Thermal maturity and petroleum potential of the Paleozoic strata in southwestern Ontario

The Ordovician Sherman Fall, Lindsay (Collingwood Member) and Blue Mountain formations, Silurian Guelph Formation (Eramosa Member), and Devonian Marcellus and Kettle Point formations of southwestern Ontario were examined using Rock-Eval pyrolysis, gas chromatography, gas chromatography-mass spectrometry and incident-light microscopy in order to evaluate their thermal maturity and hydrocarbon generating potential. All units contain sufficient amount of oil-prone (Type II), predominantly marine organic matter to be considered as potential source rocks. Unstructured bituminite with varying proportions of unicellular alginite (Tasmanites and Leiosphaeridia telalganite) are the dominant dispersed organic matter macerals. Accumulation of the organic matter occurred in low-energy, marine environments grading from shallow shelf into deep shelf and open basinal setting. High plankton productivity and progressing anoxia, controlled by a subtropical location, diminished water circulation, stratification of the water column and a depressed pycnocline resulted in high preservation rates.; Optical and geochemical parameters indicate that the Sherman Fail and Blue Mountain strata are within the zone of prolific oil generation throughout the whole area of study. The Collingwood shales are mature with respect to hydrocarbon generation in the Toronto area, while in the Georgian Bay area these shales are only marginally mature. The Eramosa and Marcellus strata have reached the early stages of hydrocarbon generation and expulsion whereas the Kettle Point Formation is thermally immature.; Three geochemically and stratigraphically distinct oil families are found in southwestern Ontario: the Cambro-Ordovician, Silurian and Devonian families. The Cambro-Ordovician and Devonian families share many compositional affinities in distributions and concentrations of normal alkanes, acyclic isoprenoids, terpanes and steranes. However, C{dollar}sb{lcub}30{rcub}{dollar} 4-desmethyl steranes, present in the Devonian oils, are absent in the Ordovician oils. The geochemical character of the Silurian oil family is distinctly different compared to the other families. Isomerizadon of C{dollar}sb{lcub}29{rcub}{dollar} steranes, and hence maturity, increases from Devonian through Silurian reaching equilibrium in Ordovician oils. The composition of the oils is typical for oils derived from marine organic matter, indicating a carbonate source rock and a strongly reducing environment for the Silurian oils, and a clastic source formed under dysoxic to anoxic conditions for the Cambro-Ordovician and Devonian oils.; All oils show a close geochemical relationship with the kerogen found in the corresponding stratigraphic intervals. Thin discontinuous organic-rich laminae within the Sherman Fall carbonates are recognized as the principal source of the Ordovician oils. Accordingly, the Upper Trenton interval is defined as a self-sourcing system with oils migrating over relatively short distances. Geochemical and geological evidence indicate a stratigraphically younger and less mature source for the Devonian oils. Since Devonian strata in the study area are marginally mature, oil migration from more mature Devonian units, located downdip in the Appalachian Basin appears to be the most plausible explanation. The Silurian Eramosa extracts show some geochemical relationship with the Silurian oils. However, a high variability of geochemical composition within the Silurian oil family indicates short-range migration and localized sources, a conclusion supported by geological evidence. Therefore Silurian strata comprising organic facies similar to those within the Eramosa dolostone, and located in proximity to Silurian reservoirs, are likely sources of these oils.