PROCESSES OF DOLOMITIZATION IMPORTANT FACTORS INFLUENCING PORE GEOMETRY AND RESERVOIR QUALITY IN MISSISSIPPIAN INNER SHELF CARBONATES OF SOUTHEASTERN SASKATCHEWAN

The origin of the dolomites that form hydrocarbon reservoirs in inner shelf lime mudstones and wackestone of the Mississippian Midale and Ratcliffe Beds of southeastern Saskatchewan is not clearly understood. However, studies of the geometry of pore systems using capillary pressure mercury injection analyses supplemented by scanning electron microscopy aid elucidation of the processes that created the dolomitized reservoir rocks. There is fairly good evidence that entry pressures obtained from capillary pressure mercury injection analyses are markedly influenced by the "calcareousness" of the carbonate res.ervoir rock; the greater the proportion of dolomite present, the lower the entry pressure. In addition, it has been shown that the best dolomite reservoir is one in which 7590 to 9090 of the rock has been dolomitized (inhibition boundary stage of crystal growth). When the degree of dolomitization increases beyond 9090 the dolomite cyrstals enter a compromise boundary stage of growth, resulting in a reduction of pore volume and permeability, Attainment of the inhibition boundary stage is not sufficient to produce a good reservoir; it must be accompanied by dissolution of the remnant host rock, creating porosities ranging from 25Vo to 3890 and permeabilities on the order tens of millidarcies. An additional factor that creates good reservoir rock in the inhibition boundary stage ofcrystal growth is the uniform size of crystals and pore throats, easily recognized on capillary pressure mercury saturation plots. The most likely dolomitizing mechanism by which both inhibition boundary stage and remnant host rock dissolution could be facilitated is the mixing zone process. Fresh meteoric waters capable of dissolving the remnant host rock would enter the dolomitized rock proceeding the passage of the mixing zone through it. In contrast, reflux dolomitization would be more likely to cause either a compromise boundary stage of dolomitization or produce a calcareous dolomite.