Oil-bearing CO 2–CH 4–H 2O fluid inclusions: oil survival since the Palaeoproterozoic after high temperature entrapment

Abstract

The 2.45-Ga fluvial quartz arenites and conglomerates of the Matinenda Formation at Elliot Lake, Canada, contain evidence for two episodes of oil migration with entrapment of oil-bearing fluid inclusions. The first episode was during diagenesis, the second during a subsequent sub-greenschist facies metamorphic event at ca. 2.2 Ga at temperatures exceeding 250 °C. Remnants of the migrating hydrocarbon fluids are preserved as radiogenic pyrobitumen nodules occupying former inter-granular pore spaces and as complex fluorescing oil-bearing fluid inclusions. The latter include two dominant types: (1) aqueous-oil inclusions with a minor oil phase, located within original detrital grains and clasts and within rare quartz overgrowths, and (2) aqueous–carbonic inclusions with a minor oil phase, light hydrocarbons and solids, located within microfractures in detrital quartz grains and clasts. The majority of Type 1 inclusions were trapped with a high salinity fluid (up to 25 wt.% NaCl equivalent) at relatively low temperatures between 80 and 200 °C and pressures of less than 2 kbar early in the burial history of the Matinenda Formation. Type 2 inclusions were trapped during metamorphism with a much lower salinity aqueous–carbonic fluid (1.2 to 17 wt.% NaCl equivalent) at temperatures between 280 and 350 °C and pressures between 1 and 1.5 kbar. Under these conditions oil may have been miscible in the mixed fluid. δ 13C of −25.5‰ and −24.9‰ for the pyrobitumen nodules and fluid inclusion hydrocarbon gas indicate a biogenic origin for the petroleum. δ 13C of −16.2‰ for the bulk inclusion gas suggests a value of −12‰ for the carbonic phase, hence a mix of inorganic CO 2 and organic CO 2 from either thermal alteration of organic matter or in situ chemical oxidation of hydrocarbons. This study has implications for the depth and temperature at which oil may be present, suggesting that deep, old reservoirs may not be completely barren of petroleum. The fact that hydrocarbons in the Matinenda Formation fluoresce indicates that aromatic compounds are able to survive temperatures of up to 350 °C in a suitable geological environment.