ISOTOPIC CONSTRAINTS ON THE MIGRATION OF HYDROCARBON AND HELIUM GASESOF SOUTHWESTERN ONTARIO

Natural gas samples from wells located in Ordovician and Cambrian stra ta of southwestern Ontario have been characterized isotopically and co mpositionally. Together with data from Silurian natural gas wells (Bar ker and Pollack, 1984), these results provide important constraints on gas formation and migration within the Lower Paleozoic strata of sout hwestern Ontario. Compositionally these gases are ''wet'', composed pr edominantly of CH4 (57-92% by volume), with significant concentrations of higher hydrocarbons (C2+ = 5-33%) and N2 (1-16%). Notably, gases i n the Ordovician (Black River Group) and Cambrian strata that form the contact rock with the Precambrian basement have very high He concentr ations (up to 1.2 % by volume) compared to concentrations of < 0.15% i n overlying strata. CH4 isotopic results indicate that the gases are t hermogenic in origin, with a significant bacterial component (10-45%) in several Silurian reservoirs. Isotopic and compositional thermal mat urity indicators support the conclusion that the gases originated in m ature to overmature source rocks. Such high maturation levels are inco nsistent with in situ gas production in the immature to marginally mat ure southwestern Ontario strata, and suggest that gases migrated into southwestern Ontario from more mature source rocks outside the region. The distribution of estimated gas formation temperatures (based on ca rbon isotope geothermometers) indicates a source in the Appalachian Ba sin to the southeast of the study area. Migration pathways over hundre ds of kilometres appear to be controlled by the Cambrian strata and th eir contact with the Algonquin Arch. He-3/He-4 ratios for gases from C ambrian wells range from 3 x 10(-8) to > 8x10(-8) (R/Ra = 0.02-0.06) - values typical of radiogenically produced crustal He. The highest He- 4 concentrations and He-3/He-4 ratios are consistently found in the Ca mbrian and Ordovician wells located close to the Precambrian basement. An observed correlation between He-4 concentrations and He-3/He-4 rat ios may reflect a mixing trend between in situ gas and a deeper end-me mber enriched in both He-4 and He-3/He-4. At present, it is not possib le to determine whether this end-member originates in the underlying P recambrian basement, or like the hydrocarbon component, originates in the Appalachian Basin and has migrated into southwestern Ontario follo wing pathways controlled by Cambrian strata and the Algonquin Arch.