--> Abstract: Are the Silurian Reef-Hosted Oils Locally Sourced in Ontario?, #90907 (2000)

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ABSTRACT: Are the Silurian Reef-Hosted Oils Locally Sourced in Ontario?

OBERMAJER, MARK, FOWLER, MARTIN G., SNOWDON, LLOYD R. and MACQUEEN, ROGER W., Geological Survey of Canada, Calgary, AB, Canada

The reefal carbonates of the Guelph Formation in southern Ontario have a long history of hydrocarbon exploration and production, with the first commercial oil production established at the end of 19th century. The majority of crude oils found in reef reservoirs have an aromatic character, intermediate density and elevated sulfur content, and as such are similar to other Silurian-reservoired oils from the central part of the Michigan Basin. Distinct geochemical composition of these oils (e.g. high concentrations of acyclic isoprenoids with phytane dominating over pristane, predominance of Tm over Ts, abundant gammacerane, prominent C34 and C35 homohopanes) not only indicates a single genetic oil family, but also suggests a carbonate source rock deposited in a hypersaline, strongly reducing environment.

Geochemical analyses of the organic extracts from brown laminated dolostone lithofacies occurring in the stratigraphically adjacent Guelph-Salina strata show that they have similar geochemical composition to majority of the reef-hosted oils. These rocks have good source potential but based on geochemical maturity indicators are only marginally mature. However, the presence of bitumen and hydrocarbon fluid inclusions in the Guelph samples suggest that early generation of hydrocarbons has occurred in this stratigraphic interval. Some heterogeneity in oil composition and the high concentrations of benzene and toluene in the gasoline range fraction of these oils suggest very limited secondary migration and sources occurring in close proximity to reservoirs. Also noteworthy is that some oils share compositional similarities with oils found in Trenton reservoirs, perhaps indicating that the Ordovician system in Ontario is not fully closed.

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Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada