Generation and Migration of Petroleum from Abnormally Pressured Fluid Compartments

John M. Hunt

Much of the world's oil and gas has been generated from source rocks inside deep (> 3,000 m) pressure compartments. The quantity and composition of the kerogen and the burial history of the rocks in the compartments determine the volumes of petroleum generated. But migration in an oil and gas phase is a pressure-driven process in which flow direction is controlled by the configuration and internal pressures of the fluid compartment. Many sedimentary basins have a layered arrangement of two or more superimposed hydrogeological systems. The shallow systems are usually basin wide in extent and exhibit normal hydrostatic pressures. The deeper systems where the oil is generated are not basin wide and are abnormally overpressured. They generally consist of a series of indivi ual fluid compartments which are not in pressure communication with each other nor with the overlying hydrodynamic regime.

The generation of oil and gas within the compartments plus the thermal expansion of pore fluids eventually causes fracturing of the top compartment seal. Hydrocarbons and pore fluids move vertically into the overlying lower pressured sediments where they accumulate in conventional structural and stratigraphic traps. Seal breakout causes a pressure drop, but the compartment eventually reseals and pressure builds up to another breakout. The process appears to be episodic, comparable to the pulsed dewatering mechanism proposed for Mississippi Valley-type lead-zinc deposits, with resealing and breakout cycles measured in thousands of years.

Crude oil-source rock correlations have demonstrated that about half the world's petroleum has moved vertically from source to reservoir rock. About 75% of the petroleum reserves in the United States Gulf Coast are in reservoirs within or just above the seal. In the Central graben of the North Sea, the Kimmeridgian source rock is in a pressure compartment under a seal that is horizontal for over 200 km. Oil and gas have broken through the seal and accumulated in Cretaceous reservoirs above. The concept of episodic dewatering of deep-basin pressure compartments needs to be considered in any basin modeling program where the bulk of the oil generation occurs in the compartmented overpressured section of the basin.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.