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Biogenic Gases in the Western Canada Sedimentary Basin*

Barbara Tilley1 and Karlis Muehlenbachs1

Search and Discovery Article #80017 (2008)
Posted July 31, 2008

*Adapted from oral presentation at AAPG Annual Convention, San Antonio, TX, April 20-23, 2008

1Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada. ([email protected])

Abstract

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.

Selected Figures

 

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¨ Abstract
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¨ Conclusions
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Carbon isotope ratios of methane in gases of the Western Canada Sedimentary Basin. 

 

 

Location of the Western Canada Sedimentary Basin.

 

 

Plot showing that pure natural methane does not exist in the basin.  

 

 

Analyzed “microbial” gases in Cretaceous sediments in the Western Canada Sedimentary Basin, considering only production gases with δ13C methane < -55 and showing narrow range of methane values

 


Analyzed “microbial” gases in Cretaceous sediments in the Western Canada Sedimentary Basin, showing that in crossover zone carbon isotopes of methane cannot distinguish microbial from thermogenic methane.

Conclusions

·         δ13C methane is a poor discriminator of microbial versus thermogenic.
·         Need to look at δ13C of ethane, propane and butane.
·         For a suite of gas samples:

§  can often estimate relative abundance of microbial versus thermogenic methane.
§  δ13C of C2+ components can suggest whether the microbial gas generation was early or late.

·         Uncertainties

§  Must be very careful to assess for biodegradation of C2.
§  In crossover zone, carbon isotopes alone are not very useful.

References

Chung, C.J.F., and W.A. Spirito, 1990, Estimation of distribution parameters from data with observations below detection limit with an example from South Nahanni River area, District of Mackenzie: Geological Survey of Canada Paper #89-09, p. 233-242.

Galimov, E.M., 1967, Isotopic enrichment of methane during passage through rocks: Translated from the Russian, Geokhimiya, v. 12, p. 1504-1505.

Masters, J.A., 1984, Lower Cretaceous oil and gas in western Canada; in Elmworth-case study of a deep basin gas field: AAPG Memoir 38, p. 1-33.

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