Contribution of non-coal facies to the total gas-in-place in Mannville coal measures, Central Alberta

• Published in: International journal of coal geology 2016-01, Vol.154-155, p.69-81
• Main Authors: Bustin, A.M.M., Bustin, R.M.
• Format: Article
• Language: English
• Subjects: Coal; Coalbed methane; Desorption; Gas content; Isotherms; Logs; Methane; Mudstone; Reservoir facies; Reservoirs; Resource; Seams; Shale; Wells
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2015.12.002

The contribution of non-coal, organic rich, fine grained lithofacies to total gas-in-place and producible gas in the Upper Mannville Group coal measures, has been investigated in the producing and prospective coalbed methane fairway in Alberta. Petrophysical well logs, core, fluid, and gas analyses of organic rich rocks have been evaluated in order to quantify the total gas-in-place resource that may be accessed by a horizontal well(s) completed in the thickest coal seam. Within a ±20m stratigraphic interval of the completed seam, significant additional gas storage exists in coal seams too thin to warrant separate completions and in organic rich, fine grained facies. Such gas, although not currently attributed to producible reserves, can reasonably be expected to be produced. Three non-coal, organic reservoir facies are recognised based on core desorption and laboratory analysis and are identified on log, through mainly a combination of the bulk density and gamma ray logs. The reservoir facies are carbonaceous mudstone (1.5–2.0g/cm3), organic rich mudstone (2.0–2.2g/cm3), and organic mudstone (2.2g/cm3–2.4g/cm3). The gas storage in the mudstone reservoir facies, similar to the coal, is mainly in the adsorbed state with minor amounts in solution within the water/brine, and in the free state within the pores and fractures. The average gas content from canister desorption of carbonaceous mudstone is 147scf/t,111cm3/g (SI unit) ≈ 34scf/t (oil field units, 15.6°C and 101kPs). organic rich mudstone is 83scf/t, and organic mudstone is 54scf/t. Gas in solution ranges from about 5scf/t for the carbonaceous mudstones to a low of 0.5scf/t for the organic mudstone reservoir facies. The measured as received porosity of the mudstones ranges from 2.5 to 14%. The organic matter in all lithologies is dominated by particulate carbonaceous material derived from vascular plants (Type III kerogen/vitrinite). The gas capacity of non-coal facies, as determined by adsorption isotherms, and gas content as determined by desorption analysis of core, increase with thermal maturity (rank), and percentage organic matter. Values for the carbonaceous mudstone approach those of coal. Regionally, the gas capacity and content of the coals and non-coal reservoir facies increases from northeast to southwest in parallel with the increase in current depth of burial and level of organic maturation (coal rank), although local variations exist. The gas composition of the coal and non-coal reservoi