Occurrence Characteristics And Accumulation Mechanism Of The Fluids In Upper Paleozoic Natural Gas Reservoirs, The Ordos Basin

The Ordos basin is one of the main bases for energy production in China. The natural gas resource in this basin is mainly derived from the Upper Paleozoic coal-bearing strata, which is mainly belonged to tight sandstone gas reservoir. According to exploration and production practical in recent years, the distribution and occurrence characteristics of natural gas in this basin is very complicated, for water production phenomenon is common in many production areas, so it’s difficult to clarify the distribution of natural gas and formation water underground. The phenomenon of high formation water production not only seriously constrains the improvement of natural gas production, but also blocks the arrangement of exploration disposition. Seeking for the occurrence rule of fluids in tight sandstone is one of the hot topics in the field of petroleum geology, which can provide key evidences for exposing the mechanism of natural gas accumulation. Therefore, this study is important either in practical aspect or in theoretical aspect.The idea to combine the dynamic accumulation processes and static microcosmic pore framework of reservoir is employed in this study. Two case study areas i.e. Otog gas field in the west and Yulin-Zizhou gas field in the eastern Ordos basin are chosen to anatomizing and comparing study. The main technical data include drilling, wire-line logging, gas test and dynamic data based on more than1000boreholes, as well as core description, water geochemistry analysis, physical parameters examination for rock and so on. Macro and micro distribution characteristics of different type of fluids in natural gas reservoir, as well as the factors affecting the distribution of fluid are analyzed. As a result the forming mechanism of natural gas reservoir in Upper Paleozoic in the Ordos basin is discussed.It’s concluded that the fluids distribution in the Upper Paleozoic reservoirs show obvious heterogeneity in macro aspect. For the lower accumulation association, the gas-producing area is mainly continuously distributed in the eastern Yishan slope, while the major water-producing areas are located in the Yimeng uplift and to the southwest of the main gas-producing area. For the middle accumulation association, the main gas-producing area is located in the western Yishan slope, while the major water-producing areas are located in the Yimeng uplift and to the west of the main gas-producing area. The main gas-producing area in the upper accumulation association is sparsely distributed in the western Yishan slope.It’s showed that the water production derived from the Upper Paleozoic in the Ordos basin mainly belongs to normal formation water. Combined the hydrogen and oxygen isotope examination and the rare bromine element analysis, it’s indicated that the formation water in the Otog Banner of the western Ordos basin, is derived from continental origination, which experienced intense water-rock interaction and evaporation-concretion processes. The characteristics of formation water chemistry reveal that the formation water in this area has good closed condition and is propitious to natural gas accumulation and conservation. Combined with the analysis of hydrocarbon accumulation elements, the formation water type in the Otog Banner area mainly belongs to residual formation water forming in the weak accumulation processes.Compared the differentiation of hosted sandstones for natural gas and formation water in aspects of sedimentary facies, diagenesis facies, strata pressure and micro pore framework of reservoir, two conclusions are showed as follow. With respect to the second member of Shanxi Formation in Zizhou area, eastern Ordos basin, natural gas mainly hosted in pore framework with medium size. The mean pore throat radius is0.51μm. A few reservoirs have large pore throat radius while bad sizing. The formation water is mainly hosted in the down-dip part of major channel sand bodies with mean pore throat radius of1.Oμm and well sizing. The type of formation water is mainly belonged to free water and served as edge-bottom water. With respect to the eighth member of Lower Shihezi Formatin, in Otog area, western Ordos basin, there has similarity between the gas-producing wells and water-producing wells in aspects of medium pore throat radius, porosity and permeability. In the gas-producing areas, the better physical properities are corresponded with higher gas saturation. The water-producing wells are usually found to produce gas and water together in which the capillary water is considered to be the major water-producing type.It’s indicated that the characteristics of vertical migration and accumulation are widely developed in various gas-bearing accociations. There is as high as5.99MPa overpressure difference between the source rock and reservoir in the second Member of Shanxi Formation in lower gas-bearing association in the east. There are strong filling power combined with better pore framework in this area. With respect of the Eighth Member of Lower Shihezi Formation in Otog area in the western Ordos basin, there is only4.76MPa overpressure difference between the source rock and reservoir. The relative lower filling power accompanied with worse pore framework in reservoir. Therefore the variety between filling power and reservoir physical properities result in the difference of fluid distribution between two case study areas.