ABSTRACT

Macro- and micro-facies of the Mississippian rocks of southern Saskatchewan include shelf, basin slope or basin deposits. The most effective original porosity was present in the crinoidal and mixed skeletal grainstones and packstones of the shelf to basin slope transition zone, as well as in the pisolitic and oolitic grainstones and packstones of the shelf crest. Much finer and somewhat less effective original porosity existed in the wackestones and mudstones of the shallow subtidal shelf, although skeletal and non-skeletal grainstone and packstone lenses and bank-like accumulations probably had fairly good interparticle porosity. Diagenetic studies show that the original porosity was greatly reduced by cementation, neomorphism and compaction, and enhanced by dolomitization and solution. Solutions passing through the rock removed undolomitized constituents and provided cementing material that fills original porosity.

Capillary-pressure mercury-injection analyses show that the geometry of present pore systems is related more to the effects of diagenesis than to their occurrence in specific macro- or micro-facies. The porosity belongs broadly to two categories: intercrystal and meso- or micro-cavity. In addition, there are various combinations of these two that create a mixed grouping. Intercrystal porosity is characterized by: 1) shallow slopes on mercury injection curves; 2) relatively uniform pore throat sizes; 3) relatively uniform crystal sizes; 4) variable entry pressures (seldom less than 2 kg/cm2); and 5) relatively high withdrawal efficiences. On the other hand, cavity porosity has: 1) steep slopes on injection curves; 2) generally poor uniformity of pore throat sizes; 3) varied constituent sizes; 4) entry pressures that are commonly less than 2 kg/cm2; and 5) generally low withdrawal efficiencies. The mixed grouping has characteristics intermediate to the two extremes. Intercrystal porosity is commonly found in the reservoirs of the Midale Beds and generally is the most productive form of porosity. Meso- and micro-cavity porosity are prominent in the shelf crestal rocks, such as those of the Frobisher Beds, and have more variable productivity. Reservoirs in the Ratcliffe Beds are mainly intercrystal, though some belong to the mixed grouping.

Data from the Benson Oilfield in southeastern Saskatchewan suggest that local variations in the yield capabilities of the main reservoir rocks may be correlated with geographic and stratigraphic differences in the capillary pressure factors. Consequently, the determination of these factors for other oilfields should provide valuable information for the design of enhanced recovery schemes.