Methane occurrences in the Canadian Shield

Abstract

Methane is ubiquitous and discharging freely from numerous exploration boreholes in the Canadian Shield. Methane concentrations in the free gas phase vary from a few % to over 80%. The gas occurs in association with He (up to 20%), Ar (up to 4%) and H 2 (up to 30%), as well as with N 2 (up to 80%) and minor amounts of noble gases and higher hydrocarbons. The δ 13C-values of the methane vary between ∼ −56 and ∼ −25‰ PDB, whereas deuterium contents range from δ D = −130‰ to values as low as −450‰ No equilibration with associated fluids (usually CaNaCl brines) or known carbon compounds has occurred. Chemical and isotopic analyses tentatively indicate an abiogenic origin for these gases. In each mining district the observed isotopic compositions are unique, and tightly grouped, suggesting local gas production under specific environmental conditions. A positive correlation between δ 13C and δD in each mining district is not readily explainable in terms of carbon sources and formation mechanisms, or in terms of migration and mixing of gases from more than one source. He isotope results typical of crustal He indicate that a mantle origin for methane is unlikely; but as yet there are no definitive indications of the role of secondary processes such as diffusion and bacterial oxidation in establishing the observed isotopic signatures. The occurrence of high levels of free hydrogen gas in certain mining districts indicates that hydrogen may be involved in kinetically controlled, inorganic processes of methane formation. Reactions involving light hydrogen typical of that generated in the Canadian Shield and in other ultramafic-mafic environments could explain the deuterium-depleted nature of the Canadian Shield methanes. However, no specific generating mechanism has yet been identified.