Heavy oil accumulations in A1ississipian carbonate rocks, immediately subjacentto the sub-A1esozoic unconformity in southwestern Saskatchewan, have been knownfor more than forty years. Until late 1993, sporadic attempts using verticalwells to recover oil trapped at Battle Creek have attained about 17, 000m3 in 15 years of production. However, recently drilled horizontalwell indicate there is potential for much higher yields over shorter timeperiods.
The reservoir rocks were deposited on a ramp that prograded over olderMississippian deeper water and basin slope carbonates. The ramp rocks includecrinoidal grains/ones and packs/ones, interbedded with lime muds/ones andcrinoid-rich wackestones
About 15–20 m below the unconformity the grainstones and packstones have beensealed by calcite Cf!JlJeI1B. However subjacent to it, they were extensivelyaltered. Skeletal components of the grainstunes and packstones were micritized, and in places, preexisting particles and interparticle cements have been wholly01' partically dissolved, exhuming original porosity or creating enlarged vugs.Furthermore, the paleotopograplucally positive features on the unconformityhave been dolomized. Lithologies affected include: micritized skeletalcomponents of grains/ones and packstones and micrites of packstones, wackestones and lime mudstones.
Diagenesis has created two suites of non-reservoir rocks and three of reservoirgrade. Porosities of 8–41%, permeabilities of 8–5/2 md, and entry pressuresless than 4 kg/m3 make up the reservoir rock properties found atBattle Creek and several other poorly exploited localities
The Mississippian rocks of southwestern Saskatchewan unlike their counterpartson the eastern flank of the Williston Basin, have been known to exclusivelycontain heavy oil shows, The shows in cores, drill cuttings and drillstem testshave been recognized since the early days of drilling in both southeastern Alberta and southwestern Saskatchewan. However, their producibility wascommonly overlooked because of the extremely low gravity of the oil (in theorder of 11 ° API). In a study for the Saskatchewan Department of Energy andMines, Kent and MacEachern 1 identified twenty-four heavy oil showsin Mississippian core and drillstem tests from the area between range 20 westof the third meridian and the Alberta/Saskatchewan Interprovincial Boundary andtownship 1 to township 15, inclusive. The most impressive of these shows was adrill stem lest from the Coop et al.. Battle Creek 1–27 well (Lsdl27-3-26w3)thaI recovered 1262 m (4140 ft) of 11 ° API oil from an interval inMississippian rocks immediately beneath the sub-Mesozoic unconformity.Subsequent tests in the same well yielded mainly water and it was eventuallyabandoned. However, this discovery identified the Battle Creek area as apotential heavy oil producing area and in 1966. Western Decalta offset the 1-27well with two boreholes at IA-27-3-26W3 and 6-27-3-26W3. The former wasabandoned shortly after completion but the latter produced oil from theMadison at the same stratigraphic level as the discovery well for about one anda half years (1966 to 1968) until a fire at the treated caused the operator toabandon the well.
