Biogenic Gases in the Western Canada Sedimentary Basin

Barbara Tilley and Karlis Muehlenbachs
Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada

Carbon isotope ratios of methane are frequently used to distinguish biogenic gases from thermogenic gases. Out of 1200 production gases from our gas carbon isotope database for the WCSB (δ13C methane -76 to -29 per mil), 430 fit the definition of biogenic gases (δ13C methane <-55 per mil), indicating a high proportion of biogenic gas in the WCSB. These include samples from the shallow biogenic gas system in southeast Alberta, CBM production in central Alberta, the Colorado Group, the Mannville Group heavy oil sands, and various pre-Cretaceous formations. We also have a database of 450 carbon isotope analyses of biogenic methane in domestic water wells.

C2-C4 gas components suggest an origin much more complicated than simply microbial. Chung natural gas plots (1/n vs δ13Cn) and mass balance mixing calculations have helped to unravel the gas history. The CBM is a mixture of in situ thermogenic gas and more pervasive biogenic gas. Some Colorado Group gas may be comprised solely of very low maturity thermogenic gas, not biogenic gas at all. The majority of the heavy-oil-associated Mannville Group gases originated as thermogenic gas that mixed with biogenic gas and has been biodegraded to varying degrees. A small proportion may be a recent low maturity thermogenic gas. Some domestic water wells contain purely microbial methane, whereas a significant number have a deeper thermogenic gas component.

Our analyses show that many gases in the WCSB that may be identified as purely biogenic based on their methane signature, are at least partially thermogenic and may have been altered by biodegradation. Our presentation illustrates that caution should be used in classifying natural gases.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas