ABSTRACT

The Virden and Daly oilfields of southwestern Manitoba have produced more than 25 million m3 (157 million barrels) of light (33-37° API) oil since their initial discovery. Limestone of the Early Mississippian (Kinderhookian) Lodgepole Formation is the principal reservoir rock; oil is structurally and stratigraphically trapped immediately downdip of the subcrop edge of this limestone.

Six stratigraphic units within the Lodgepole can be correlated across the study area. In the Virden area, a succession of bioclastic and oolitic grainstones and shales, recording shallow shelf deposition, onlap a prominent exposure surface capping a deeper water lime mudstone. To the west, in the Daly area, bioclastic limy shales are interbedded with thick packstone-wackestone accumulations, which represent some type of channel or mound feature, within a slope setting. Base-of-slope Waulsortian mud mounds have been described in eastern Saskatchewan immediately seaward of the shelf/slope package identified in this study. Detailed log correlations within the lower and middle part of the Lodgepole Formation reveal classic sigmoidal clinoform morphology which, coupled with the sedimentological data, suggests that the Lodgepole in the study area records the progradation of a westward-dipping carbonate ramp. The north-south orientation of the shelf/slope break in Manitoba is controlled by basement lineaments which have focused dissolution of Devonian salts and influenced subsidence rates during deposition of Lodgepole strata.