--> Abstract: Mapping the Distribution of Overpressure in the Beaufort-Mackenzie Basin, Northern Canada, by Dale R. Issler and Kezhen Hu; #90039 (2005)

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Mapping the Distribution of Overpressure in the Beaufort-Mackenzie Basin, Northern Canada

Dale R. Issler1 and Kezhen Hu2
1 Natural Resources Canada, Calgary, AB
2 Geological Survey of Canada, Calgary

The Beaufort-Mackenzie Basin, a dynamically-compacting, Late Cretaceous-Cenozoic post-rift basin comprised of folded and faulted deltaic complexes, is characterized by extensive overpressure within compositionally uniform sediments deposited under highly variable rates. Primary overpressure is linked to the basin stress history that, in turn, resulted in characteristic compaction fabrics that are detectable using well logs. Classic log signatures include an abrupt increase in sonic transit-time and abrupt decreases in bulk density and resistivity near the top of overpressure. Low density, organic-rich Cretaceous shales display similar log features but are normally pressured. Sonic, density and resistivity log data were compiled for shale intervals from 250 wells and used to map the depth to overpressure. Mud weight and pressure data were compiled for these wells and used to confirm the log-based interpretations.

Overpressure is associated with thick Tertiary successions and therefore is confined to the Richards, Ellice and Langley islands area and to offshore regions. Depth to overpressure varies by >3 km, cross-cutting rocks of Paleocene (onshore) to Pliocene (offshore) age. This complicated pattern probably results from multiple generations of overpressure associated with the deposition and deformation of the various deltaic complexes. In far offshore regions, the top of overpressure coincides with the base of the rapidly deposited Plio-Pleistocene Iperk sequence. Elsewhere, overpressure conforms to structural trends: deep overpressures coincide with normal faulting and shallow overpressures occur on structural highs. Although there is strong evidence for vertical fluid flow (pressure-charged shallow reservoirs), faults appear to act as horizontal barriers that compartmentalize the pressure distribution.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005