ABSTRACT: Geochemistry and Origin of Oils in Ordovician Reservoirs of Ontario, Canada

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

The platformal carbonates of the Ordovician Trenton-Black River reservoirs host the most prolific oil fields in southwestern Ontario. Saturate fraction GC of oils occurring in these strata are characterized by smooth n-alkane envelope with low concentration of C20+ members, odd/even predominance in C13-C19 range, low abundance of pristane (Pr) and phytane (Ph) relative to n-alkanes and Pr/Ph ratios greater than 1.0. These compositional characteristics are known to be unique for oils derived from Ordovician sources worldwide, indicating the enigmatic microorganism Gloeocapsomorpha prisca as an important component of the source kerogen. Moreover, the biomarker signatures of the oils suggest a clastic source rock facies deposited under dysoxic conditions and normal water salinity, with significant primary bacterial input or microbial reworking. Geochemical thermal maturity parameters (degree of paraffinicity, isomerization of regular steranes, triaromatic steroids ratios) all indicate that the oils are mid-late mature.

Detailed organic petrological and geochemical analyses of rock samples from the corresponding stratigraphic interval (Trenton - Collingwood - Blue Mountain) demonstrate that the Ordovician rocks in southern Ontario not only contain sufficient oil-prone kerogen with good to excellent petroleum source potential, but are also thermally mature with respect to hydrocarbon generation. Although organic extracts from all analyzed Ordovician units correlate positively with oils from Ordovician reservoirs in southern Ontario, the organic-rich Trenton laminae show the closest geochemical characteristics and appear to be the principle source of these oils, a conclusion supported by their stratigraphic position (below oil reservoirs) and geographic location (proximity to oil pools).

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