MICROBIAL ORIGIN OF WILCOX GROUP GAS IN NORTHERN LOUISIANA

New coal gas exploration and production in northern Louisiana (N-LA), Gulf of Mexico basin, is focused on the Wilcox Group (WG, Paleocene-Eocene), where the depth to the targeted subbituminous C to high-volatile C bituminous coal beds ranges from 980 to 5,500 ft, and individual coal beds have a maximum thickness of about 20 ft.  Measured gas content of the coal beds ranges from <12 scf/t (raw basis; or 38 scf/t dry, ash free basis, daf) at depths <1,300 ft, to >228 scf/t (280.7 scf/t daf) in deeper (>4,900 ft) parts of the basin.  LA State public data from 19 WG coal gas wells in N-LA indicate a production range from 7,000 to 229,000 ft³ of gas/day and 0 to 550 barrels of water/day.  Cumulative gas production (as of 11/06) from these wells is approximately 878 million ft³. 

To determine the origin of the WG coal gases in N-LA, samples of gas, water, and oil were collected from shallow (<5,000 ft) WG coal and sandstone reservoirs and from under- and overlying Late Cretaceous and Eocene carbonate and sandstone reservoirs.  Isotopic data from 21 WG coal-gas samples have average d¹³C_CH4 values of -62.2‰ VPDB and average dD_CH4 values of -197.4‰ VSMOW.  In seven samples, the value of d¹³C_CO2 ranges from -25.4 to -0.73‰ VPDB.  Produced WG saline water collected from oil, conventional gas, and coal-bed gas wells have dD_H2O values that range from -27.3 to -15.9‰ VSMOW.  These data suggest that the coal gases are primarily generated in saline formation water by bacterial reduction of CO₂. 

Molecular and isotopic analysis of ten gas samples collected from conventional gas and oil wells suggest that both biogenic and thermogenic gases are present in and adjacent to the WG intervals that contain biogenic coal gases.  Oil, probably sourced from thermally mature, down-structural-dip parts of the WG, is produced from sandstones within the coal-bearing interval.  Four gas chromatograms of the C₁₀₊ saturated hydrocarbons from WG oils show a depletion of n-alkanes probably resulting from biodegradation of the oil.  The isotopic composition of the gases associated with the oils is transitional between themogenic and biogenic origin (average d¹³C_CH4 = -44.4‰ VPDB, and average dD_CH4 = -182.4‰ VSMOW).  Natural gas produced from the WG interval appears to have a significant biogenic gas component.  WG reservoirs extend across the northern Gulf of Mexico basin suggesting that biogenic gases may constitute a significant gas resource.