Reservoir Architecture Of A Fine-Grained Turbidite System: Lower Triassic Montney Formation, Western Canada Sedimentary Basin

Moslow, T.E., Ulster Petroleums Ltd., 1900, 144 - 4th Avenue S.W., Calgary, Alberta T2P 3N4

Abstract

The Lower Triassic Montney Formation of west-central Alberta has been widely recognised as the first and only documented turbidite reservoir in the western Canada Basin. First discovered and exploited in 1993, this play has yielded over 1.5 TCF of gas, plus liquids, and continues to be an active and expanding exploration play. Extensive well and core control provides the basis for constructing a sequence stratigraphic framework and reservoir architecture of these fine-grained sandstone turbidites. Within the lowstand systems tract of the mid-Montney, turbidite channel, lobe/sheet-sand and levee-overbank facies associations are recognised. Reservoir quality and heterogeneity of these facies are predominately a product of hydrodynamic processes of deposition.

A distinct ramp-"edge" or break-in-slope trends north-northwest/south-southeast and defines the up-dip depositional limit of turbidite facies. The ramp-edge orientation is fault controlled and marks the onset of rapid and abrupt thickening of lowstand facies associations. Due to the up-dip headward retreat of turbidite channels, the ramp-edge is highly modified by mass-wasting processes. Turbidite channels coalesce down-dip towards the base of slope and then grade into turbidite lobes b asinward. Turbidite channels are associated with significant lateral discontinuity along depositional strike due to cross-cutting but have lateral continuity along dispositional dip. Turbidite channel facies associations can be amalgamated to thicknesses of 30 m, due to synsedimentary tectonism within localized grabens or half-grabens. The distribution of levee overbank or channel-margin facies associations constrains the width of turbidite channels to a few hundred meters. In addition to inferring proximity, a levee-overbank facies association can be used to infer the direction of the associated turbidite channel through dip-meter log patterns. Turbidite lobes have a broad aerial extent, up to 58 km² in size, occur at the down-dip depositional limit of turbidite facies deposition and thin and grade basinward to shaly siltstones.

Outcrop exposures of channel and lobe/sheet-sand facies associations provide the basis for accurate sedimentologic interpretation of cored successions and prediction of reservoir geometries in the subsurface.