AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Gas Geochemistry in Tight-Gas-Sand Reservoirs of the Rocky Mountains; Results of a Multivariate Field Study
(1) Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.
(2) ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, OK.
(3) University of Manchester, Manchester, United Kingdom.
Gas compositions in Upper Cretaceous tight-gas-sand reservoirs of the Rocky Mountains vary significantly both in an areal and a stratigraphic sense. We have investigated gas geochemistry at three fields - Jonah Field, Green River Basin; the Mamm Creek-Rulison-Parachute-Grand Valley complex in the Piceance Basin; Greater Natural Buttes, Uinta Basin, using a combination of mud gas and production well data. These three fields display significant internal variability in bulk hydrocarbon gas, isotopic and noble gas composition. We interpret these data in the context of source rock, maturation and migration models.
Jonah appears to be the simplest system. Gases in the Lance and Mesaverde reservoirs become systematically wetter with depth, and the carbon isotopic composition of each gas becomes more negative. Gases from the transition between overpressured Lance reservoirs and adjacent normally pressured Lance appear to be wetter than gases at similar depth within the Jonah structure. Compositions are consistent with gas associated with condensate and gases derived from primary or secondary cracking of oil than gases derived from coally sources. Closely spaced (~2-3 kms) wells commonly produce methane of very different isotopic composition, as much as 10-12‰ different, suggesting that these wells produce from very different parts of the section. Noble gas data on a number of cross-plots are tightly clustered, indicating relative uniform gas sources and little fractionation.
In contrast, gases from Mesaverde reservoirs in the Piceance Basin show a wide variety of compositions and no obvious trend with depth. Wetness generally ranges from 0.00 to 15% C2+ fraction, and isotopic compositions for methane mostly fall in the range of -38 to -40‰. Cross-plots suggest that the fraction of gas derived from coally sources is relatively small and the fraction of gas derived from marine sources such as the Mancos is relatively large, somewhat surprising given the abundance of coal in the Mesaverde section. Noble gas compositions show a much wider range, suggesting mixing of different gas sources or fractionation during migration and indicating that the Piceance Basin petroleum system has a much more complex generation and migration history than does Jonah Field.
Basin and migration modeling will be applied to test gas fractionation during generation and migration.