Wide-Spread Underpressures in the Cretaceous Formations of West-Central Alberta, Canada

Benjamin J. Rostron, Jozsef Toth

Underpressures, termed also as subhydrostatic, subnormal, or abnormally-low formation pressures, are fluid pressures in the voids of geological formations that area less than hydrostatic for their depth of occurrence. Despite their common presence in the world's geologic basins, underpressured formations have received relatively little attention and many fundamental questions remain poorly understood or controversial. Such unanswered questions include the mechanics of the origin and maintenance of underpressures, geographical and geological distribution, and most importantly, their possible effects on the location of hydrocarbon accumulations. The Western Canada Sedimentary Basin presents an ideal opportunity for the study of the above questions because of the areally and stratigraphically wide-spread presence of strong underpressures, the abundant hydrocarbon fields, and the large publicly available database. In order to improve the understanding of the cause, distribution, and relations between underpressuring and hydrocarbon accumulations, a petroleum hydrogeological study was conducted on an 18,000 km² area in west-central Alberta.

Underpressuring is wide-spread throughout most of the Cretaceous strata in the study area, in particular in the aquifers of the Mannville Group, Viking Formation, and Belly River Group. Subnormal pressures appear to occur on two scales. First, a regional scale underpressuring, with fluid potentials declining, and thus suggesting fluid flow, in a direction of increasing elevation of the overlying topographic surface. Second, three-dimensionally closed potentiometric depressions of extremely low formation pressures covering areas in excess of several hundred square kilometres. In both cases, pressure-to-depth ratios as low as 4.3 kPa/m (0.19 psi/ft) and hydraulic heads of 116 metres (as compared with theoretically normal values for the area of 10.1 kPa/m and 700 to 900 metres, respectiv ly) can be found.

Local-scale positions of hydrocarbon accumulations and areas of closed potentiometric depressions do not seem to be correlated. This lack of correlation combined with the magnitude of the pressure anomalies is suggestive of a flow pattern today which is different from that prevailing at the time of hydrocarbon emplacement and, in addition, is still in a state of transience. Based on various considerations, the probable cause for the disequilibrium appears to be erosional rebound-induced pore dilation.

The observed underpressures, interpreted to represent disequilibrium flow conditions, point to the importance of understanding the paleohydrogeology of the basin to improve prediction of petroleum accumulation sites. Further work on the permeability and rock-mechanical properties of shales and erosional and ice-loading history of the basin is needed to improve our understanding of underpressures and their relations to hydrocarbon accumulations in this, and other basins.

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