Characterization of Burrow-Mottled Dolomites in the Devonian Wabamun Group, Pine Creek Area, West-Central Alberta, Canada

Baniak, Greg ¹; Gingras, Murray ¹; Pemberton, George ¹
(1)Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.

The Devonian (Famennian) Wabamun Group provides the main producing reservoir interval for the Stettler Formation in the Pine Creek gas field of west-central Alberta (Township 56-58, Range 19-20W5M). In the studied intervals, the Wabamun Group is comprised of a series of highly bioturbated carbonate fabrics interpreted to represent a series of shallow water ramp deposits. These intervals provide an example of a reservoir within which the petrophysics are largely dictated by the distribution of trace fossils.

The detection of biogenically modified fabrics within the Wabamun Group suggests paleobiological controls on reservoir development. The utilization of microtomography (Micro-CT) was coupled with spot-permeametry experiments to provide a detailed characterization of the bioturbated media. Micro-CT modeling shows intricate ichnofabrics with complex interconnectivity between the burrows due to their geometry and density. Evidence of high levels of bioturbation is also common within these CT-scans. Through this, it can be concluded that bioturbation factors such as burrow geometry and density may have a greater influence on reservoir parameters such permeability, tortuosity, and dispersivity than originally believed. Spot-permeametry of these samples indicated that the burrows are preferentially higher in permeability relative to the matrix. Results imply that interaction between the flow paths within the burrows is extensive and complex. It can therefore be suggested that diagenetic textural heterogeneities within the Wabamun Group result in a dual permeability system (i.e. permeability differs more than two orders of magnitude between the burrows and matrix).

In conjunction with analysis of the petrophysics of the bioturbated units, the scanning electron microscope (SEM) and ion microprobe are also being used to provide a detailed characterization of the controls influencing dolomitization within the reservoir units. Analysis of SEM micrographs indicate that preferential dissolution and precipitation of planar dolomite crystals within and adjacent to burrow fabrics occurred. Preliminary ion microprobe analysis of the carbon and oxygen isotopes within these dolomite crystals suggests an enrichment of δ18O and δ13C relative to calcite values. Isotopic values thereby suggest that the sites of bioturbation influenced the pore-water geochemistry and in so doing dolomitization.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.