Mississippian Southwestern Alberta

Mississippian Study, Southwestern Alberta and Northwestern Montana

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Summary:
Location: T1-20, R20W4-30W4
Strata: Mississippian and Deeper
Year of Study: 1994

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Introduction

A regional assessment of logs, cores and tests from approximately 1,400 wells in southwestern Alberta and northwestern Montana, augmented by outcrop study in north-central Montana and aeromagnetic, gravity and seismic data, was undertaken in order to document the controls on pooling within Mississippian units and to recommend new areas for exploration. The subsurface component of the evaluation covers Townships 36 and 37, Ranges 5W – 11W in Montana and Townships 1-20, Ranges 20 – 30 W4M in Alberta.

The structural architecture of the region contains two major Precambrian blocks; the Matzihwin High and the Medicine Hat Block, which are separated by the southwest-northeast trending Vulcan Low. South of the Vulcan Low, structure takes the form of several northwest-striking.

Features which, when analyzed in detail, appear as northwest-plunging antiforms which are commonly fault-bounded. Additional economically significant, but subtle structural features are small rollovers found on the up-plunge (i.e., to the southeast) side of the antiforms.

The Mississippian package is formed of a lower muddy, locally shaley, marine shelf package (i.e., Banff, Lodgepole). Small isolated bioherms are found at the base, and are overlain by several stacked parasequence sets of alternating burrowed mudstones and rippled to cross-bedded packstones and grainstones.

The upper Mississippian package (Livingstone, Rundle, Mission Canyon, Sun River) contains complexly interbedded mudstones, wackestones, and packstones with rare grainstones. These deposits formed in subtidal to peritidal environments. Consistent regional correlations are not discernable within these rocks, which are further complicated by the presence of brecciated, muddy, solution collapse zones. The strata are organized into both small- and large-scale clinoforms with northerly and southerly dips. This stratigraphic architecture precludes a ‘layer cake’ assessment of stratigraphy and complicates the application of entrenched stratigraphic terminology.

South of the Vulcan Low, the upper portion of the Mississippian section is dolomitized, which results in a blanket of porous and permeable rock. North of the Vulcan Low, both limestone and dolomite are encountered, but porous and permeable rocks are still present within virtually every well.

Other major alteration productions include silicification of burrow fills in the Lodgepole-Banff, and bitumen linings of many pores. Bitumen and pyrobitumen are so abundant at some locales that they constitute a major destroyer of reservoir quality (e.g. Del Bonita). Fracturing, both tectonic and that related to polyphase karsting, has also affected reservoir development by adding a component of heterogeneous vertical permeability to the succession.

The systematics of fluids within the Mississippian sequence take the following forms:

1. Oil gravity and gas content increase to the west and south;
2. Downdip water flow occurs from uplifted areas in the south, localized foothills recharge sites, and from eastern topographic highs;
3. A trend of wells with bypassed pay within the Mississippian is located due east of the Lower Cretaceous ‘Deep Basin’.

The Banff/Lodgepole is characterized by isolated reservoirs and aquifers, whereas the Rundle/Livingstone/Mission Canyon displays widespread vertical and lateral reservoirs and aquifers. A total of 54 wells display bypassed or damaged petroliferous zones within the Mississippian.

To date few wells have been completed within the Banff/Lodgepole due to limited reservoir development and modest oil recoveries. In contrast, the widespread blanket development of porous and permeable rock within the Rundle/Livingstone/Mission Canyon causes structure to have a significant influence on pooling, with pool reserves up to 1-10 MMBO encountered in the study area. Complicating the role of structure are stratigraphic complications such as lenses of bioclastic units, fractures, bitumen-plugging, karst development, Rierdon shale thickness, and Mesozoic valley fills.

The best pools are located where secondary migration of oil, via hydrodynamic flow, has focused petroleum accumulations into the vicinity of structural highs (e.g., Reagan, Del Bonita-Landslide Butte, Claresholm).

South of the Vulcan Low the vertical seals to Mississippian pools are Jurassic shales. Where the shales are less than 25 metres thick, hydrocarbons have breached the seal and charged Lower Cretaceous reservoirs (e.g., Cut Bank, Long Coulee, Carmangay). In some cases heavier oils (25-28o API) are still produced from the Mississippian where Jurassic shale thicknesses range from 0-25 metres (e.g. Blackfoot, Red Creek). Where Jurassic shales exceed 25 metres, light oils (37-40o API) are encountered within Mississippian reservoirs (e.g. Reagan, Del Bonita, Claresholm).

North of the Vulcan Low, light oil and gas are found within Mississippian units which lack any Jurassic shales as a vertical seal (e.g., Gladys, Herronton). However, these pools possess intra-Mississippian permeability barriers such as tight brecciated units, which trap hydrocarbons significantly below the top of the Mississippian.

Several play types have been identified for the Mississippian succession in the study area. A conceptual play type can be found within the bioherms (Waulsortian mounds) of the Lodgepole/Banif. These features are anticipated to occur at the base of major clinoforms near major basement structures. In these deposits the flank facies, formed of crinoidal grainstone, are the prospective reservoirs, whereas the central mound units, though forming paleotopographic highs, are formed of tight, fractured mudstones. The dominant play type is formed in the porous and permeable blankets of Rundle/Livingstone/Mission Canyon units where they intersect northwest-plunging antiforms. Up-plunge rollovers (i.e. to the southeast) on these structures are necessary for entrapment. Where a regional Mississippian hydraulic low coincides with these structures and Jurassic shale thicknesses are greater than 25 metres, the best potential trends occur. Multiple pools should be found along the strike of these trends (e.g. Landslide Butte – Del Bonita – Spring Coulee trend). In the vicinity of the Mississippian hydraulic low, 54 bypassed and/or damaged Mississippian zones have been identified. Furthermore, many wells have tested Cretaceous targets on prominent structures where the Rierdon is over 25 metres thick, but have terminated drilling in the Mesozoic. These locations may represent potential Mississippian leads.