--> --> Origin and Character of Biogenic Coal Gas in North Central Louisiana, by Peter D. Warwick, Paul C. Hackley, and F. Clayton Breland; #90052 (2006)

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Origin and Character of Biogenic Coal Gas in North Central Louisiana

Peter D. Warwick1, Paul C. Hackley1, and F. Clayton Breland2
1 U.S. Geological Survey, Reston, VA
2 Louisiana State University, Baton Rouge, LA

Wilcox Group (Paleocene-Eocene) coal-bed gas in north-central Louisiana is potentially a significant resource; however, additional investigation is needed to evaluate its economic potential. Gas-bearing Wilcox coal beds in the area are at depths from 450 to 1,525 m (1,500–5,000 ft); individual coal beds have a maximum thickness of about 6 m (20 ft). Measured coal-bed gas content is as much as 8 cm3/g (280 scf/t daf). Coals are lignitic at the surface and subbituminous A, or greater rank, at depths of approximately 760 m (2,500 ft). Comparison of desorbed gas content with CH4 isotherms indicates that shallower coal samples (<600 m or 2,000 ft) are CH4 under-saturated, containing about 40 % less gas than CH4 saturation at reservoir P-T. Deeper samples (>600 m or 2,000 ft) are slightly under-saturated or over-saturated. The distinction between shallower, gas-under-saturated coals and deeper, saturated coals may be the result of increased competition for adsorption space by water in the shallow, higher moisture-content coals.

Isotopic data obtained from coal-bed gas and produced water samples indicate that the gases are primarily produced by bacterial reduction of CO2 and, in some places, these gases may be mixed with migrated thermal gases. Gas samples have δ13CCH4 values that range from -60.73 to -65.57 ‰ PDB, and δDCH4 values that range from -196 to -186.7 ‰ SMOW. Samples of produced saline water have δDH2O values of -27.3 to -18 ‰ SMOW, δ18OH2O values of -4.38 to -0.54 ‰ SMOW, and δ13 CDissolved Inorganic Carbon values of 4.87 to 12.64 ‰ PDB.