Abstract

The occurrence of coal bed gas (CBG) in four shallow wells drilled into the Cretaceous Mesaverde and Meeteetse Formations is empirically related to a geochemical signature interpreted as paleoweathering of the coal and the presence of a Paleogene stratigraphic seal above the coal. Weathering is indicated by a reduction in hydrocarbon content and an increase in the oxygen content of coals as measured by Rock-Eval analysis, and a tendency toward a reduced aromaticity as indicated by nuclear magnetic resonance (NMR) studies. Burial history reconstruction shows that CBG-bearing coals were exhumed to the near surface in the Paleogene.

Coalbeds from the CBM-2 (Hudson) well in the Cretaceous Mesaverde Formation from depths of 470 to 590 ft have a hydrogen index (HI) averaging 101 and 115 mg hydrocarbon (HC) / gC, and a desorbable CBG content averaging 1.8 ml / g. Coal cores from CBM-8 (Pilot Butte) in Mesaverde strata at depths of 402-829 ft have a HI averaging 163 mg HC / gC and a desorbable CBG content averaging 2.2 ml / g. The coal cores barren of CBG from the wells at CBM-1 (Alkali Butte) in the Mesaverde and CBM-4 (Welton) in the Meeteetse have HI averaging 192 mgHC / gC at CBM-1 and 128 to 175 mg HC / gC at CBM-4. Coals from CBM-2 and CBM-8 have average oxygen indices (OI) ranging from 35 to 37 and 31 mg CO₂ / gC, respectively. CBM-1 and CBM-4 coals have an average OI of 32 and 14 to 20 mg CO₂ / gC, respectively. S₁ in the CBG-bearing coals averages less than 0.3 mg HC / g rock. Barren coals have an S₁ averaging from 1.5 mg HC / g rock in CBM-1 and 0.04 mg HC / g rock in CBM-4. The Rock-Eval results, although somewhat overlapping, indicate the CBG-bearing coals and the barren coals have different geochemical signatures. These changes in geochemistry are not related to differences in thermal maturity because vitrinite reflectance and Tmax data show that these coals have a uniform subbituminous rank.

¹³C cross polarization magic angle spinning NMR studies indicate that CBG-bearing wells, except for CBM-8, have an aromaticity of <0.6. This difference in aromaticity of the coals, which are of equal rank, seems attributable to the influence of the maceral composition and (or) their degree of weathering. Maceral proportions in these coals are roughly similar, so the degree of weathering is the favored explanation.

The tendency towards a reduced S₁ peak and HI along with an increased OI and a reduced aromaticity in the CBG-bearing coals suggests they are more weathered compared to the barren coals. CBG in CBM-2 occurs only 30 ft below a major Paleocene unconformity and in CBM-8 CBG first occurs 140 ft below a major Eocene unconformity. Coals in CBM-1 and CBM-4, first occurring at depths of 578 and 964 ft, respectively, are much farther below any Tertiary or present day unconformity.

Burial history reconstruction shows that the CBG-bearing coals in the shallow wells drilled around the southwestern basin edge are the result of moderate burial to reach post-Eocene peak temperatures of 70 to 80°C. This burial history suggests that an extended time period was available for biogenic gas generation, as well as for later thermogenic gas generation. The CBG apparently originated after Paleogene weathering and renewed burial diagenesis during post-early Eocene time.