ABSTRACT

The association of evaporites with a large percentage of the world's oil and gas reservoirs makes these rocks economically significant. This study was intended to increase our knowledge of environments of deposition and diagenesis of these rocks in a basin in which good core control was available -- the Middle Devonian Elk Point basin, Alberta.

We have recognized four major environments of anhydrite deposition -- supratidal, shallow-water, deep-water, and replacement. Supratidal anhydrite includes gypsum pseudomorphs and most of the nodular, nodular-mosaic, and mosaic types. These are identified by comparison with the anhydrite in the sabkha sequence of the Trucial Coast. Bedded-massive anhydrite is interpreted to have been deposited in shallow water under conditions comparable to those in the Pekelmeer of Bonaire Island. Tiny nodules and thin laminae of anhydrite which are usually within or interbredded with laminated limestone are interpreted as deep-water in origin, from their stratigraphic position in the deeper parts of the Elk Point basin. No modern analogues of these types are known. Some massive, nodular, nodular-mosaic, and mosaic anhydrites are considered to be postlithification replacement types. They are commonly located on the edges of shoals, banks, and reefs and are characterized by their relatively greater thickness and by the inclusion of carbonate fragments of the host rock within the anhydrite.

By detailed core studies of the anhydrite environments and their order of occurrence in three subbasins, we have been able to recognize eleven major sedimentary events. By correlating these events between subbasins, with the aid of additional core and density-log control, the history of the Middle Devonian basin has been reconstructed.