Print this pageGeochemical and Stable Isotope Analysis of the Wilcox Group Coal-Bed Gas and Related Petroleum Systems of Northern Louisiana, U.S.A.
Peter D. Warwick1 and F. Clayton Breland2
1 U.S. Geological Survey, Reston, VA
2 Basin Research Institute, Baton Rouge, LA
Samples of gas, oil, and water were collected from shallow (<1,051 m)
Wilcox Group (Paleocene-Eocene) coal and sandstone reservoirs and from under-
and overlying Late Cretaceous and Eocene carbonate and sandstone reservoirs in
northern Louisiana. Isotopic data from seven Wilcox coal-gas samples
indicate that the coal gases are primarily generated by bacterial reduction of
CO2. The gas samples have average d13CCH4 values of -62.05‰ PDB, and
average dDCH4 values of -187.9‰ SMOW.
Eight gas samples collected from conventional gas and oil wells suggest that
both biogenic and thermogenic gases are present in and adjacent to the
stratigraphic intervals that contain biogenic coal gases. The mixed gases
have d13CCH4 values that range
from -59.4 to -32.72‰ PDB, and dDCH4 values
that range from -180.8 to -131.9‰ SMOW.
Two Wilcox oil samples produced from sandstones within the coal-bearing
interval have geochemical characteristics comparable to published Wilcox oil
data of the region including a pristane/phytane ratio of about 2, and similar
d13C values for the saturate and aromatic
fractions. The oils are probably sourced from thermally mature,
down-structural-dip parts of the Wilcox Group. Gas chromatograms of the
C10+ saturated hydrocarbons show a depletion of n-alkanes that
probably indicate groundwater washing of the oil in these shallow reservoirs.
Isotopic data from four samples of produced saline water
collected from oil, conventional gas, and coal-bed gas wells have dDH2O values that range from -27.3 to -15.9‰ SMOW,
and when compared to dDCH4 values from the
same wells, suggest that CO2 reduction may be an important gas
generation process in this system.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005