Abnormal formation pressures in the West Siberian Mega-basin, Russia

The West Siberia Low Lands, a single gigantic basin-like depression filled by Mesozoic and Cenozoic sediments, is one of the most prolific petroleum provinces of the world. It is divided into two regions: Northern and Central-Southern. In the former the Mesozoic and Cenozoic sedimentary sequence ranges between 7 and 10 km thickness, whilst in the latter it is between 3 and 5 km. The present hydrogeological pressure regimes to be found in West Siberia (i.e. normal hydrostatic, abnormally high, abnormally low) could possibly reflect the different geodynamic evolution of basement mega-blocks. However, following a review of some Jurassic and Cretaceous reservoirs with abnormally high and low pressures, it is possible to observe that volume expansion of fluids stored in confined closed reservoirs seems the most likely cause of the abnormally high formation pressure. Abnormally low formation pressures are considered in some cases to be due to the cooling effect of permafrost, in others to the closeness of the reservoir to fractured and weathered basement.