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The AAPG/Datapages Combined Publications Database

CSPG Bulletin


Bulletin of Canadian Petroleum Geology
Vol. 32 (1984), No. 3. (September), Pages 313-326

The Geochemistry and Origin of Natural Gases in Southern Ontario

J.F. Barker, S.J. Pollock


Up to 1980, about 2.3 times.gif (834 bytes) 1010 m3 of natural gas was produced from about 15 stratigraphic units of Cambrian to Silurian age in southern Ontario. Reservoirs are found in both the Appalachian and Michigan Basin segments of southern Ontario. Lower Silurian sandstones yielded 54% and Middle and Upper Silurian reefs about 40% of production in 1980.

The natural gases are generally wet, with N2 being the dominant nonhydrocarbon component. Gases from the Appalachian and Michigan basins can be distinguished on the basis of a ratio (R) obtained by dividing the ethane/propane ratio by the ratio of isobutane to normal butane. An interpretation of the gas chemistry indicates either an immature or a very mature to overmature source. The neighbouring rocks are immature to barely mature.

Carbon and hydrogen isotopes generally indicate an overmature source, at least for the methane component of many of these natural gases. This is difficult to reconcile with the less mature nature of southern Ontario Paleozoic strata. A reinterpretation of carbon isotope ratios, emphasizing kinetic processes, reconciles the apparent overmature isotopic indicators by having gas generation from thermally immature but old sources, proximal to the gas pools. Alternatively, migration of methane from sources deeper in the basins or from underlying crustal rocks is suggested.



Les reservoirs de gaz naturel des bassins du Michigan et des Appalaches du Sud de l'Ontario ont produit 23 milliards de m3 de gaz jusqu'en 1980.

Les accumulations d'hydrocarbures sont reparties dans 15 unites lithologiques distinctes d'age cambrien a silurien.

Les gres du Silurien inferieur et superieur ont produit a eux seuls respectivement 54% et 40% de la production totale de l'annee 1980.

Le gaz naturel est generalement humide et contient de l'Azote. Le rapport (C1/C2)/(i-C4/n-C4) permet de distinguer entre les deux bassins d'origine (Michigan ou Appalaches).

D'apres la chimie de ces gaz, les roches-meres sont soit immatures soit supramatures (overmature). Les roches encaissantes sont immatures ou marginalement matures.

Les isotopes mesures sur le methane indiquent que la plupart de ces gaz proviennent de source supramature. La difference de maturite entre les roches encaissantes paleozoiques et la maturite avancee des roches-meres sont donc en apparente contradiction.

En tenant compte de la cinetique de la Previous HitmaturationTop des hydrocarbures il se pourrait que le gaz puisse avoir ete genere a partir de roches-meres immatures, mais tres anciennes, situees a proximite des accumulations. L'alternative serait que le gaz, en particulier le methane, provienne de roches-meres plus profondes ou meme du socle.

Traduit par F. Monnier

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