Abstract

The Late Devonian Canol Formation and associated stratal units in the Northwest Territories of Canada illustrate the expression of sequence-stratigraphic surfaces and units in a distal Paleozoic carbonate shelf to relatively deep-basin setting. These Devonian strata are time equivalent and quite analogous in depositional conditions and petroleum-system characteristics to the proven shale-gas deposits in the Horn River Basin of northeastern British Columbia.

The Canol Depositional Sequence spans the upper part of the Ramparts Formation, the Canol Formation, and the lowermost part of the Imperial Formation. This depositional sequence comprises a range of facies associations, spanning siliceous, calcareous, and argillaceous fine mudstone to sandy mudstone. Eight distinct facies associations can be identified, correlated, and mapped within the study area. These form the building blocks of a predictive understanding of the depositional system that is critical for resource identification, delineation, and development. These facies associations vary in a systematic and predictable manner both vertically and laterally within a sequence-stratigraphic framework. The strata record the interplay of intrabasin organic production (phytoplankton) and associated growth of zooplankton (mainly radiolaria and tentaculitids) versus extrabasinal detrital siliciclastic input (dominated by clay minerals), as well as input of carbonate sediment-gravity flows from pre-existing or age-equivalent platforms and buildups along the basin edges. The lowstand systems tract of the Canol Depositional Sequence contains mostly argillaceous-calcareous mudstone facies association with some transported benthic-carbonate material. The transgressive and lower highstand systems tracts contain the best reservoir potential as their composition is dominated by biogenic silica and organic matter. The upper highstand systems tract is influenced increasingly through dilution by detrital clay and silt, resulting in lower reservoir potential and higher seal potential. Laterally, time-equivalent strata are more biosiliceous away from input points of terrigenous clay minerals into the basin.

Insights gained from constructing the sequence-stratigraphic framework were applied across the stratigraphic hierarchy from bed to sequence set, allowing integration of data from thin-section to seismic scale. Sequence-stratigraphic analysis was an important part of seismic inversion and rock-property mapping because it reveals genetically related rocks. This enables application of process-based models and incorporation of regional context and information about sedimentary provenance, paleogeography, and paleo-oceanography that are not discernible from seismic data alone.