Distribution and Behaviour of Oil and Water in Mississippian Limestone Reservoirs, Southeastern Saskatchewan

In the Mississippian of southeastern Saskatchewan, variations in performanceof individual wells or segments of limestone oil fields commonly result frommarked lateral variations in the amount and type of porosity together with thepresence of numerous impermeable barriers within the reservoir unit. Porosity in limestones is classified asinterparticulate,vuggy andfracture. Interparticulate porosity includes intergranular (characteristicof calcarenites) and intercrystalline (typical of secondary dolomites). Vugsare cavities that are distinctly larger than the size of particles comprisingthe rock matrix. Unfractured, well sorted calcarenites finer than the Wentworth fine grainsize invariably have insufficient permeability for an effective oil reservoir.In contrast, the minimum crystal size for an effective unfractured sucrosicdolomite reservoir falls in the Wentworth very fine size range. Interstitial water content of Mississippian limestone oil reservoirs ofsoutheastern Saskatchewan exhibiting good interparticulate porosity commonlyrange from 20 to 40%. However, where the limestone is characterized by vugsassociated with dense lime mudstone matrix, interstitial water saturations aregenerally much higher despite the presence of relatively good permeability. Thehigh interstitial water content of such vuggy rocks is due to inability of oilacting under much of the water, as many vugs are not connected from beneath.Water trapped in the bottom of such vugs is free to move laterally duringdrillstem and production tests. Interstitial water is commonly at a minimum along the axes ofanticlinal noses and increases markedly along the flanks even where thereservoir bed is structurally above the free oil-water contact. The increasedwater saturation along flanks of anticlinal structures is believed due tonatural oil flooding which, at the time of oil accumulation, proceeds from thecrests of the anticinal nose laterally down the flanks and has an efficiencysomewhat similar to that of induced water flooding, as considerable water isby-passed in the reservoir.