from degradation of oil provides partly gas source for shallow gas reservoir. The caprock controls the vertical distribution of gas reservoir and the prerequisite for natural gas accumulation is residual pressure in gas reservoir less than breakthrough pressure of caprock. The mudstone caprock begins to have sealing gland ability from 800m of burial depth. The faults are the main pathway of gas vertical migration. The gravity segregation in oil?gas reservoirs, as well as migration segregation along the fault and unconformable surface is the mechanism of shallow?middle gas reservoir forming. So the distribution of shallow?middle gas reservoir in basins is mainly controlled by fault. Two types of differential accumulation will occur and oil?gas reservoir distributions will be complex according to closed mechanism of traps when oil?gas sources are sufficient and a series of connecting traps along dip direction exist.