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Geological Controls on Coalbed Methane Reservoir in East Bokaro Coalfield, Jharkhand, India

Abstract

Coalbed methane desorption and other reservoir parameters were evaluated for coal seams in East Bokaro coalfield comprising blocks such as Tenughat, Saram, Tulbul, Koiyotanr, Gomia and Kathara of Bokaro district, Jharkhand. The main objectives of the investigations were to decipher the in-situ gas content, sorption time (τ), geochemical properties, sorption capacity (VL), vitrinite reflectance (VRo%), molecular gas composition, petrographic constituents, etc. East Bokaro coalfield belongs to Peninsular Gondwana Basins in India occupies an area of about 237sq.km. between Latitudes 23°44′N and 23°49′N and Longitudes 85°42′E and 86°04′30″E. The coalfield has 24–27 coal seams worth for CBM exploration with thickness as high as 35m with cumulative thickness of 105–150m and occurring to a maximum depth of 1300m. Coal core samples were collected from exploratory boreholes in air tight canisters. In-situ gas content of coal core samples ranges from 1.7–22.9m3/t and decreases laterally from East to West of the coalfield. The molecular gas concentrations of C1, C2, C3, N2 and CO2 varies from 34.3–99.4, 0.1–30.0, 0.1–4.0, 0.2–65.7 and 0.4–58.8 respectively. It is observed that methane is the main component of desorbed gas (in average >74.4%). The observed δ13C1 (0/00) lie in the range of –70.5 to –22.4. The shallower samples (<600m) are lighter with δ13C1 values mostly in the range of –70.5 to –45.0, whereas deeper samples become heavier with depth and δ13C1 values vary from –42.6 to –22.4. The shallower samples are of mixed origin with substantial biogenic methane and relatively higher amount of N2. The sorption time (τ), moisture, ash, volatile matter and VRo% obtained as 0.08–10.8days, 0.5–7.4, 11.2–68.0, 16.2–38.7wt.% and 0.83–1.50 respectively, which indicates the coal seams are sufficiently matured. Sorption capacity (VL) varies from 14.8–24.6m3/t with Langmuir pressure (PL) as 2865–3716kPa. The sorption data compared with gas content reveals, near saturation (>65%) of coal seams. Liptinite has been found to constitute up to 12.1% by volume, hydrogen rich liptinite coals are capable of generating higher hydrocarbons at higher maturation level. This may be attributed to mixing with or dominance of biogenic methane generated during interaction of bacteria carried by recharging water in to the coal seams. Desorption and other properties of coal seams are highly encouraging for production and development of CBM reservoir in East Bokaro coalfield.