Anomalous Formation Pressure Patterns in Rocky Mountain Basins, Their Probable Causes and Relation to Hydrocarbon Accumulation

Fred F. Meissner

Anomalous pore fluid over- and under-pressures are present in parts of many Rocky Mountain sedimentary basins. Areas of anomalous pressures are associated with fluid systems that are dominated by either 1) groundwater or 2 hydrocarbons.

Most pressure anomalies in groundwater dominated systems are related to losses in mechanical potential energy produced by hydrodynamic flow between outcrop charge and discharge areas and to variations in surface topography between them. Hydrocarbon accumulations in these areas are characterized by tilted bottom-water contacts and constructive or destructive effects on capillary seal capacities.

Anomalous areas of both over- and under-pressure that are characterized by the presence of hydrocarbon-saturated reservoir fluid systems are present in the deeper parts of several basins. These pressure anomalies form "cells" around a "core" of mature source rocks. Over-pressures in this setting are created during active hydrocarbon generation by volume changes produced during the conversion of immature solid kerogen into potentially expellable fluid hydrocarbons and kerogen residue. The amount of over-pressure created is governed by Darcy's Law and depends mostly on generation/expulsion rates and effective permeabilities. Although hydrocarbon-phase over-pressures may be maintained by stratigraphically-controlled "capillary seals", the presence of generation over-pressure may also be time-transient. In the transient case, when active high-rate generation ceases, over-pressu e created by the process will diminish if the hydrocarbons are able to leak off and migrate away from their deep basin position. As leak-off occurs, formation water will be imbibed into formation porosity, and this leads to conditions of under-pressure in the region formerly characterized by over-pressure. Under-pressures will remain until all mechanically unstable hydrocarbons have left the synclinal deep-basin position. When this occurs, the dynamic pressure regime will return to normal. As long as the pressure cycle related to hydrocarbon generation and migration from the deep-basin position is in a state of either over- or under-pressure, economically viable accumulations of the "deep basin type" may exist in the anomalous pressure cell.

Examples of the types of fluid pressure anomalies describe above and their control on specific oil and gas accumulations in several typical Rocky Mountain basins will be presented.

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