Analysis on mechanism of abnormal low pressure in Sulige gas field, Ordos Basin

The effects of temperature decrease and gas dissipation on pore pressure was studied by using the SRK state equation. It was found that temperature decrease and gas dissipation were main mechanism of abnormal low formation pressure in the Sulige gas field, which resulting in the decrease of formation pressure and pressure coefficient. The temperature decrease of 60°C. results in formation pressure decrease by about 22% while gas dissipation about 17%~24%, resulting in formation pressure decrease by about 23%~32% after Early Cretaceous (100 Ma). It showed that the higher paleotemperature of the reservoirs the greater pressure decrease due to gas dissipation and the more gas diffusion of the gas field the smaller pressure drop causing by temperature decrease. The burial depth of water table and density difference between gas and water has so little contribution to the abnormal low formation pressure that they were not the mechanism of abnormal low formation pressure in Sulige gas field. The pressure evolution history of main reservoir was resumed using coupled fluid flow compaction model, combined with the main mechanism of abnormal formation pressure, constrained in measured formation pressure and paleopressure determined by means of fluid inclusion. The evolution of formation pressure can be divided into three stages: 1) normal pressure before 200 Ma; 2) pressure increase and accumulation of abnormal high formation pressure from 200 Ma to 100 Ma. The formation pressure and pressure coefficient gradually increased and reached maximum at 100 Ma; 3) pressure release after 100 Ma. Temperature decrease and gas dissipation caused by tectonic uplift and denudation resulted in formation pressure decrease and finally evolved to the current state of abnormal low formation pressure. With the characteristics of low permeability and low porosity, the development of abnormal high formation pressure has great significance to the hydrocarbon migration and accumulation in this area. The decrease of formation pressure at later period was beneficial to gas preservation.