Coal Characterization

The data on the coal seams presented in the previous section provide the means to evaluate the available coalbed methane resources in the Ardley Coal Zone of the study area. For the coal resource calculations, only coal and shaly coal intervals were used. The ash contents of coaly shale are too high for this rock to be a suitable thermal coal or CBM reservoir. The densities of the various lithologies can be estimated from the bulk density logs of wells in the study area. From this, a good density estimate for coal in the area is 1.4 g/cc and for shaly coal 1.6 g/cc. Tonnage per square kilometre can be calculated from these densities and coal thicknesses (including shaly coal).

Coal Quality

Coal quality has been reported from mining areas along the Ardley Coal Zone subcrop, and from shallow exploration drillholes (up to 250 m) immediately west of the subcrop (Nurkowsky, 1985; and Strobl et al., 1989). Petrography data are presented by Kalkreuth and Langenberg (2002). Coal quality and reflectance data suggest a coal rank of sub-bituminous B within the subcrop area, with a slight increase in rank to the boundary between sub-bituminous B and high volatile C bituminous coal. Ash content is low, typically 16%. Coal from the Highvale mine near Lake Wabamun often contains mineral matter filling coal fractures and cleat (carbonates and/or gypsum).

Several deeper drillholes were evaluated for coalbed methane potential of the Ardley Coal Zone northwest and west of the study area (Dawson et al., 2000). The Ardley Coal Zone was sampled approximately four ranges to the west of the study area, to depths in the range of 500 to 800 m. Here, the coal has a rank range from high volatile C to high volatile B bituminous, and an ash content average of 17%. Further northwest of the study area (Township 48, Range14W5M), the Ardley Coal Zone was intersected at a depth of 710 to 820 metres. Coal rank ranged from high volatile C to high volatile A bituminous, with average ash concentrations of 19%.

Rottenfusser et al. (1991) presented two vitrinite reflectance determinations from rock chips collected from standard petroleum wells of the Ardley Coal Zone, with values ranging from 0.50% to 0.55% Ro (random) vitrinite reflectance. These values indicate a rank of high volatile C bituminous, indicating the onset of thermogenic methane generation. Cleat characteristics are uncertain; however, Ardley zone coal from Township 56, Range 19W5M shows well developed cleat in bright coal lithotypes, as does the coal from the Wabamun Lake mine areas. The rank of the coal in the mine areas is slightly lower and is sub-bituminous, as indicated by 0.40% to 0.44% Ro (random) vitrinite reflectance (Kalkreuth and Langenberg, 2002).

Gas Content

Very few wells have been drilled to test reservoirs of the Ardley Coal Zone for gas production (Dawson et al., 2000). Eight wells have desorption test data from core samples. Only two wells have had any formation tests done. Both wells (AEC 1-34-45-14W5 and BHP 14-15-46-10W5) are located a bit further west than the study area of the present report. However, it is interesting to note that the 14-15-46-10W5 well, which shows reasonable permeabilities (4-7 md), is close to the present study area. Over the last few years, several CBM wells have been drilled in the Buck Lake area; unfortunately, no data on these wells have been released yet.

The available data can be used to obtain some estimates for gas content calculations. Contents of 4 cc/g for coal and 2.5 cc/g for shaly coal were used for gas in place (GIP) calculations. These gas contents are consistent with the available observed data (Dawson et al., 2000), and estimates based on random vitrinite reflectance of 0.5% to 0.55% (Eddy et al., 1982).

Total GIP equals the product of tonnage of coal and gas content per unit weight of coal. Because we are using density in g/cc units and gas content in cc/g units, the formula for the GIP calculation simplifies to the product of the volume of coal and a constant. Consequently, the formula we used is

GIP = Constant x Volume

Where: Constant = Density (in g/cc) x Gas content (in cc/g)

The constant is different for each type of coal and is calculated as the product of the density in g/cc and the gas content in cc/g. Coal in the area has a density of 1.4 g/cc and with a gas content of 4 cc/g, this constant is 5.6. For shaly coal (density of 1.6 g/cc and gas content of 2.5 cc/g), the constant is 4.

The map of Figure 17 (Gas content in million cubic metres per square kilometre) was obtained using this formula and shows gas contents of 52 million to 85 million cubic metres of methane per square kilometre. The map pattern is similar to the pattern shown by the total cumulative coal, because the volume of coal is largely dependent on the thickness of coal.

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