Geochemistry of Oils and Source Rocks in the Ontario Portion of the Appalachian Basin

Obermajer, Mark, Martin G. Fowler, and Lloyd R. Snowdon
Geological Survey of Canada, Calgary, AB

Organic geochemical data indicate that at least two of the several oil families occurring in Ontario show stratigraphic and genetic relationships with the Appalachian Basin. The first oil family occurs predominantly in the Middle Ordovician Trenton-Black River and, to a lesser extent, Cambrian strata, with only minor oil found in Middle Silurian strata. The second family occurs in Middle Devonian reservoirs. Both families have compositions typical for oils derived from marine organic matter. Although these families share some compositional similarities, especially in the distributions of n-alkanes, acyclic isoprenoids and terpanes, C₃₀ 4-desmethylsteranes have been detected only in oils from Devonian reservoirs, indicating a Devonian or younger source. Thermal maturity parameters (sterane isomerization, triaromatic steranes ratio and paraffin indices) indicate that Cambro-Ordovician oils are consistently more mature than Devonian oils.

Both families show good genetic correlation to sources occurring in close stratigraphic proximity to the oil reservoirs. The Trenton Group platformal carbonates often contain thin (up to few cm), organic-rich (up to 9%) shaly intercalations while the organic matter content of the lower part (15 m) of the Blue Mountain Formation falls within 0.7-2.7 % range. These units contain type II kerogen which was deposited under dysoxic conditions and normal marine water salinity, with significant primary bacterial input or microbial reworking. The Middle Devonian Marcellus Formation contains two thin (less than 1 m) but distinctively organic-rich (up to 11% TOC) intervals, the “upper and lower organic shales.” These shales were deposited in an algal-dominated environment and contain type II organic matter dominated by unicellular alginite with some amorphous kerogen.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004