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The AAPG/Datapages Combined Publications Database
CSPG Bulletin
Abstract
Turbidite Reservoir Facies in the Lower Triassic Montney Formation, West-Central Alberta
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 constitues 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 ramp-edge 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.
RESUME
La Formation Montney du Trias inferieur du centre-ouest de l'Alberta est divisee en deux sequences majeures de troisieme ordre, separees par une limite de sequence
qui est correlative d'une chute eustatique globale du niveau marin au Trias precoce (Smithien-Dienerien). Cette chute du niveau marin a accentue les processus de mouvement de masse responsables de la formation d'un chenal a turbidites et d'un complexe de lobes a l'interieur du cortege de depots associe a un bas niveau marin de la
sequence
de Montney superieur. Cet assemblage de facies constitue le reservoir principal des champs Valhella-La Glace. La plupart des reservoirs du facies de turbidites sont derives du remaniement d'un epaississement de depots progradant vers le sud-ouest, designe sous le nom de "Lobe Cindy" de la
sequence
inferieure de Montney qui forme une protuberance en direction du bassin, a partir de l'extension ouest du graben de Cindy.
Le facies du reservoir Valhalla-La Glace est un complexe a chenaux de turbidites et de lobes a pendage aval, depose sur la plate-forme subaquatique, et un littoral de bas niveau marin en progradation, situe immediatement en direction de la mer sur le bris de pente d'une marge continentale. Le "bord" de la rampe s'oriente NNW-SSE a travers la region et definit la limite de depot amont pendage du facies de turbidites. L'orientation du bord de la rampe est probablement controlee par faille et marque le debut de l'epaississement rapide et abrupt des associations de facies de bas niveau marin.
End_Page 507------------------------
L'origine de ces facies est attribuee au mouvement de masse et a la generation de coulees sedimentaires de gravite dues a l'instabilite du substrat au bris de la pente. Les chenaux individuels de turbidites ont probablement subi un recul a leur tete, se deplacant en amont de pente et creant leur propre source de sediment de coulee de turbidites.
Les relations laterales de facies exprimees par les facies de chenaux de turbidites, de marge de chenal et de levee/plaine d'inondation fournissent un modele predictif pour determiner la proximite de l'axe du chenal de turbidites. L'association du facies de chenal de turbidites contient les reservoirs de la meilleure qualite en termes de porosite et de permeabilite. Les chenaux de turbidites sont amalgames et se recoupent les uns les autres; par ce fait, elles induisent des barrieres de permeabilite laterale et verticale et l'heterogeneite du reservoir. Toutefois, il y a une continuite laterale des lithofacies le long de la direction et du pendage de depot. Les facies de reservoir sont plus continus le long du pendage de depot. Les associations de facies de chenal de turbidites, de marge de chenal de turbidites et de lobe de turbidites sont reconnus dans la region d'etude. Le dernier facies se retrouve presqu' exclusivement a l'interieur du champ de Glacier.
Les associations de facies et leur variabilite laterale a l'interieur de la region d'etude sont analogues aux chenaux de turbidites et les lobes de la plate-forme subaquatique du delta de la riviere Fraser, qui fournit l'analogue du processus sedimentaire et geomorphologique des facies de reservoir du champ Valhalla-La Glace.
Traduit par Lynn Gagnon
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