Coalbed Methane Potential of Upper Cretaceous-Tertiary Strata, Alberta Plains

This study was undertaken as a joint project of the Alberta Scientific Research Authority (ASRA) and the Alberta Geological Survey (AGS) to evaluate the coal resources and potential coalbed methane resources within Upper Cretaceous-Tertiary strata of the Plains region of Alberta. The uppermost Cretaceous-Lower Tertiary stratigraphic interval in the Alberta Plains can be divided into seven discrete groups and formations: the Lea Park Formation, Belly River Group Bearpaw Formation, Horseshoe Canyon Formation, Whitemud and Battle formations, Scollard Formation and Paskapoo Formation. The Lea Park, Bearpaw and Whitemud Battle formations consist of relatively fine-grained siliciclastic sediments deposited in a marginal marine or lacustrine environment. The Belly River Group and the Horseshoe Canyon, Scollard and Paskapoo formations consist of interbedded sandstone, siltstone and mudstone of terrestrial origin.

Within the Uppermost Cretaceous-Lower Tertiary formations, seven coal zones were identified and correlated. The McKay Coal Zone lies in the lowest part of the Belly River Group. The Taber Coal Zone occurs in the upper part of the Belly River Group (Foremost Fm.), below the finer grained uppermost Belly River Group (Oldman Fm.). The Lethbridge Coal Zone is located in the uppermost Belly River, below the Bearpaw Formation in the southern Plains. The lower Horseshoe Canyon Formation contains the Drumheller Coal Zone. The upper Horseshoe Canyon contains the Daly-Weaver Coal Zone in its lower part and the Carbon-Thompson Coal Zone near the top. The Ardley Coal Zone, one of the most prospective coal zones in central and northern Alberta, occurs within the upper Scollard Formation. There are also some minor coals in the lower Paskapoo Formation.

Maps presenting coal-zone distribution and thickness, net coal and depth to top of each coal zone were generated from the AGS CBM-coal database (EUB/AGS 2002), an extensive database including more than 7500 oil and gas wells, 32 000 coal picks, 1500 vitrinite reflectance results and 16 000 stratigraphic picks compiled under the current ASRA-AGS joint project. Cross-sections showing basin geometry, stratigraphy and coal-zone distribution were constructed to verify coal zone correlations (not included in this report).

Gas potential within the basin is poorly understood, with less than 20 wells having publicly available gas-concentration data for the Upper Cretaceous-Tertiary strata in the Plains. Utilizing existing public-domain data, and data collected under the current ASRA-AGS collaborative project, gas-concentration and resource distribution maps and summations were generated for each of the seven coal zones within the study area.

Potential gas resources for the Upper Cretaceous-Tertiary strata of the Alberta Plains total 186 trillion cubic feet (Tcf), (5.27*10¹² m3). This number includes all potential coal-gas resources contained within the seven coal zones evaluated, regardless of depth and net coal thickness. Shallow coals generally have lower gas potential than deeper coals, although biogenic methane may be a contributor to shallow coal-gas resources. Furthermore, net coal thicknesses of less than 1 m are unlikely candidates for production completions. Considering these two factors, resources were also calculated by omitting coals less than 1 m in net thickness and less than 200 m in depth to provide a 'constrained' gas-resource potential of 147 Tcf. (4.16*10¹² m3). No data are currently available to evaluate gas potential of interbedded clastic rocks associated with the coal zones.

The majority of the potential gas resources are held within the Horseshoe Canyon Formation (40%); however, only 22% of the potential gas resources occur within the Drumheller Coal Zone. The Ardley Coal Zone of the Scollard Formation contains 34% of the total potential gas resources, and the McKay Coal Zone of the Belly River Group contains 13% of the total potential gas resources. Maps of gas-resources potential indicate numerous areas of potential interest for coalbed methane (CBM) exploration within the Alberta Plains.