Recognition and sources of secondary biogenic gases in the oil sand areas, Western Canada Sedimentary Basin

Abstract

Abstract Western Canada Sedimentary Basin oil sand deposits are derived from light oils generated in southwestern Alberta, which migrated to the north and east for more than 100 km. Biodegradation is the primary process that transformed the original light crude oil into heavy oil and bitumen, although other alteration mechanisms were present. Biodegradation levels increase from the southwest (non-biodegraded) to the northeast (extremely biodegraded) and are associated with decreasing reservoir paleo-temperature that plays the primary role in controlling the biodegradation regime. Compositional gradients and variable biodegradation within a single reservoir column indicate that water-leg size is a critical local control on vertical variations of biodegradation degree and oil physical properties. Secondary biogenic gas is a by-product of the formation of heavy oils and bitumens by anaerobic biodegradation, and “gas over bitumen” is a common feature of the bitumen accumulations. Observed 13C-depleted methane and 13C-enriched CO2 provides direct evidence for methanogenic biodegradation and biogenic methane generation. Supplemental evidence for anaerobic biodegradation is found in the geochemistry of associated formation water. The gases associated with anaerobic biodegradation are easily differentiated from primary biogenic gases using the isotopic signature of C2+ alkane components. Based on the observed crude oil biodegradation levels, the stoichiometry of methanogenic alkane biodegradation, and assuming a conversion rate of carbon dioxide to methane, approximately 141.3 × 1012 m3 (4991 Tcf) of secondary biogenic methane was generated accompanying the biodegradation of these petroleum accumulations. However, assessing how much secondary biogenic gas is preserved currently in the subsurface is difficult. Gas resource assessments performed prior to and independent of biodegradation studies suggests that McMurray Formation is expected to contain approximately 58.7 × 109 m3 natural gas in place and that all Mannville Group reservoirs associated with the oil sand regions (Athabasca and Lloydminster) are expected to contain approximately 608.5 × 109 m3 of raw gas in place. Even if the estimates of retained gas are conservative, it is apparent that the vast majority of the secondary biogenic methane generated during biodegradation is leaked into the overburden, dissolved in formation water, or escaped into the atmosphere.