--> ABSTRACT: Geologic Controls on Subsurface Pore Pressure Distribution, by D. R. Converse, G. G. Gray, P. H. Nicholson, G. T. Cayley; #91020 (1995).

Datapages, Inc.Print this page

Geologic Controls on Subsurface Pore Pressure Distribution

D. R. Converse, G. G. Gray, P. H. Nicholson, G. T. Cayley

Prediction of subsurface pressures is critical to the successful drilling, exploration, and exploitation of hydrocarbons. A knowledge of fluid pressures is invaluable in predicting top seal performance, trap integrity, reservoir quality, and hydrocarbon migration. A global compilation of overpressured reservoirs indicates that commercial accumulations of hydrocarbons occur in many different basin settings. The optimum prediction method depends on the basin geologic framework and history.

In the Mesozoic reservoirs of the North Sea, standard wireline log methods do not adquately predict subsurface pressures. Accurate prediction requires the integration of geological, drilling, and engineering data to determine the limits of large-scale hydrologic domains, and small-scale pressure cells.The boundaries of the hydrologic domains and pressure cells can be either stratigraphic or structural as shown by a series of examples. Slow fluid transport between pressure cells is also documented - in these cases the boundaries have low but geologically significant permeabilities.

In a number of examples, the subsurface pressures are limited by rock failure criteria. Rock failure is shown by the presence of gas chimneys and hydrocarbon shows through the top seals. Failure may occur along a fracture network or along seismically observable faults. Rock failure may occur passively as a result of fluid pressure rise and hydrocarbon accumulation or with the addition of active deformation due, for example, to salt movement.

The interrelationship between subsurface pressures and temperatures is presented via a comparison of the Northern North Sea and Central North Sea Basins. Each basin has a different average temperature and pressure profile with depth, but the shift to higher pressures occurs at a similar temperature (ca. 120-130°C) in each basin.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995