ISOTOPIC GEOCHEMISTRY OF GASES FROM THE HORSE CANYON FORMATION COALS IN ALBERTA, CANADA

Commercial development of CBM began in Alberta, Canada in 2002. Production is predominantly from dry coals in the Late Cretaceous Horseshoe Canyon Formation. There are at least seven coal-bearing intervals with a total coal thickness of 30m ranging in depth from 200m to less than 1000m. Gas is generally produced from different intervals in the same wellbore, a process described as co-mingling. We have analyzed 21 CBM gases, half of which are from specific coal-bearing intervals. The carbon isotope geochemistry is complex. Methane values vary from -60.4 to -56.3, ethane -47.8 to -39.0, propane -36.7 to -28.5, n-butane -35.6 to -25.8 per mil (PDB). The variation in methane δ¹³C is surprisingly small compared to the much larger variations in ethane and propane. The methane values do not vary systematically with depth, whereas ethane and propane show strong correlations with depth, with the shallowest coals having the most negative ethane and propane δ¹³C. Considering only the methane values, these gases would be classified as biogenic. On the other hand when the C1-C4 gas components are plotted on a Chung natural gas plot (1/n vs delta;¹³Cn), some of the gases are consistent with having a thermogenic origin at low maturity accompanied by selective bioalteration of C3. Gases from the shallowest coals are least linear on the Chung plot. A much lower δ¹³C methane than is observed would be predicted from extrapolation of the C2+ trend. The shallower coal gases are isotopically distinct in having lower δ¹³C C2+ values but higher than expected methane δ¹³C values that are identical to those of the deeper coals. Two mechanisms might explain the observed isotopic results. One is that thermogenic and biogenic methane is homogenized throughout the formation, whereas C2+ components formed by thermogenesis are selectively retained in the coal. The other is that the coal sequence contains thermogenic gas that has formed in situ, with the addition/development of biogenic gas in the upper coals.