Generation and Migration of Petroleum from Abnormally Pressured Fluid Compartments

ABSTRACT Much of the world’s oil and gas has been generated from source rocks inside deep ( 3,000 m or 9,840 ft) seal-bounded fluid compartments. The quantity and composition of the kerogen and the burial history of the source rocks determine the volumes of petroleum generated; however, the migration from the compartments in an oil and gas phase is a pressure-driven process in which the flow direction is controlled by the configuration and internal pressures of the fluid compartments. Many sedimentary basins contain layers 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 usually consist of a series of individual fluid compartments that are not in hydraulic pressure communication with each other nor with the overlying hydrodynamic regime. Tops of fluid compartments in currently sinking basins do not always follow a specific stratigraphic horizon. They frequently have planar tops and subsurface temperatures ranging from 90 ° to 100 °C (194 ° to 212 °F). The tops in clastic sediments appear to be caused by carbonate mineralization along a thermocline. In the North Sea, the depth to the top of the deepest seal changes with the geothermal gradient. The seal is deeper where the gradient is lower. The generation of oil and gas within the compartments plus the thermal expansion of pore fluids eventually causes fracturing of the top compartment seal during periods of basin sinking. Hydrocarbons and other pore fluids then move vertically into the overlying lower pressured sediments and accumulate in the nearest structural and stratigraphic traps. Seal fracturing causes a pressure drop with compartment fluids rushing to the breakout point. The compartment then reseals and pressure builds to another breakout. This episodic process continues with resealing and breakout cycles probably occurring in intervals of thousands of years in rapidly sinking basins such as the United States Gulf Coast. This concept of episodic dewatering of deep-basin fluid 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 and the oil moves vertically into the normally pressured rocks above.