Compositional variability of crude oils and source kerogen in the Siluriancarbonate-evaporite sequences of the eastern Michigan Basin, Ontario, Canada

Biomarker analyses were conducted on a suite of oil and rock samples from t he Middle Silurian Guelph-Salina interval in southern Ontario. The oils occ urring 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 C-34 and C-35 homohopanes) which not only indicates a single oil family, but also suggests a carbonate sourc e rock deposited in a hypersaline, strongly reducing environment. Analyses of the extracts of rock samples from brown, laminated dolostone li thofacies 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 kero gen (TOC values ranging from 0.5 to 3.5%) and have good source potential. T hough most of the organic matter is amorphous in nature, a structured, iden tifiable algal component is also common. A broad distribution of n-paraffin s, with a relatively high concentration of C-214 members and abundant acycl ic isoprenoids are typical in the organic extracts, suggesting a contributi on from microbial organisms. A strong predominance of phytane over pristane in the Salina extracts suggests highly anoxic bottom waters and/or hypersa line conditions. Reducing conditions are further suggested by a broad C-31- C-35 extended hopane profile with prominent C-34 homohopane and a high conc entration of gammacerane. The overall geochemical similarities among the ex amined rock samples indicate periods of recurrence of back-reef lagoonal an d inter-biohermal paleodepositional conditions during the deposition of bot h the Guelph Platform and Salina Group carbonates. Geochemical correlation of the extracts and oils indicates that they are ge netically related. Although geochemical maturity parameters suggest that th e kerogen is only marginally mature, the presence of bitumen and hydrocarbo n fluid inclusions in the Guelph samples suggest that early generation of h ydrocarbons has occurred in this stratigraphic interval. The Salina extract s occurring in close proximity to reefs. show a closer resemblance to the o ils (Pr/Ph<1.0, C-35 prominence, higher gammacerane) indicating that the or ganic-rich laminated dolostones of the: Salina A-1 Formation are the most l ikely sources of the majority of the reef-hosted oils. Moreover. some oils share compositional similarities with those found in Trenton reservoirs, do cumenting that the Ordovician system in Ontario is not fully closed.