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Bedded Salt in Ontario: Geology, mining, hydrocarbon storage, and oil and gas reservoirs

Abstract

The Upper Silurian Salina Group underlies large parts of Michigan, southern Ontario, Ohio and New York State within the Michigan and Appalachian basins and is one of the most extensive and best-known salt deposits in the world. It is characterised by a thick cyclic succession of limestones, dolostones, shale, anhydrite, gypsum and halite with a maximum aggregate thickness of 750 metres in the middle of the Michigan Basin and 420 metres in southern Ontario. There are nine named formations in the Salina Group in southern Ontario; A-0 Unit, A-1 Unit, A-2 Unit, B Unit, C Unit, D Unit, E Unit, F Unit and G Unit. Within Ontario four of these formations contain beds of halite; the A-2, B, D and F units, at depths ranging from 275 to 825 metres. The B Salt is the thickest and most regionally extensive salt unit in Ontario, ranging upwards to 90 m in thickness with numerous interbeds of limestone, dolostone and anhydrite. Salt is mined by conventional underground mining methods from the F Unit in the city of Windsor and from the A-2 Unit in the town of Goderich. The Sifto salt mine in Goderich is the largest salt mine in the world, producing over 7 million tonnes of salt annually. Salt is also mined by the solution-mining method from the B Unit at both these locations. Liquified hydrocarbons are stored in solution-mined caverns in the B Unit and also in the A-2 Unit in the Sarnia area and in the city of Windsor. There are a total of 104 cavern storage wells servicing 71 storage caverns with active storage capacity for over 22 million barrels of hydrocarbons, valued in excess of $2 billion when the caverns are full. The caverns provide safe, economical, high-volume pressurized storage capacity for the extensive petrochemical complex in the Sarnia area and are a major factor in location of these industries in this area. Differential post-depositional dissolution of halite beds has had a controlling influence on the thickness and structure in overlying strata and formed structural domes hosting shallow Devonian oil fields, including the famous Oil Springs and Petrolia oil pools. The Salina Group also forms the cap rocks for reservoirs of oil and natural gas in pinnacle reefs of the underlying Guelph Formation. Most of these reservoirs are depleted with 34 now being utilized for storage of natural gas. Over 25% of the gas consumed annually in Ontario has been stored temporarily in a designated storage area.