The Gulf of St. Lawrence is the largest embayment on the Atlantic Coast of North America between Florida and the Labrador Sea. Surface geology, geophysical data, and subsurface well control define four structural units and extend them from the land areas across the gulf. Beds having possibilities for oil and gas production are present in early Paleozoic formations overlying Precambrian basement in the Anticosti basin and western Newfoundland fault belt; in middle Paleozoic beds overlying Taconic basement in the Gaspe fold belt; and in late Paleozoic formations overlying Acadian basement in the Maritimes basin. The southward continuation of this Acadian basement is exposed on various uplands in Nova Scotia, and also underlies the Sydney, Fundy, and other basins, and extend beneath the great Mesozoic and Tertiary sedimentary wedge of the Scotian basin of the continental shelf.

Production has not been obtained from wells drilled in the Anticosti basin--a relatively stable monocline. Many gas and oil shows have been found throughout Ordovician rocks; Silurian rocks may be productive in deeper parts of the basin.

The Gaspe fold belt is thrust over the south edge of the Anticosti basin and is overlapped by the north edge of the Maritimes basin. Many seeps and shows of oil, but only insignificant recovery, have come from structurally complex Devonian beds in the likely part of the fold belt. In places, the Silurian beds may be productive, too.

The allochthonous or klippe sequence of the western Newfoundland fault belt has been complexly thrust over the autochthonous sequence at the eastern margin of the Anticosti basin. Porosity and shows have been found in Ordovician formations in both structural sequences; very minor oil production has been obtained from shallow wells.

The Maritimes basin, a large complex intermontane basin of Middle Devonian to Permian sedimentary rocks, has one small oil and gas field producing from Mississippian sandstones. The presence, at the southeast, of a thick sedimentary section, sizeable structures, porous sandstones and carbonate rocks, many shows and seeps, oil shales, and albertite encourage exploration, especially in the Mississippian Horton and Windsor Groups and the Pennsylvanian "coarse-fluvial facies."

Although estimated values for oil and gas in place in these four structural units may be quite large, the Paleozoic age of the sedimentary section, the low-grade metamorphism in some parts of the area, the generally poor porosity at depth, and the large area under water where operations are very expensive all reduce the commercial potential.