ABSTRACT: Highly Porous and Permeable Waulsortian-like Buildups of the Subsurface Pekisko Formation (West-Central Alberta)

KIRKBY, KENT C., University of Wisconsin-Madison, Madison, WI

Anomalously porous (to 25%) and permeable (to 10.2 darcys) carbonate build-ups occur along the Pekisko (Lower Carboniferous) shelf margin of west-central Alberta. These excellent reservoirs contradict the prevailing "tight" image of Lower Carboniferous mounds and provide an unusual opportunity to study porosity evolution in limestone Waulsortian-like mounds that have not undergone extensive weathering or near-surface alteration.The remarkable preservation of these build-ups provides a wealth of fine textural detail that is atypical of outcrop studies.

A combination of core, seismic, and dipmeter data can be used to construct a "composite" Pekisko build-up model. Preliminary core examination suggests that most porosity and permeability in these reservoirs are primary, with some secondary enhancement by fractures and dissolution. These reservoirs are exceptional since much of the porosity occurs in crinoid-rich strata that are highly susceptible to syntaxial calcite overgrowths. Possible contributing factors to porosity preservation include the mounds' deep-water origins, burial history, and substrate control of cementation by mineralogy or organic sheaths. An understanding of large- and small-scale controls on porosity and permeability in the Pekisko mounds is applicable to an array of limestone reservoirs (crinoidal, deep-water, et .) and is not limited to the temporal and spatial setting of these specific buildups.

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