Evaporites are an important factor as the seal for hydrocarbon accumulations in carbonate provinces throughout North America. In the Silurian of the Michigan basin, dolomitized Niagaran pinnacle-reefs have been effectively sealed with evaporite cycles composed of anhydritic sabkha deposits of the Ruff Formation and cleaner evaporites of the Salina units. In the Williston basin of North Dakota and Saskatchewan, successive regressions of the Mississippian sea have resulted in the subsequent basinward migration of anhydritic sabkha deposits over algal-pelletal limestone banks that were formerly shoals at the seaward edge of the sabkhas. The anhydrite has halted the updip migration of hydrocarbons in these limestones. In the Permian of west Texas, over 731 million bbl of oil ave been produced from the San Andres Formation against a porosity barrier along the eastern side of the Central Basin platform where anhydrite has plugged the porosity of the dolomite. The McElroy field is an excellent example of this important trend.

The environmental conditions that provide the typical setting for the formation of shallow-marine carbonates are also ideal for the formation of evaporites whenever marine waters become sufficiently restricted. The occurrence of excellent evaporite seals in close proximity to porous carbonate reservoirs can provide many opportunities for entrapment of

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hydrocarbons in this province. However, a thorough understanding of facies relations is essential for success in the search for this type of subtle stratigraphic trap.

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