Southwestern Ontario lies between the Appalachian area and the Michigan basin. The area is thus marginal to two oil and gas provinces of the United States; marginal geographically and marginal in yield.

Early drilling (1860) disclosed notable oil in the Middle Devonian (Dundee), and records from these fields and later widely scattered drilling gave thousands of depths to the top of the Dundee from which relatively good contour maps were constructed on this near-surface Devonian formation.

In some parts of the area discussed productive Silurian strata (mostly gas-yielding) are more or less conformable with the Devonian strata, but over most of the area this does not hold, since here between the Silurian productive strata and the Devonian are salt deposits whose thicknesses are locally very variable. The result is that at places low structures on near-surface strata overlie high structures on Silurian formations, and any other combination between that and the normal may occur. In the Silurian, seven productive reefs, or buried hills, as they were first called, have been found. These reefs are small in area. One of 120 acres had 200 feet of pay and yielded close to 10 billion cubic feet of gas.

The result has been to emphasize the importance of isopach maps and sedimentation maps in outlining favorable areas.

Contour maps, isopach maps, and sedimentation maps illustrate the problems, and structure sections illustrate some anomalous structures resulting from salt thickness variations and picture the known reefs.

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