Abstract

The Cutbank Sandstone, a basal member of the Mannville Group (Lower Cretaceous) rests unconformably on the Rierdon shale (Jurassic) in the Horsefly Lake Pool, southern Alberta, Canada. The sandstone is a predominantly light-grey, medium to coarse-grained, crossbedded, poor to moderately sorted litharenite. The sandstone contains fining-upward sequences comprising four distinct lithofacies:

a) C1: Conglomeratic Sandstone

b) C2: Crossbedded Sandstone

c) C3: Rippled Sandstone and

d) C4: Rooted Mudstone, Siltstone and Fine Sandstone.

Detailed examination of cores, logs, isopach maps and correlation of cross-sections reveals a distinct fluvial sandstone body geometry. Deposition within braided channels was by aggradation of longitudinal bars resulting in discontinuous elongate sandstone bodies scattered throughout the Cutbank Valley. The Cutbank Sandstone forms a belt up to 24 km wide and more than 160 km long, extending from Great Falls (Montana) to north of Retlaw (Alberta).

Oil production in the Horsefly Lake Pool is obtained from the Cutbank Sandstone reservoir. Reservoir anatomy suggests that it is extremely heterogeneous and contains large quantities of clay clasts and matrix. Internal heterogeneities are related to the deposition of the fine sediments. In this paper, great emphasis is placed on the presence or absence of clay clasts in the lithofacies which directly influences fluid migration in the reservoir sandstone.