Abstract

Biomarker analyses were conducted on a suite of oil and rock samples from the Middle Silurian Guelph-Salina interval in southern Ontario. The oils occurring in the Guelph reef reservoirs have a distinct biomarker composition (e.g. high concentrations of acyclic isoprenoids with phytane greater than pristane, abundant gammacerane, prominent C34 and C35 homohopanes) which not only indicates a single oil family, but also suggests a carbonate source rock deposited in a hypersaline, strongly reducing environment.

Analyses of the extracts of rock samples from brown, laminated dolostone lithofacies occurring in the stratigraphically adjacent Guelph-Salina strata show that they have compositional similarities to the majority of the reef-hosted oils. These rocks contain significant amounts of marine Type II kerogen (TOC values ranging from 0.5 to 3.5%) and have good source potential. Though most of the organic matter is amorphous in nature, a structured, identifiable algal component is also common. A broad distribution of n-paraffins, with a relatively high concentration of C21+ members and abundant acyclic isoprenoids are typical in the organic extracts, suggesting a contribution from microbial organisms. A strong predominance of phytane over pristane in the Salina extracts suggests highly anoxic bottom waters and/or hypersaline conditions. Reducing conditions are further suggested by a broad C31-C35 extended hopane profile with prominent C34 homohopane and a high concentration of gammacerane. The overall geochemical similarities among the examined rock samples indicate periods of recurrence of back-reef lagoonal and inter-biohermal paleodepositional conditions during the deposition of both the Guelph Platform and Salina Group carbonates.

Geochemical correlation of the extracts and oils indicates that they are genetically related. Although geochemical maturity parameters suggest that the kerogen is only marginally mature, the presence of bitumen and hydrocarbon fluid inclusions in the Guelph samples suggest that early generation of hydrocarbons has occurred in this stratigraphic interval. The Salina extracts occurring in close proximity to reefs show a closer resemblance to the oils (Pr/Ph<l.0, C35 prominence, higher gammacerane) indicating that the organic-rich laminated dolostones of the Salina A-1 Formation are the most likely sources of the majority of the reef-hosted oils. Moreover, some oils share compositional similarities with those found in Trenton reservoirs, documenting that the Ordovician system in Ontario is not fully closed.

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