ABSTRACT

The lower member of the Amaranth Formation in the Manitoba portion of the Williston Basin produces oil from sandstone subunits discontinuously developed in its lower part. In the Coulter-Pierson area current Lower Amaranth production represents about 7 percent of Manitoba's total oil production.

The Lower Amaranth is separated from underlying Mississippian strata by a marked angular unconformity, and is overlain by Jurassic Upper Amaranth evaporites. The member is subdivided into an upper shaly unit consisting mainly of mudstones and a lower sandy unit composed of siltstones and primarily fine- to medium-grained sandstones. The lower sandy unit comprises four sandstone subunits, designated in ascending order as 'Lower' Sand, 'Main' Sand, 'A' Sand and 'B' Sand.

The Lower Amaranth sediments were deposited in a low energy, tidal flat environment. Three generalized lithofacies are recognized: 1) supratidal mudstone facies; 2) intertidal siltstone facies; and 3) subtidal channel-fill sandstone facies.

The sandstone unit is the reservoir rock and the 'Main' Sand is the primary producer (2-5 m thick). Porosity is generally intergranular and variable due to anhydrite occlusion. Entrapment is primarily stratigraphic. Oil is trapped within the porous sandstones by porosity - permeability pinch-out resulting from lateral and vertical variations in lithofacies. Reservoir characteristics are heterogeneous and are controlled by Mississippian topography, depositional environment, compaction and anhydritization through diagenetic processes.