Factors affecting the development of the pressure differential in Upper Paleozoic gas reservoirs in the Sulige and Yulin areas of the Ordos Basin, China

Abstract

Abstract The Sulige gas field and the Yulin gas field are located in the north of the Ordos Basin. Reservoir pressure in the Sulige area is subnormal, whereas reservoirs in the Yulin area have normal hydrostatic pressure. This paper provides an explanation of this difference. The characteristics of reservoir sediment and formation water chemistry in the gas reservoirs of these two areas were analyzed. The results showed that both reservoirs have good sealing ability. Data of numerical simulation showed that tectonic uplift resulted in a great reduction in formation pressure in the northern Ordos Basin during the late Cretaceous period, and the decrease was greater in the Yulin gas field than in the Sulige gas field. However, because the total hydrocarbon-generating intensity of the Sulige area ( 32 × 108 m3/km2), the gas supply intensity of the two areas was different. Fluid inclusion data indicate three stages of hydrocarbon charge in the 2nd member of the Shanxi formation in the Yulin area, but only two in the 8th member of the Xiashihezi formation in the Sulige area. Consequently, although the formation pressure of the Yulin area decreased significantly during the late Cretaceous period, its relatively high initial formation pressure ensured that the present pressure of the 2nd member of the Shanxi formation in the Yulin area is higher than that of the 8th member of the Xiashihezi formation in the Sulige area.