ABSTRACT: Canadian Offshore Formation Pressure Geology: Geopressures and Hydrodynamics of the Grand Banks, Scotia Shelf and Beaufort-Mackenzie Basin

DAHLBERG, ERIC C., ECD Geological Specialists Ltd., Calgary, Alberta, Canada

The understanding of relationships among formation pressures, compaction history, present-day geology, and the abundance of, producibility of, and migration and accumulation of associated hydrocarbons is useful and important for reasons of safety and economics in offshore and frontier exploration regions not only of Canada but of the entire world.

Drill-stem tests, repeat formation tests, leak-off tests, and production tests (to some extent) yield measured pressure values, but only at isolated depths and over limited intervals in the section. These pressures are, since measured, "real". Logging tools respond to variations in formation attributes (porosity) that reflect anomalous pore pressures, but also to those that don't. Mud weight information and pressures estimated from it aids in differentiating "real" from "false" pressure anomalies.

Examination of these data from dozens of wells from Canadian offshore areas discloses that:

1. Geopressured zone boundaries don't predictably correspond with formation tops, lithology transitions, faults, or unconformities.
2. Not all compactional anomalies identified as depth trend breaks constitute abnormally pressured zones susceptible to blowout, etc. However, most geopressured zones verified by pressure measurements do in fact show up prominently as compactional anomalies!
3. Compactional anomalies interpreted from SoniTranTM plots for 75 Grand Banks wells occur commonly near 1500 m, 3000 m, and 4500 m well depths.
4. Pressure-depth relationships in the Beaufort-MacKenzie basin differ significantly from those of the Atlantic offshore regions in that the former display attributes of a possibly hydrodynamic, open geobasinal system favoring long-range fluid migration, large accumulations where hydro effects on entrapment are positive, and strong natural water drives. The latter appears conversely compartmentalized with natural barriers and limited regional flow with localized pressure cells.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)