ABSTRACT

A number of reservoir styles, producing lithologies, and associated porosity systems have been identified in the Mississippian Ratcliffe Beds along a transect from the Oungre Field, through the Flat Lake Field, to the Lake Alma Field. Scanning electron microscopy (SEM) and mercury- injection capillary pressure analysis (MICP) are important techniques in studying these carbonate rocks.

At the Oungre Field (Township 2, Range 14W2M) a stratigraphic trap is created within the Oungre zone by the change from non-argillaceous microsucrosic dolomite to argillaceous carbonate up-dip. Porosity is homogeneous, very fine intercrystalline, with minor remnant micrite as observed by SEM. MICP analyses show a high degree of uniformity in pore throat sizes, and high ejection efficiencies.

In the Flat Lake Field (Township 1, Range 16W2M) oil is trapped in the upper Ratcliffe Beds by a combination of structural and stratigraphic elements. Reservoir lithologies include oolitic-pelletoidal-skeletal wackestones to packstones, lime mudstones, and dolomitic mudstones. The wackestone-packstone lithofacies developed as low-relief shoals near the edge of the Hummingbird Trough. Intergranular and intercrystalline porosity types are present in reservoir strata. MICP analyses indicate heterogeneous pore networks in Flat Lake reservoir rocks, though ejection efficiencies are comparable to those at the Oungre Field.

In the Lake Alma Field (Township 1, Range 17W2M) oil is trapped by structural closure (over 40 m on the Ratcliffe Beds) related to dissolution of salt in the Prairie Evaporite (Devonian). As in the Oungre Field, excellent intercrystalline porosity is developed in microsucrosic dolomites of the Oungre zone. In addition, microcrystalline limestones serve as reservoir rock to a much lesser extent. Microvugs and moldic porosity cause greater heterogeneity of pore systems, though ejection efficiencies remain high.

The quality of these Ratcliffe Beds reservoirs is controlled by depositional factors which influence the extent of dolomitization.