Capillary Seals as a Cause of Pressure Compartmentation in Sedimentary Basins

A new hypothesis that pressure compartmentation in basins can be caused by capillary seals is presented and discussed. Capillary seals form when a non-wetting fluid phase is generated within or introduced into grain-size layered sediments that are finegrained enough that gravitational separation of the phases does not occur. Experiments show that such seals can be formed in the laboratory. These seals have many important implications: The changes in basin permeability that allow fluid overpressuring may occur when non-wetting phases are generated in the basin (CO2 or hydrocarbon). Basin seals can be dynamic; they can move through the sediment column. Porosity profiles reflect such movement. Hydrocarbons and water may follow the trajectory of seal rupture and move along the same migration routes. The flow of both water and hydrocarbons may be directed to areas where the top of overpressure is covered by the least sediments. We have not proven the capillary seal hypothesis, but diverse evidence supports it and it has important implications.