ABSTRACT

The Alida Beds in the Pheasant Rump Pool represent an example of the complex diagenetic history that affected some Mississippian carbonates at their subcrop in southeastern Saskatchewan. Here, secondary porous, often "chalky" pack/wackestone alternate with secondary porous crinoidal grainstones to form a complex reservoir with significant vertical heterogeneity.

Deposition of the principal Alida reservoir beds occurred at or slightly below fair-weather wave base forming part of a retrogradational (transgressive) parasequence set. Low energy packstone and wackestone alternate with crinoidal grainstone tempestite.

Important byproducts of initial burial diagenesis were the formation of syntaxial calcite cement in the grainstones and matrix dolomite in the pack/wackestone. Subsequent dolomitization and anhydritization are associated with the pre-Watrous erosion surface and created the impervious "caprock". Further anhydritization, particularly the emplacement of aggressive cements in the grainstone beds, took place during early reburial.

Significant alteration of the carbonates and development of effective secondary porosity resulted from late burial diagenesis involving dedolomitization (and concomitant carbonate leaching) in the pack/wackestone, and the dissolution of anhydrite cement in the grainstone. Late stage recompaction and production of mobile fines accompanied this diagenesis. Flow of pore fluids through the Madison Aquifer was the driving mechanism.