Secondary anhydrites in deeply buried Devonian carbonates of the Alberta Basin, Canada

Abstract

Observations from about 500 drill cores from Devonian reefs and carbonate platforms in the Alberta Basin, Canada, reveal that the Upper Devonian strata across this basin contain about 20 petrographically distinct types of anhydrite that can be placed into two groups: ‘primary’ and ‘secondary’ anhydrites. ‘Primary’ anhydrites formed syndepositionally in essentially unlithified sediments, either directly from evaporitic brines or within a few years to perhaps hundreds of years during very shallow burial (centimeters to meters). In contrast, ‘secondary’ anhydrites formed in essentially lithified sediments at burial depths of meters to thousands of meters. Some secondary anhydrites formed from gypsum at depths of less than about 600–1,100 m, others formed directly as anhydrite at greater depths. Some remobilized anhydrite invaded sour gas fields in which anhydrite had been removed previously by thermochemical sulfate reduction. In the Leduc, Swan Hills, and Nisku reefs and platform carbonates of the Alberta subsurface, and in the strata surrounding these carbonates, no primary anhydrites have been found. Rather, a few types of secondary anhydrite predominate. The sources appear to be threefold: (1) brines migrating from Middle to Late Devonian evaporite-bearing strata; (2) seawater and mesohaline brines that formed gypsum and/or anhydrite as a by-product of dolomitization; (3) redistribution of both primary and secondary anhydrites, in some cases demonstrably after maximum burial and sour gas formation. These findings are of interest to the petroleum industry because anhydrites often reduce porosity and permeability significantly.