ABSTRACT

The Virden-Roselea and North Virden fields are located on the northeast flank of the Williston Basin. They were discovered in 1953 and development is still in progress. The reservoir rocks are largely stable shelf, clastic limestone members of the Mississippian Lodgepole formation. A combination of:

(1) local structural warping and

(2) permeability barriers, from secondary effects, have formed the trap.

These secondary processes in order of occurrence are:

(1) leaching of limestone at the post-Mississippian unconformity.

(2) dolomitization of the leached limestone during initial evaporite deposition of Amaranth (Middle Jurassic) age, resulting in a major decrease in permeability and a minor decrease in porosity.

(3) anhydrite infilling during later Amaranth deposition resulting in reduced porosity and locally in reduced permeability.

Within any separate accumulation each individual permeable zone in the reservoir has its oil water interface which, once up dip from edge water, becomes the base of the permeable zone and thus follows the structure.

It is probable that the main source of the oil is from the down dip shale and argillaceous limestone facies of the clastic limestone reservoir rock.