Petroleum Geology of Lower Mannville and Jurassic formations, Manyberries Area, Southeastern Alberta and Southwestern Saskatchewan

Location: T1-10, R25W3-9W4
Strata: Mannville / Swift / Sawtooth
Year of Study: 1989


Regional analysis of Jurassic and Lower Mannville strata of southeastern Alberta and southwestern Saskatchewan reveals that a complex interplay of depositional facies, structure and hydrodynamics controls the location of major hydrocarbon accumulations.

Jurassic sediments are almost entirely marine and thicken in an eastward sense towards the centre of the Williston Basin. The Sawtooth Formation, a major oil reservoir in the area, is composed of quartzose shoreface sandstones in the west which grade eastward into more open marine calcareous Rocks. Productive units at Battle Creek (2-26W3M) are formed of coquinas. Sawtooth sedimentation was followed by widespread marine deposition of Rierdon Formation shales which were, in turn, succeeded by progradational prodeltaic sandy lithofacies of the Swift Formation. Syn-sedimentary tectonic movements during Jurassic deposition includes salt removal from Devonian strata (Elk Point Group, Beaverhill Lake Formation, Wabamun Formation) and vertical movements associated with block faulting over the Sweetgrass Arch. The most significant tectonic activity in the study area appears to have ended by the end of lower Mannville time.

The Lower Mannville units of the area are the Cut Bank and Sunburst Formations, with the former restricted to the northwesternmost corner. Cut Bank sediments comprise mainly valley fill deposits found within a tributary to the master northwest-oriented Cut Bank Valley, or Spirit River Channel. Cherty, quartzose Sunburst sediments were laid down upon valley floors within southwesterly flowing fluvial systems which were tributary to the master northwest-flowing drainage. The Manyberries trend is part of a regional network of southwesterly flowing rivers which include exposed Sunburst strata in the Great Falls area of Montana, the productive Kevin-Sunburst trend of northern Montana (which may extend into southwestern Saskatchewan) and many other trends found north of the study area. Sunburst valleys filled in response to the Clearwater-Ostracode transgression which culminated in the deposition of the shaley 'brown sand' which caps and flanks major Sunburst sandstones.

Structures on shallower surfaces (Base Fish Scales, Second White Specks) display relatively less relief than on older units. Nonetheless, these younger structures mimic many deeper trends.

The Jurassic-Lower Mannville sequence in the Manyberries area can be divided into two hydrodynamic units - Sawtooth and Swift/Sunburst - separated by Rierdon shales. The Sawtooth flow unit displays continuity between the surface and subsurface with a general northward orientation of flow. Oil is trapped in southwestern Saskatchewan under artesian conditions within conquinas located upon late, high relief structures. Gas, which exsolved during secondary hydrocarbon migration, is generally found along the western perimeter in capillary and structure traps. The Swift/Sunburst flow unit displays several enclosed potentiometric lows and is characterized by progressively lighter oils and more brackish waters towards the centre of the oil accumulation. Oil and gas distribution is complicated by the variations in Swift/Sunburst reservoirs and present structure.

Integration of all variables illustratesseveral prospective areas. For example, Sawtooth prospects can be located upon a relatively untested structure north of Battle Creek and within a potentiometric low found between two cypress Hills potentiometric highs. A major Sunburst fairway extends from Manyberries west to Pakowki Lake. All play types observed here can be extended into northern Montana.

Seismic modelling illustrates that porosity variations associated with Sawtooth and Sunburst reservoirs are resolvable only if high signal-to-noise ratios are obtained. Structural plays, like Battle Creek, are easily viewed on seismic. Nonetheless, potential reservoir trends on structures within potentiometric lows with NaCl-rich waters are highly prospective.

For more information contact:

Leslie Sears
Petrel Robertson Consulting Ltd.
500, 736- 8th Avenue S.W.
Calgary, Alberta
T2P 1H4

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