Controlling factors of underpressure reservoirs in the Sulige gas field, Ordos Basin

Based on the study of low-pressure distribution characteristics, the main factors that control the formation of low-pressure reservoirs in the Sulige gas field are analyzed from several aspects, such as structural evolution, sedimentary characteristics and formation fluid features. It is suggested that the gas pool pressure is low and affected greatly by buried depth. Its roof and floor developed several layers of uncompacted mudstone, indicative of good property and pressure sealing conditions. The formation water also indicates favorable preservation conditions. The reservoir pressure of the Sulige gas field experienced 4 major evolutionary stages: the normal pressure stage of the Late Triassic-early Jurassic, the pressure rising stage of the Middle Jurassic-Late Jurassic, the sustainable pressure increasing stage of the Early Cretaceous, and the pressure decreasing stage of the late Early Cretaceous. The pore rebound and temperature decrease caused by intense tectonic uplift after the late period of Early Cretaceous, caused the reservoir pressure of the Sulige gas field to reduce by 0.673 MPa and 23.08% of the original strata pressure, respectively. The low pressure of the Sulige gas field is related to sedimentary assemblage, tectonic evolution and hydrocarbon accumulation.