Discussion on Types of Upper Paleozoic Gas Reservoirs in Ordos Basin

In order to estimate whether the Upper Paleozoic gas reservoirs in Ordos Basin are deep basin gas reservoirs or not,the accumulation conditions,accumulation periods and distribution regularity were studied and compared with those of the deep basin gas reservoirs in Alberta Basin.The results show that the Upper Paleozoic gas reservoirs in Ordos Basin have the characteristics of extensive distribution of source rocks,unobvious gas generation center,low gas generation rate,bad persistent and decentralized gas-providing condition,so that the force of gas driving water is not enough to form deep basin gas reservoirs for the Upper Paleozoic natural gas.For the low porosity and permeability,strong heterogeneity,similar distribution direction between bodies and regional structure,surrounded by mudstones in the updip direction,the Upper Paleozoic reservoirs in Ordos Basin were different from those of deep basin gas reservoirs characterized by porosity and permeability being better and better along the updip direction.The source-reservoir assemblage styles,with limited contact areas between source and reservoirs due to small scale and discontinuous bodies,are self-generation self-accumulation and sand bodies superposed on source rocks and are different from those of deep basin gas reservoirs which have great scale contact areas.The Upper Paleozoic gas reservoirs have three accumulation periods including the burial and uplift periods of basin,experienced strong late reformation,and don't have the conservation condition of deep basin gas reservoirs.Gas and water,controlled by the physical properties of reservoirs and the regional structure,have complicated distribution,unobvious partition and no gas-water inversion interface resulted from gas driving water in the reservoirs.The characteristics of the Upper Paleozoic gas reservoirs in Ordos Basin show that the Upper Paleozoic gas reservoirs are the stratigraphic gas reservoirs mainly controlled by the physical properties of reservoirs and not deep basin gas reservoirs.This indicates that the study of favorable reservoirs should be paid more attention in the future petroleum exploration.