Events

Sedimentology, Stratigraphy and Reservoir Characteristics of the Mississippian Frobisher Beds in Southeastern Saskatchewan

Holly Shoulak – Geologist, Saskatchewan Geological Survey

Abstract

The Mississippian Frobisher beds in southeast Saskatchewan have been a prolific oil producer since their discovery is the 1950s. Situated along the northeastern edge of the Williston Basin, these strata consist of carbonates, evaporites and siliciclastic lenses. The purpose of this study was to determine the lithofacies distribution and a depositional model of the Frobisher Beds in the study area, as well as their control on oil distribution in the Sherwood Frobisher, Elmore Frobisher and Workman Frobisher oil pools.

The study area extends from Township 1, Range 30 west of the First Meridian (Tp. 1, Rge. 30W1M) in the southeast to Tp. 2, Rge. 32W1M in the northwest. Core from 78 wells were described, 54 thin sections from 10 wells were prepared and evaluated, and over 600 geophysical well logs were interpreted. Reservoir characterization was completed for five oil pools in the study area: Elmore Frobisher, Sherwood Frobisher, Workman Frobisher, Winmore Frobisher and Gainsborough West Frobisher.

Analysis identified 16 lithologies (L) that were grouped into 5 lithofacies associations (LFA) based on their depositional environments. These lithofacies associations cyclically recur and consist of marker beds (LFA1 and LFA2) that separate carbonate (LFA3 and LFA4) and evaporite (LFA5) sequences. Marker beds are dolomitic mudstones and are characterized by high gamma ray response on geophysical logs. Carbonate sequences are comprised of microbial and coated grain mudstones to grainstones. Evaporite sequences are comprised of laminated, nodular or massive anhydrite. Based on areal distribution and stacking patterns these lithologies and lithofacies associations are indicative of a cyclical intertidal to supratidal depositional setting with lagoon, embayment, saline lake, sabkha and mudflats environments.

The Frobisher Beds in the study area contain multiple stacked reservoirs due to a complex depositional and diagenetic history. This has resulted in multiple productive horizons and traps within the same oil pool. The porosity values of the carbonate sequences and the marker beds are comparable, but the carbonate sequences have higher permeability values. The evaporites as well as the alteration zone at the sub-Mesozoic unconformity create effective seals for these reservoirs. Oil accumulation in these reservoirs is controlled by structural, stratigraphic and diagenetic traps or a combination of these mechanisms.

Bio

Holly Shoulak is a geologist with the Saskatchewan Geological Survey within the Ministry of Energy and Resources. She earned her B.Sc. in Geology and a Diploma in Business Administration from the University of Regina in 2018 and completed her M.Sc. in Geology in 2025. Her current work at the Saskatchewan Geological Survey focuses on the sedimentology, stratigraphy, and reservoir characterization of the Mississippian Frobisher Beds in southeastern Saskatchewan. Her previous work with the Survey includes studies of the Bakken Formation, Groundwater Protection and the Shaunavon Formation.