THE POTENTIAL FOR MAINTAINING FLUID OVERPRESSURES IN A GEOCHEMICALLY "OPEN" BASIN

Pore fluid pressures (overpressures) in excess of hydrostatic pressure, approximately 0.45 psi/ft or 10.2 kPa/m, occur in sedimentary basins worldwide and can influence a variety processes. The expulsion of hydrocarbons from an overpressured region is an example of this influence. There are many fundamental questions about overpressures that have not as yet been resolved. One of the most important is how overpressures are maintained over geologic time intervals (>10⁶ years). Many studies consider the source and the maintenance of fluid overpressurization as separate aspects of this problem. A common mechanism proposed to explain the maintenance of overpressures is a "seal" of low permeability strata that acts as a significant barrier to fluid movement over geologic time. This concept implies poor hydrologic communication between an overpressurized compartment and strata adjacent to the compartment. There is, however, geochemical and isotopic evidence that there is, or has been, hydrogeologic communication between the overpressurized and normally pressurized regions in the Gulf of Mexico basin. This study proposes a 'hydraulic' pressure seal that could maintain excess pore fluid pressures. The sealing mechanism proposed in this paper does not adhere to the traditional definition of a pressure seal because there is no requirement for a lack of hydrogeologic communication between an overpressured and hydrostatically pressured region. The model suggests that multiple fluid sources may be capable of producing a fluid pressure to depth relationship similar to that observed in almost any sedimentary basin in the world. Additional study is required to determine if the proposed 'hydraulic' pressure seal can produce a reasonable simulation of the fluid pressure patterns observed in a specific sedimentary basin. Any fluid expulsion mechanism, such as the conversion of smectite to illite may produce a 'hydraulic' pressure seal.