PRESSURES AND HYDROGEOLOGY IN PETROLEUM BASINS

In most deep sedimentary basins in the world, there is a layered arrangement of at least two superimposed hydrogeological systems. The shallowest hydrogeological system can extend to great depths; however, in many basins it extends from the surface down to only about 10,000 ft greatest historical depth of burial. The shallow hydrogeological systems generally are basin-wide in extent and exhibit normal pressures. The pore water apparently is free to migrate approximately in accordance with the theory of fluid migration proposed by Munn (1909) and quantified by Hubbert (1940 and 1953). The deeper hydrogeological systems usually are not basin-wide in extent and exhibit abnormal pressures. They generally consist of a layer of individual fluid compartments which are sealed off from each other and from the overlying system. In some basins, mainly in the onshore U.S., there is an even deeper, near normally pressured noncompartmented section. The compartmented layer in those basins generally is in the sequence of rocks which were deposited during the period of most rapid deposition. The underlying noncompartmented layer, where present, usually is in prebasin shelf deposits and basement rock. The uppermost noncompartmented layer usually is in rocks which were deposited during the last 10,000 ft in basin filling. Correlations of petroleum in reservoirs with the rocks from which the petroleum was generated has imposed a system of mainly upwards paths for the first several thousand feet of migration of petroleum from source rocks largely within the compartmented layer to pools generally in the shallow hydrogeological system. The migration paths of petroleum and water may not be coincident. © 1990.