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Isotopic Evidence for Fault-Induced Gas Mixing in Sweet Spots of the Sukunka Gas Field, Western Canadian Foothills

Tilley, Barbara 1; Bhatnagar, Pradeep 2; McLellan, Scott 2; Quartero, Bob 2; Veilleux, Byron 2; Muehlenbachs, Karlis 1
1 Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.
2 Talisman-Energy Inc., Calgary, AB, Canada.

Extremely unusual carbon isotope compositions are found in gases from some of western Canada’s most prolific gas fields, located at the very western edge of the Western Canada Sedimentary Basin, in the foothills of northeast British Columbia. Isotopic data suggest that the sweet spots in these fields result from mixing of two sources of gas controlled by the tectonic setting. The mixing end members are difficult to identify but must include a deep source (Tmax>600) and an even deeper source. The deep source shows a carbon isotope fingerprint typical of an overmature gas (ratios of approximately -30 to -26 per mil for methane and -26 to -24 per mil for ethane) whereas the extra deep source has the unusual isotopically reversed ratios of <-35 per mil for methane and <-40 per mil for ethane. The origin of isotopically reversed gases is controversial but can involve oil-cracking, Fischer-Tropsch abiotic synthesis, or mixing of immature and mature gas.

The tectonic evolution of faulting and folding in the gas fields likely allows mixing of the conventional deep overmature gas and the extra deep gas that is isotopically reversed. The isotopically-reversed gas is believed to be from a very deep organic-rich metamorphosed source rock. The conduits for this extra deep gas may be 1) deeply rooted thrust faults associated with Laramide Orogenesis and/or 2) normal basement faults which are associated with nearby grabens. The greatest production may be related to structures that are linked with deeply rooted thrust faults and/or basement faults.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009