Abstract

A detailed facies and architectural characterization of a slope-turbidite channel-complex set is presented (Isaac Channel 5, Neoproterozoic Isaac Formation, Castle Creek area, southern Canadian Cordillera). Isaac Channel 5 outcrops discontinuously across an ~5-km (~3-mi)-wide section, and the exposure selected for the detailed outcrop characterization (termed Castle Creek South) covers an area 675 m (2215 ft) wide by 75–100 m (246–328 ft) thick, or about 5.8 x 10⁴ sq m (~6.2 x 10⁵ sq ft).

In Castle Creek South, Isaac Channel 5 is comprised of three main facies assemblages: 1) sandstone-dominated channel-fill facies, 2) mudstone-dominated, thin-bedded overbank facies, and 3) muddy debrites and slump deposits. Channel-fill facies make up 68% of total exposed strata and occur in three vertically stacked, multistory channel complexes (8–30 m [26–98 ft] thick). Strata are dominated by amalgamated sandstone (56%) having the best reservoir characteristics, with lesser interbedded sandstone and postdepositionally brecciated mudstone-rich layers (26%), interbedded sandstone- and mudstone-rich strata (15%), and structureless sandstone (3%). Thin-bedded overbank facies cover 23% of the exposed area. Most of these strata (71%) consist of laterally persistent turbidite beds a few cm (in.) thick, having moderate to poor reservoir characteristics. This facies drapes the channel complexes and exhibits a fining- and thinning-upward trend, inferred to reflect local channel-complex abandonment. Less commonly (18%), thin-bedded overbank facies are observed to interfinger with channel-fill facies. These strata, which are interpreted to be inner-bend-levee deposits, are muddier and thinner than turbidites in the abandonment facies and have poor reservoir characteristics. This mudstone-rich facies occurs adjacent to channel fills and should be considered when evaluating levee deposits as potential reservoir targets. Muddy debrites and slump deposits are good stratigraphic markers within Isaac Channel 5 and cover the remaining 9% of the exposed area. These unstratified, mudstone-rich units have poor to no reservoir potential and very likely represent impermeable barriers to fluid flow.

Sandstone-rich channel-fill facies within a channel complex are uncommonly vertically separated by barrier-type facies that are represented by thin-bedded facies and slump deposits. Where present, these barrier-type facies do not extend across the full length of the outcrop. Composite channel-fill sandstone within a discrete channel complex, therefore, forms a laterally and vertically contiguous reservoir unit interrupted locally by discontinuous (<500 m [<1640 ft] long) permeability barriers. Individual channel complexes, however, are vertically separated (compartmentalized) by laterally persistent muddy debrites and thin-bedded deposits that would constitute kilometer-scale barrier-type facies within a multilayer reservoir (~100 m [~330 ft] thick).

Although this paper focuses on Isaac Channel 5 facies distribution and architecture in the Castle Creek South area, preliminary observations outside this area suggest that neither stratal architecture nor facies proportion change significantly across an inferred ~2.5-km (~1.6-mi)-wide axis-to-margin transect.