Abstract

Overpressured basins are found throughout the world, in sediments young and old, petroliferous and not. Several groupings of basins are identified. Stratigraphically young basins with active, rapid deposition are characterised by increasing amounts of overpressure with increasing depth. Many examples exist including the Gulf of Mexico, many Tertiary basins in SE Asia, and the Niger Delta. Overpressure is also found in stratigraphically old sediments, which have undergone their major phase of burial some time ago, perhaps with later uplift. The overpressure tends to be restricted to mud-prone successions, for example in the Delaware Basin. A third group are stratigraphically and/or structurally complex, for example in overthrust rocks in Papua New Guinea and South America. In each case the magnitude of overpressure and its distribution (as seen from pressure transition zones on pressure vs. depth plots) relate directly to the mechanism creating the overpressure, the distribution of reservoirs and seals, and the timing of the overpressure relative to other basin processes. There is a high degree of variability which often masks the underlying causes and can make pressure prediction problematical.

Overpressure has a strong influence on the petroleum system. Trap integrity is related to pore pressure history and fracture pressure evolution during burial and uplift, although in many basins pore pressure history is poorly constrained. Reservoir quality can be shown to be controlled in part by overpressure. For example, in the Gulf of Thailand and the North Sea, the availability of quartz cement is related to effective stress at grain contacts, and the highly overpressured sandstones contain loss quartz cement. Timing of maturation of petroleum is also influenced by overpressure, for example in the South China Sea where retardation of maturation indices is found in overpressured source rocks. Finally, primary migration of petroleum from source rock to carrier bed is controlled by pressure. Regional overpressure of major reservoirs acts to retard expulsion, and may also influence migration direction.