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Analysis of Bioturbated Dolomudstones in the Mississippian Debolt Formation, Northwestern Alberta, Canada

Baniak, Greg *1; Amskold, Larry 2; Muehlenbachs, Karlis 1; Pemberton, George 1; Gingras, Murray 1
(1) Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.
(2) SRK Consulting (Canada) Inc., Vancouver, BC, Canada.

Sedimentological and ichnological analysis of the Mississippian (Visean) Debolt Formation suggests deposition in a low-gradient inner ramp carbonate setting. This assessment is arrived at through the recognition of inner ramp sub-environments such as sabkha, hypersaline lagoons, restricted subtidal lagoons, intertidal mud flats, and peloidal shoals. Monospecific ichnofossil assemblages, such as Chondrites and Planolites, present within the inner ramp suggest ecological stresses such as hypersalinity and anoxia. These physico-chemical stresses, in conjunction with sulfate reducing bacteria, appear to have mediated dolomitization within the burrows. Isotopic analysis revealed that the dolomite was enriched in δ18O and δ12C relative to average calcite values. Similar enrichment has been seen in laboratory and field-based studies where dolomite has been found in association with microbial reduction of sulfate. Appreciable amounts of organic carbon, an essential component required to help set off a series of reactions that involves sulfate reduction, are believed to have been sourced from the organisms and their burrows.

Due to dolomitization within the burrows, widespread development of diagenetic textural heterogeneities is observed. As a result, development of porosity and permeability within these bioturbated facies are a function of primary sedimentation, biogenic reworking, and early facies-selective dolomitization of the burrow fabric. Spot-permeametry confirms the presence of a dual porosity flow media, where the permeability difference between the burrows and matrix are less than two orders of magnitude. Because the matrix typically has permeabilities less than 1mD, significant reserves of natural gas can remain trapped within the low-permeability matrix. Micro-CT analysis suggests that high levels of interconnectivity occur between the ichnofossils due to a high initial burrow density. As a result, the interconnected burrow networks help increase the surface area of flow conduits within the matrix. Thus, natural gas may be produced from the matrix via localized permeability streaks provided by the burrows. Given these conditions, the term Ichnofossil Hosted Gas (IHG) is proposed for biogenic reservoirs, such as in the Debolt Formation, wherein significant volumes of natural gas are produced from the low-permeability matrix via burrow networks.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California