Gas
Origin
in Coals of the Blackhawk Formation, Castlegate Coalbed Methane Field, Utah*
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Abstract
Coals of the
Blackhawk Formation locally contain a world class in-situ methane resource base
that is estimated to exceed 30 billion standard cubic feet of gas
per sq mile.
Following recent coring of the primary coal groups at the Castlegate CBM field,
detailed carbon isotopic investigations of desorbed gases with mixed composition
(C1, C2, C3, CO2) while strongly
indicating a thermogenic origin, suggest that not all
gas
within the coal is
internally sourced from humic kerogens. Rather, the isotopic signatures and
gas
composition indicate that the
gas
in place has a mixed origin from both internal
and external sources, the latter likely having been generated by type I & II
kerogens from the underlying Mancos Shales, and adsorbed during uplift.
Detailed study of both subsurface
and outcrop data indicate that the migration pathway for secondary thermogenic
enrichment is the likely combination of coal seam positioning relative to large
progradational parasequences of the Spring Canyon, Aberdeen, Kenilworth, and
Sunnyside members and vertically connecting Laramide natural fracture networks.
Collectively, the petroleum system of the Castlegate CBM field demonstrates the
role that can be played by basin
-
centered
source rocks in enriching low to
medium ranked coal groups.