Outcrop characterization of a passive-margin, channel-complex set: Isaac Channel 5, Neoproterozoic Isaac Formation, British Columbia, Canada

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 crops out discontinuously across a ~ 5-km (3.1-mi)-wide section, and the exposure selected for the detailed outcrop characterization (termed Castle Creek south) covers an area of 675 m (2210 ft) wide by 100-75 m (330-246 ft) thick, or about 5.8 x 104 sq m (~ 6.3 X 105 sq ft). In Castle Creek south, Isaac Channel 5 is made up 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, multi-story channel complexes (8-30 m (25-100 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%), which consist mostly of a few cm (0.4-9 in)-thick, laterally persistent turbidite beds with moderate to poor reservoir characteristics, drape the channel complexes and exhibit an upward-fining and -thinning 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 taken into account 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 units (i.e., barriers) to fluid flow. Sandstone-rich channel-fill facies within a channel complex are uncommonly vertically separated by barrier-type facies (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 long) permeability barriers. Individual channel complexes, on the other hand, are vertically separated (compartimentalized) by laterally persistent muddy debrites and thin-bedded deposits that would constitute kilometer-scale barrier-type facies within a multi-layer reservoir (~ 100 m (330 ft) thick). Although this contribution focuses on Isaac Channel 5 facies distribution and architecture in the Castle Creek south area, preliminary observations outside this area suggest that stratal architecture nor facies proportion change significantly laterally across an inferred ~ 2.5-km (1.6-mi)-wide axis-to-margin transect.