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

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.