Origin and Character of Biogenic Coal Gas in North Central Louisiana

Peter D. Warwick¹, Paul C. Hackley¹, and F. Clayton Breland^2
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 δ¹³C_CH4 values that range from -60.73 to -65.57 ‰ PDB, and δD_CH4 values that range from -196 to -186.7 ‰ SMOW. Samples of produced saline water have δD_H2O values of -27.3 to -18 ‰ SMOW, δ¹⁸O_H2O values of -4.38 to -0.54 ‰ SMOW, and δ¹³ C_{Dissolved Inorganic Carbon} values of 4.87 to 12.64 ‰ PDB.