Sub-Mississippian Formations, Southwestern Manitoba and Southeastern Saskatchewan

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Summary:
Location: T1-14, R20W1-26W2
Strata: Ordovician / Silurian / Devonian
Year of Study: 1999

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Introduction

The study area lies within that portion of southwestern Manitoba and southeastern Saskatchewan underlain by the Williston Basin. It covers an area of approximately 550 townships, lying between Townships 1-14, and Ranges 20W1M to 26W2M. As originally conceived, the project was designed to give a comprehensive overview of the petroleum potential of seven sub-Mississippian sedimentary packages as an aid to evaluating acreage surrendered under the Saskatchewan Government’s Deep Rights Reversion legislation which came into effect in April, 1998.

 

Table 1: Horizons Examined in the SASKMAN Study

 

Bakken Formation (Three Forks Group)

The Bakken Formation is a widespread clastic succession that straddles the Mississippian/Devonian boundary in the Williston Basin and corresponds to the Exshaw Formation west of the Sweetgrass Arch. A tripartite division of the formation exists, with each of the members onlapping the one below. The Lower and Upper members are black to brownish-black organic mudstones for which numerous depositional environments have been proposed, including a shallow sea with a stratified water column (lending itself to anaerobic bottom water conditions), marine swamps, and deep marine (>200 metres). The Middle Member (sandstone/siltstone/ mudstone) is believed to be dominantly a transgressive deposit that formed in a shallow marine environment.

Reservoir quality is moderate with relatively low porosity and permeability; however, hydraulic fracturing and acidization significantly enhance the production of Bakken oil which, within the study area, is light and sweet. The Middle Bakken is a regionally continuous aquifer, sealed above and below by the Upper and Lower members (where present). The Bakken dips and thickens to the south and thins towards the northern erosional edge of the Williston Basin. Four significant oil fields exist within the confines of the study area, with pools averaging one section in areal extent. Structure, stratigraphy, and hydrodynamic flow all play a role in developing an economic accumulation. There is huge potential for new discoveries of Bakken oil in the underexplored hinterlands of southeastern Saskatchewan and southwestern Manitoba, particularly with the increased drilling that is currently taking place for deeper targets, and the advent of horizontal technology.

The study consists, in part, of a regional overview composed of Introduction (Chapter A), Regional Geology (Chapter B), Potential Field (Aeromagnetics and Gravity) Study (Chapter C), and Hydrodynamics Summary (Chapter D). This overview is followed by Chapter E, which includes a sub-chapter on each of the seven units studied (Chapters E-1 to E-7). Included in this analysis are the following general topics: age, previous work, stratigraphy, sedimentology, diagenesis, structure, hydrodynamics, and petroleum geology. The last includes information on reservoir type and quality, source rock and maturity data, play types, pool studies (where applicable), seismic modeling (where applicable) and future petroleum potential. A “tops” data base (hard copy and diskette), along with a suite of representative core descriptions; cross-sections and maps round out the individual sub-chapters.

Winnipeg Group

The Winnipeg Group is a 50-60 metre thick Middle Ordovician clastic succession that overlies either Cambrian Deadwood clastics or Precambrian basement rocks, and is overlain by carbonates of the Red River Group. It consists of a basal sandstone unit (Black Island Formation) and an overlying shale unit (Icebox Formation). The sandstones have excellent reservoir potential, with porosity often exceeding 25%. Until 1998, no hydrocarbons had been produced from the Winnipeg in the study area, but the Founders Hartaven 7-2-10-9W2M well (415 BOPD of 53° API crude) has proven that the unit is productive. This trap is apparently associated with basement-related structures of the Minton-Midale type, and shows the way for a new phase of deeper drilling.

Red River Group

The Red River Group consists of approximately 150 metres of ?Middle and Upper Ordovician carbonates and minor evaporites overlying clastics of the Winnipeg Group, and underlying carbonates and shales of the Upper Ordovician Stony Mountain Formation. The unit forms an almost continuous carbonate blanket over the entire Williston Basin. It is divisible into a lower burrow-mottled limestone and dolomite sequence (Yeoman Formation) overlain by two to three carbonate-evaporite couplets of the Herald Formation.

Excellent quality reservoir rocks are present in the lower (burrow-mottled) portions of each carbonate evaporite couplet, with vertical seals provided by the overlying anhydrites. At the inception of the SASKMAN study, there were only handful of marginal fields producing from the Red River; the Berkley Midale (1996) discovery was still confidential. Since that time, some 150 wells have been drilled in search of analogous features, and dozens of pools have been located. These small, basement-controlled structures are currently the most significant deep play type in the Canadian portion of the Williston Basin.

Interlake Group

The Silurian-aged Interlake Group consists of a thick package of predominantly dolomitized shallow marine to supratidal mudstones, wackestones and skeletal (bioclastic) carbonates. The unit conformably overlies the Ordovician Stonewall Formation and is disconformably overlain by the Middle Devonian Ashern Formation. It is one of the more enigmatic and poorly understood successions in the Williston Basin. Although penetrated by some 300 wells, it has – to date – yielded only a single oil test on DST. The unit is a prolific producer in the United States, in structural traps associated with such features as the Nesson and Cedar Creek anticlines. Complicating factors in Canada include finding suitably porous mappable units immediately beneath the sub-Middle Devonian unconformity, and delineating sufficient local structure to produce a trap.

Winnipegosis Formation (Elk Point Group)

The Middle Devonian Winnipegosis Formation (Keg River equivalent) was deposited in the southernmost portion of the broad northwest-southeast trending Elk Point Basin. During late Winnipegosis time, this basin was divided into a deeper, interior basinal area flanked by carbonate shelves. The former area was dotted by hundreds of small (one section or less) pinnacle reefs which attained heights of up to 70 metres. The development of this pinnacle reef province is unique to the Winnipegosis Formation, and makes it the most prospective Devonian horizon in the study area. Three types of plays exist:

• dolomitized pinnacle reefs (Tableland, Hitchcock, Macoun) mantled by anhydrite and encased in Prairie Evaporite, which hold 95% of the known Winnipegosis oil reserves;
• the untested updip southern margin of the Winnipegosis shelf, where it is sealed by Prairie Evaporites;
• structural traps formed by Precambrian basement uplifts. These can occur inboard of the shelf margin (Minton) or within ?non-reefal basinal sediments (Kingsford).

Prior to the recent discovery of the prolific Minton-type of Red River structural play, Winnipegosis pinnacle reefs held greater than 90% of proven sub-Mississippian hydrocarbon reserves in the study area.

Birdbear Formation (Saskatchewan Group).

The Birdbear Formation represents the last phase of upper Devonian carbonate sedimentation in the Williston Basin, and corresponds to the Nisku Formation of the Alberta Basin. It consists of a single, shoaling-upward, carbonate-evaporite couplet that forms a broad, pancake-like deposit over the breadth of the study area. The basal two-thirds consists of open marine to intertidal, variably fossiliferous limestones and dolostones, whereas the upper third consists of interbedded calcarenite and anhydrite. The unit forms a continuous aquifer throughout the project area, and structure is a prerequisite for trap formation. Until recently (1997), there were only two pools of any commercial consequence (Hummingbird and Kisbey). These pools are found in structures formed by the removal of underlying Prairie Evaporites (halite). The recent re-completion of the Berkley et al Midale 11-35-6-11W3M as a Birdbear oil well demonstrates that the Birdbear is a also good potential reservoir in the basement-controlled Minton-type of multi zone structural play.

Bakken Formation (Three Forks Group)

The Bakken Formation is a widespread clastic succession that straddles the Mississippian/Devonian boundary in the Williston Basin and corresponds to the Exshaw Formation west of the Sweetgrass Arch. A tripartite division of the formation exists, with each of the members onlapping the one below. The Lower and Upper members are black to brownish-black organic mudstones for which numerous depositional environments have been proposed, including a shallow sea with a stratified water column (lending itself to anaerobic bottom water conditions), marine swamps, and deep marine (>200 metres). The Middle Member (sandstone/siltstone/ mudstone) is believed to be dominantly a transgressive deposit that formed in a shallow marine environment.

Reservoir quality is moderate with relatively low porosity and permeability; however, hydraulic fracturing and acidization significantly enhance the production of Bakken oil which, within the study area, is light and sweet. The Middle Bakken is a regionally continuous aquifer, sealed above and below by the Upper and Lower members (where present). The Bakken dips and thickens to the south and thins towards the northern erosional edge of the Williston Basin. Four significant oil fields exist within the confines of the study area, with pools averaging one section in areal extent. Structure, stratigraphy, and hydrodynamic flow all play a role in developing an economic accumulation. There is huge potential for new discoveries of Bakken oil in the underexplored hinterlands of southeastern Saskatchewan and southwestern Manitoba, particularly with the increased drilling that is currently taking place for deeper targets, and the advent of horizontal technology.