Geological controls on reservoir distribution in the Lower Cretaceous Basal Quartz, Chin Coulee–Horsefly Lake area, south-central Alberta
In much of southern Alberta the Lower Cretaceous Basal Quartz is a complex assemblage of mostly nonmarine sandstone and mudstone. Within the study area, which is centred on Horsefly Lake and Chin Coulee pools (twp. 7–9; rge. 14-17W4), the BQ comprises a minimum of five unconformity-bounded sequences that are differentiated by a unique assemblage of attributes, including mineralogy, micro- and macroscopic sedimentological textures and structures, reservoir-quality characteristics, and production history. In stratigraphic succession, these sequences are informally termed the “A” sandstone, “weathered Horsefly” sandstone, “unweathered Horsefly” sandstone, “old BAT” sandstone, and “young BAT” sandstone.The oldest strata consist of the “A” and “weathered Horsefly” sandstone. As a result of extensive pedogenic alteration, primary sedimentary characteristics and reservoir quality have been largely destroyed. Reservoir development in these strata is the result of diagenetic leaching, and is limited in areal extent (limited reservoirs). The younger “unweathered Horsefly” sandstone, or simply “Horsefly” sandstone, and “BAT” sandstones form the principal reservoirs in the study area. However, in spite of their different ages, a consistent vertical assemblage of lithofacies exists for each sandstone unit a braided fluvial succession several metres thick (primary reservoir strata) is overlain abruptly by a several-metre-thick meandering fluvial succession. This consistent upward change is interpreted as resulting from repetitive changes in sediment supply and its control on the nature and spatial patterns of sedimentation.Although the Basal Quartz was deposited over about 18–32 my, it is less than ~ 25 m thick within the Chin Coulee–Horsefly Lake study area. This suggests that Basal Quartz sediment accumulated under highly accommodation-limited conditions, which in turn caused net sedimentation rates to be very low. One of the most significant consequences of this limitation was the development of a particularly complex stratigraphic succession. Because of the limited accommodation space, each unconformity-bounded succession tends to incise from a similar stratigraphic horizon, which therefore requires careful regional mapping to determine relative ages. In additon, younger stratigraphic successions eroded some or most of the commonly extensively pedogenically altered older ones. This resulted in the formation of an extremely complex succession of strata and a highly compartmentalized reservoir system.