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Facies associations, sedimentary cyclicity and compartmentalization of the MC-1 Member (Mississippian), Tilston Field, southwestern Manitoba

Department of Geology, Brandon University, Brandon, MB R7A 6A9

The sedimentary rocks of the Mississippian Mission Canyon-1 (MC-1) Member (Tilston Beds) of the Tilston Field, southwestern Manitoba, Canada, were deposited on a low-relief carbonate platform along the northeastern margin of the Williston Basin. Three oil pools occur in the Tilston Field: Mission Canyon 1 A (MC-1 A), Mission Canyon 1 C (MC-1 C) and Mission Canyon 1 D (MC-1 D). The rocks of the MC-1 Member are broken into five facies associations, which stack to form five shallowing-upward subtidal carbonate cycles (TB1, TB3 to TB6) and one shallowing- to deepening-upward subtidal carbonate cycle (TB2).

Each cycle was deposited in water depths that varied from fair weather wave base to slightly below storm wave base. Well log analysis of the uppermost Lodgepole Formation indicates at least four shallowing-upward subtidal carbonate cycles (L1 to L4). Argillaceous-rich wackestone to mud-stone intervals occur in the basal part of each cycle. Each cycle of the MC-1 Member and uppermost Lodgepole Formation can be correlated across the study area. The MC-1 Member and Lodgepole cycles show a trend of increasing total porosity and decreasing argillaceous content from south to north across the Tilston Field. Cycles TB4 and TB5 have the lowest argillaceous content and the highest porosity, and are the main oil producing units in the Tilston Field. The lack of argillaceous material and the relatively high porosity suggests that reservoir compartmentalization is highly unlikely across the TB4 to TB5 boundary. In contrast, the basal parts of cycles TB3 to TB1 and L1 to L4 are argillaceous rich and characterized by fair total porosity. It is likely that these argillaceous-rich zones form barriers to the vertical movement of fluids, thus compartmentalizing the aquifer.

In the absence of vertical fractures, the dominant drive mechanism in the Tilston Field oil pools is likely to be edge water drive. Progressive erosional truncation of the MC-1 Member cycles to the north, combined with the presence of two structural saddles, effectively reduces the areal support of the edge water aquifer from south to north. The MC-1 A Pool is likely to have the strongest aquifer support, whereas the MC-1 D Pool has the weakest aquifer support. Significant differences in horizontal well productivity can be partially attributed to the relative strength of the aquifer in each pool.

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