鄂尔多斯盆地上古生界泥岩在油气成藏中的作用

Role of the Upper Palaeozoic mudstones in the hydrocarbon accumulation in the Ordos Basin

关键词:  泥岩盖层  气水倒置  地层水  超压  溶蚀孔隙

Key words:  mudstone overburden  inverse gas-water reservoir architectures  formation water  overpressure  solution openings

摘要:

泥岩可以作为烃源岩提供油气来源,也可以作为盖层保存油气。此外,泥岩在形成异常超压、控制地层流体分布方面也有着特殊的作用,对油气成藏有着非常重要的意义。鄂尔多斯盆地整体处于低压状态,但受泥岩排烃、欠压实影响,局部地层超压。由于区域上的直接盖层厚度较小,超压泥岩是区域油气保存的重要保障,预示着油气勘探的有利区带。盆地地层水外来补给较少,受厚层泥岩的封堵作用,矿化度和pH值整体较高,造成本区成岩作用阶段明显晚于同期同深度的其它区域地层。后期受泥岩中粘土矿物脱水作用影响,泥岩中的水释放使地层水矿化度有所降低,也形成了本区特殊的气水倒置的成藏结构。

Abstract:

The mudstones as the source rocks to provide oil and gas for the reservoir rocks and also as the cap rocks to preserve oil and gas have important implications for hydrocarbon accumulation due to their special role in the controls of abnormal overpressure and fluid distribution.The present paper deals with the mudstone distribution and its bearings on formation fluids.On the whole,the formation pressure is relatively low in the Ordos Basin.However,the overpressure often appears in local parts due to hydrocarbon expulsion and undercompaction.The overpressured mudstone may act as important seal rocks for regional hydrocarbon preservation,and indicate the favourable zones for oil and gas exploration.Due to the sealing of thick mudstones,higher mineralization degree and pH values are responsible for later diagenesis in this region than the equivalents in other regions.Moreover,the dehydration of clay minerals may lead to the decrease of the mineralization degree in formation water and thus contribute to the formation of the special inverse gas-water reservoir architectures in the study area.