Abstract

Underpressured formations are present in the southeast uplift of the Songliao basin, northeast China. At depths between 1540 and 3000 m, the Lower Cretaceous strata in the Shiwu depression of the basin are significantly underpressured at about 50-80% of hydrostatic pressure. Exploration for oil and gas indicates that the hydrocarbon distribution in the depression has unusual characteristics: gas pools occur in the central part of the depression, whereas oil pools occur at the marginal uplift; some gas reservoirs are located at a very shallow depth of 150-450 m. We investigate the mechanism of the abnormal pressures and the characteristics of the hydrocarbon distribution. Numerical modeling indicates that the basin was dominated by overpressure through much of its history and became underpressured only in recent geological time. The earlier overpressurization appears to be essential for the occurrence of the underpressure at the present. The origin of underpressure results primarily from uplift and erosion, as well as reduction of geothermal gradients. Our study also shows that evolution of formation pressure affects not only fluid flow but also hydrocarbon migration and accumulation and gas and oil distribution. The development of underpressure is, therefore, a major factor responsible for habitats of hydrocarbons in the study area.

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