25th International Meeting on Organic Geochemistry IMOG 2011

P-261
Natural gas generation, migration and accumulation in the
Jonah Field area, Green River Basin, Wyoming: implications
from chemical and isotopic compositions of gaseous
compounds in the Cretaceous Lance and Mesaverde formations
Qilin Xiao 1 , Paul Philp 1 , Nicholas Harris 2 , Jon Allen 1
1 School of Geology and Geophysics, University of Oklahoma, Norman, United States of America,
2 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada (corresponding
author:pphilp@ou.edu)
Jonah Field is located in Sublette County, Green
River Basin, Wyoming. Five Cretaceous source rocks
developed in this region, deposited in nonmarine,
marginal-marine and marine environments,
respectively [1]. Natural gas is produced mainly from
the Cretaceous Lance Fm, and partly from the deeper
Cretaceous Mesaverde Fm. The occurrence of
multiple gas sources in different sedimentary
environments and complex tectonic movements make
it difficult to provide an unambiguous conclusion
surrounding gas generation, migration and
accumulation in this area. Combined with basin
modelling results, chemical and isotopic compositions
of gaseous compounds in gas samples from the
Lance and Mesaverde Fms were utilized to identify
and clarify the generation, migration and
accumulation of natural gas in the Jonah Field area.
Methane is the principle component in the samples
from the Lance and Mesaverde Fms, resulting in
C1/C2+C3, C1/C1-C5 and CO2/C1-C5 ratios in the range
of 3 – 10, 80 – 90%, and 0 – 10%, respectively; δ 13 C
values are mainly -45.0‰ to -35.0‰ for methane, -
30.0‰ to -24.0‰ for ethane, -28.0‰ to -24.0‰ for
propane, -26.0‰ to -24.0‰ for butanes and
pentanes, and -15.0‰ to -12.0‰ for CO2,
respectively; δD values range from -220‰ to -160‰
for methane, -180‰ to -150‰ for ethane and
propane. Molecular and stable carbon and hydrogen
isotopic compositions of individual gaseous
compounds in samples from the Cretaceous Lance
and Mesaverde Fms suggests thermogenic gas
associated with oil and condensate.
From the shallow Lance to deep Mesaverde Fms,
the gases become progressively wetter and
isotopically lighter. Ratios from Lance are normally
>85% for C1/C1-C5,