Philip D. Heppard¹, Martin O. Traugott¹
(1) BP Amoco, Houston, TX

Abstract: Significance of Extreme Overpressure for Understanding Petroleum Systems

Extreme overpressure approaching minimum horizon stress has been observed in a number of basins around the world including parts of the Gulf of Mexico, the Eastern Venezuelan Basin, the Central Graben in the North Sea, the Nile Delta, and the South China Sea. While few wells penetrate very far into this difficult drilling environment it is reasonable to speculate that significant thicknesses of the rock section are extremely overpressured and often include the source rock section. We feel that in shale dominated basins extreme overpressure is a predictable consequence of the decrease of shale permeability and continued burial.

Within an extremely overpressured section the capacity of vertical seals to trap oil and gas is minimal and therefore is a zone of secondary migration. The buoyancy of an oil or gas column within the overpressured section tends to increase the pressure gradient at the shallowest point leading to the failure of the overlying shale seal. Seal failure is also driven by any deformation creating vertical relief of dipping beds and the redistribution of overpressure within reservoir rocks which also increases the pressure gradient at the shallowest point.

The extremely overpressured section has a predictable maximum porosity and therefore minimum velocity. This slow velocity may be overlooked in some cases leading to inaccurate depth conversions of seismic interpretations and a misinterpretation of the timing of petroleum generation and migration.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana