Abstract

The Lower Triassic Montney Formation in west-central Alberta is divided into two major third-order sequences separated by a sequence boundary that correlates to an Early Triassic (Smithian/Dienerian) global eustatic sea level fall. This drop in sea level enhanced mass-wasting processes responsible for the formation of a turbidite channel and lobe complex within the lowstand systems tract of the upper Montney sequence. This assemblage of facies constitutes the principal reservoir within the Valhalla-La Glace fields. Most of the turbidite reservoirs facies were derived from the reworking of a southwestward-prograding depositional thick, designated the "Cindy Lobe", of the lower Montney sequence that protrudes basinward from the western extension of the Cindy Graben trend. The Valhalla-La Glace reservoir facies are a complex of turbidite channels and downdip lobes deposited in the subaqueous platform of a prograding lowstand shoreline immediately seaward of a continental ramp slope break. The ramp "edge" trends NNW-SSE through the area, and defines the updip depositional limit of turbidite facies. The rampedge orientation is probably fault controlled and marks the onset of rapid and abrupt thickening of lowstand facies associations. The origin of these facies is attributed to mass-wasting and generation of sediment gravity flows due to substrate instability at the slope break. Individual turbidite channels likely have undergone headward retreat, moving upslope and creating their own supply of sediment for turbidity flows. Lateral facies relationships expressed by turbidite channel, channel margin and levee/overbank facies provide a predictive model for determining proximity to the turbidite channel axis. The turbidite channel facies association contains the highest quality reservoir in terms of porosity and permeability. Turbidite channels can be amalgamated or crosscut one another, thus inducing vertical and lateral permeability barriers and reservoir heterogeneity. However, there is a lateral continuity in lithofacies along depositional strike and dip. The reservoir facies are more continuous along depositional dip. Turbidite channel, turbidite channel margin, and turbidite lobe facies associations are recognized within the study area. The latter is found almost exclusively within the Glacier field. Facies associations and their lateral variability within the study area are analogous to turbidite channels and lobes from the subaqueous platform of the Fraser River delta, providing a process sedimentologic and geomorphic analogue for the Valhalla-La Glace field reservoir facies.

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