Offshore exploration in the Great Lakes region has resulted in a small but growing industry on the Canadian side of the international boundary. Approximately 35 billion cubic feet of natural gas has so far been produced from beneath the waters of shallow Lake Erie, where offshore activity has been concentrated.

The five Great Lakes make one of the largest fresh-water drainage system in the world and it is of interest that the underlying oil and gas reserves should now be tapped. The Lakes are an important natural asset to the 71 million population living in the province of Ontario in Canada and in the eight states of the U.S.A.; all of which touch on their shores.

Lake Erie and the southern part of Lake Huron have particularly attractive geological possibilities; Lake Michigan and Lake Ontario have geological possibilities as well, with those areas onlapping the Precambrian Shield to a progressively less degree, approaching the Shield; Lake Superior is the deepest and is unattractive, lacking adequate Paleozoic representation. So far Lake Erie and Lake St. Clair have been the only areas made available for drilling; the other lakes are suitably shallow for drilling with present tower-equipment only along their perimeters.

Water depths, greater than the arbitrary 108-foot maximum which is suggested here for bottom-based towers currently in use, could be handled by more elaborate and expensive towers, by drilling barges, or by drilling vessels of the type used in Lake Erie in 1960. Two hundred ninety-three wells have been drilled offshore in Lake Erie and Lake St. Clair to date, with two of these in the United States opposite Pennsylvania.

Offshore development in Lake Erie has been concentrated in the eastern and western sectors. Clinton-Medina gas fields almost surround the east end on land, and underlake extensions of these are being sought. The geological problem has been to trace permeability trends following discovery; a better understanding of the conditions of deposition of these strandline sands is necessary. Modern fracturing techniques have been used successfully in this region.

In the western sector, Guelph gas-bearing reefs and Salina A-2 gas fields have been successfully extended underlake. However, the success ratio in wildcatting for these biostromic structures has been disappointingly low in spite of the use of the various geophysical tools. The geological problem has been in finding a suitable means of location.

The oil discovery in the Trenton Group at Colchester in Essex County, Ontario, near the west end of Lake Erie has resulted in a renewed interest in deeper exploration. It has also resulted in the first commercial oil well in the Great Lakes (in 1959). Subsurface geology must be used to locate areas of suitably porous dolomitized Trenton Limestone.

Some offshore wells have penetrated the Cambrian sands and these beds also have oil and gas possibilities, and are of current interest.

An attempt has been made to correlate the various survey maps, where available, in order to show composite geological maps of the western and eastern Lake Erie regions. This geological information has been projected across Lake Erie and Lake St. Clair, where possible, on the basis of information obtained from offshore wells drilled to date. This has been done with due thought to the implications involved in such projections with limited information, and with the different nomenclatures which are encountered when political boundaries are crossed.