An Emerging Ellenberger and Trenton-Black River Exploration Play—Anticosti Island, Gulf of St. Lawrence, Québec, Canada

Claude Morin
Hydro-Québec Pétrole et Gaz, Ste-Foy, QC

Anticosti is a large, ESE-WNW oriented island (222 km by 55 km) in the Gulf of St. Lawrence that is situated along the Laurentia passive margin of Ordovician carbonates that extends from western Texas to Newfoundland. This margin hosts numerous, prolific petroleum reservoirs and Mississippi Valley-type Pb-Zn deposits.

Hydrothermal dolomites of the Trenton-Black River Formation and Ellenberger play-type represent the targets on Anticosti. More than 725 km of modern seismic was acquired by Shell (1998) and Hydro-Québec Pétrole et gaz (2003). A number of "sag" features at the top of Trenton-Black River platform have been identified on this data. These are associated with chaotic seismic character or complete loss of seismic signal within the Trenton-Black River interval, and coincide with zones of conductivity as identified on magneto-telluric data, low acoustic impedance (porosity) as identified by seismic inversion, and Class III AVO anomalies.

The top of the Romaine Formation, composed of mainly dolostones, is cut by an unconformity (the regional Sauk-Tippecanoe sequence boundary), that is associated with paleokarst features. Seismic data appear to indicate that these features formed through karstification, collapse, and the creation of breccia "chimneys". These coincide with disturbed and dimmed seismic signatures, structural sags, and circular shapes on time-structure maps.

The Chaloupe well, drilled in 1999, produced 1,650 barrels of salt water from the Romaine Formation (40 m of 11% Ø) when tested, and experienced losses of 270 m3 of drilling mud in the younger Trenton Formation during drilling (70 m of porosity (fractures) with good permeability).

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005