Lower Mannville

Lower Mannville Study, Southwestern Alberta and Northwestern Montana

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Summary:
Location: T1-18, R20W4-30W5
Strata: Mannville
Year of Study: 1992

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Introduction

An integrated surface, subsurface and hydrodynamic evaluation of the Lower Mannville Formation and its equivalents within southwestermost Alberta has determined the controls on existing production, as well as pointing out prospective areas for additional drilling.

The study embraces Townships 1-18, Ranges 20-30 W4M. It is augmented by the study of areally small, densely drilled areas within the Cut Bank Field of northwestern Montana, 31 outcrops located between Turner Valley and the Crowsnest Pass, and a series of outcrops found within and near the Missouri Gorge of the Great Falls, Montana, area. The subsurface data base includes logs, cores, and drill stem tests from over 1800 wells.

The structural grain of southwestern Alberta displays a dominant regional dip to the southwest and west. Superimposed upon the regional dip are a series of northwest-plunging spurs emanating from the Sweetgrass Arch. These structures form a series of second order residual highs upon which much significant production is derived from Mississippian, Lower Mannville and Bow Island reservoirs. Superimposed upon the major structures are localized sags interpreted to signify areas of karsting or salt solution.

The lowest Mannviile-age units are the western, pebbly Cadomin conglomerates which can be correlated to cherty, cross-bedded, locally pebbly sandstones found within the Cut Bank Member at the surface and in the subsurface at Cut Bank Field and also to the so-called “Sunburst” units at Long Coulee Field. These rocks represent a depositional continuum with alluvial fans and coarse grained rivers draining to the east and joining a meandering master trunk river which flowed to the north. The resulting deposits formed a regionally extensive, porous coarse clastic package located upon a dissected Jurassic surface which included prominent escarpments variously formed of impervious and permeable material. The eastern edge to the cherty sandstones and conglomerates is very pronounced.

Regional base level rise followed lowermost Mannville deposition and resulted in the following general vertical pattern of lithofacies: bentonitic mudstone and pebbly mudstone; limestone, mud rocks and calcareous sandstone; sanding-upwards cherty sandstone/shale successions. The vertical package of lithologic types can be traced northwards from Montana outcrops to the northern reaches of the study area; it is absent In western outcrops. These deposits reflect the damming of trunk streams by cross valley alluvial fan construction, which resulted in debris flow, lacustrine and crevasse splay deposition.

A regional unconformity separates this basal Mannville package from overlying tidally- and wave-influenced quartzose sandstones, which were derived from the eastern craton. Breaking up the lateral extent of these deposits in the centre of the study area are channelized estuarine deposits trending generally north-south. Other similar channelized units are observed on the northeast side of the study area. These channelized tracts were conduits of coarse clastics which were subsequently molded into tidal flats and shorefaces in the receiving basin to the north.

Subsequent to deposition of the quartzose reservoirs, fluvial tracts characterized by large, sandy channelized units were formed. These bodies cut through pre-existing units, but rarely extend down to the level of the Jurassic surface. The cross-cutting sediments are lithic, invariably tight and create up-dip stratigraphic traps within older porous units. Many of the thickest quartzose reservoir zones found at Long Coulee are located in close proximity to lithic channel fills.

The maximum period of regional depositional quiescence occurred subsequent to the creation of the lithic channel fills and is defined by bentonitic and carbonaceous mudstone sandwiching a limestone.

The stratigraphic package observed at Long Coulee and throughout much of the southwestern Alberta subsurface appears directly correlatable to surface exposures found in the Great Falls area located over 150 km to the south. At Great Falls, one observes westerly-derived cherty cross-bedded fluvial sandstones capped by a prominent limestone (Cut Bank Member) which In turn is overlain by the following succession: sanding upwards lithic sandstones (Cut Bank Member); an unconformity with locally preserved paleosols; quartzose, tidally – influenced sandstones (Sunburst Member); red sandstones and mudstone with large lithic channel fills (Red Sandstone Member); laterally extensive, thin limestones (Calcareous Member); locally developed coarsening-upwards shoreface sandstones (Moulton Member). This package can be correlated throughout the subsurface of southwesternmost Alberta where one observes the following vertical succession: fluvial, cherty cross-bedded, pebbly sandstones capped by thin calcareous units; sanding upwards lithic sandstones; an unconformity abruptly overlain by wave and tidal-influenced quartzose sandstones; muddy sediments locally interspersed with lithic channel fill sandstones; regionally extensive quiet water coaly mudstone and limestone.

A fluid continuum also exists within the cherty sandstones and conglomerates, extending from Cut Bank Field north into southwestern Alberta. Dynamic flow is suggested by large fluid potential differences, systematic variations in fluid chemistries and tilted fluid contacts. The western, deepest portions of the basin display an apparent “deep basin” configuration with tight, gas-saturated reservoirs. However, light oil recoveries from depths greater than 3000 metres indicate that this region should not be considered barren of liquid hydrocarbons. Flow cell separation within basal Mannville clastic units can be correlated to present topography, paleodrainage divides, recent (Laramide) structures and local facies changes. Oil quality is generally similar at Cut Bank and Long Coulee Fields, with the highest quality oils displaying a positive correlation to the presence of Rierdon shales. The absence of Rierdon shale generally correlates to oil degradation due to contact with waters coming out of Mississippian aquifers. Several prospective areas are present within the cherty sandstones, with the best located in the vicinity of large basement structures.

Regionally extensive play fairways are present in northwest Montana and southwest Alberta. Light oil can be anticipated to occur in the cherty basal Mannville deposits wherever: positive basement structures with four-way dips occur; high escarpments cut into Rierdon shales are oriented at an appropriate angle to regional dip; deeply cut lithic channel fills and intra – Cut Bank facies changes (i.e. abandoned channel plugs) occur. The quartzose units form regionally extensive gas reservoirs with local oil legs; traps are formed by facies changes (e.g. lensing and updip terminations against lithic channel fills). Drape over prominent basement structures may also form appropriate trapping conditions. In any zone, most gas reservoirs are associated with downdip oil columns. Several damaged and bypassed wells have been recognized throughout the lower Mannville section.