Evidence for bacterially generated hydrocarbon gas in Canadian shield and fennoscandian shield rocks

Abstract

Hydrocarbon-rich gases found in crystalline rocks on the Canadian and Fennoscandian shields are isotopically and compositionally similar, suggesting that such gases are a characteristic feature of Precambrian Shield rocks. Gases occure in association with saline groundwaters and brines in pressurized “pockets” formed by sealed fracture systems within the host rocks. When released by drilling activities, gas pressures as high as 5000 kPa have been recorded. Typical gas flow rates for individual boreholes range from 0.25 L/min to 4 L/min. The highest concentrations of CH 4 are found in the deepest levels of the boreholes associated with CaNaCl (and NaCaCl) brines. N 2 is the second major component of the gases and with CH 4 accounts for up to 80 to >90 vol%. Higher hydrocarbon (C 2 +) concentrations range from <1 to 10 vol.%, with C1/(C2 = C3) ratios from 10−1000. Isotopically the gases show a wide range of values overall ( σ 13 C = −57.5 to −41.1%; σ D = −245 to −470‰ ) but a relatively tight cluster of values within each sampling locality. The Enonkoski Mine methanes are unique with σ 13C values between −65.4 and −67.3‰ and σD values between −297 and −347‰. The shield gases are not readily reconcilable with conventional theories of methanogenesis. The range of C1/(C2 + C3) ratios for the shield gases is too low to be consistent with an entirely bacterial origin. In addition, σD CH 4 values are in general too depleted in the heavy isotope to be produced by thermogenic methanogenesis or by secondary alteration processes such as bacterial oxidation or migration. However, isotopic and compositional evidence indicates that bacterially derived gas can account for a significant component of the gas at all shield sites. Conventional bacterial gas accounts for 75–94 vol% of the occurrences at Enonkoski Mine in Finland. At each of the other shield sites, bacterial gas can account for up to 30–50 vol% of the total gas accumulation. This study and other recent evidence of active bacterial communities in deep hydrogeological environments emphasize the need for more comprehensive investigation of the role of microorganisms in the deep subsurface.