Abstract: Calcium Sulfate Diagenesis, Mississippian of Eastern Williston Basin
Alan C. Kendall
Mississippian carbonate rocks of the eastern Williston basin are significant hydrocarbon producers. Anhydrite within the carbonate rocks may form stratigraphic traps, or it may adversely affect reservoir quality and performance. Many anhydrite types are present in the Mississippian carbonate rocks. The anhydrites range from Mississippian to Holocene in age.
Depositional to early-diagenetic calcium sulfate was precipitated in sabkha environments as nodular anhydrite and as replacements of halite and gypsum crystals (and is associated with early-diagenetic celestine), or was precipitated subaqueously as large gypsum crystals or as laminites. Subaqueous sulfates commonly suffered bacterial reduction (with formation of patterned dolomites) or were dehydrated in slightly later sabkha diagenetic environments. All surviving gypsum subsequently was converted to anhydrite during burial.
Beneath the unconformity surface that truncates the Mississippian, some of the anhydrite was hydrated to porphyroblastic, alabastrine, and satin-spar vein gypsum and is associated with late-diagenetic celestine. After burial of the Mississippian beneath Jurassic-Triassic red beds, the carbonate materials directly beneath the unconformity surface were dolomitized and all porosity was plugged with gypsum. Gypsum also was introduced massively as satin-spar veins. Locally, however, this alteration zone is absent and, at such locations, Mississippian anhydrites (and their gypsified equivalents) have been calcified.
Renewed deep burial of the Mississippian again converted all gypsum to anhydrite and at some time during the Late Cretaceous (coincident with introduction of hydrocarbons) much of the Mississippian carbonate material was replaced by euhedral anhydrite porphyroblasts.
In the northern part of the Mississippian subcrop (at depths less than 2,000 ft; 600 m) the anhydrite is being converted back into gypsum by present-day groundwaters and a third generation of satin-spar veins is forming. Elsewhere in the subsurface of Saskatchewan vugs are filled by huge gypsum crystals at depths of up to 6,000 ft (1,800 m) and are in association with concentrated brines. These crystals possibly are relict from the Pleistocene (some are being converted to anhydrite) when development of a deep permafrost caused a temperature decrease and a pressure change within groundwaters that allowed gypsum to form.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma