Abstract

The Rocky Brook Formation (Deer Lake Group) includes lacustrine and fluvial mudstone, sandstone, dolostone and off shale deposited in the Early Carboniferous fault-bounded Deer Lake Subbasin. Over the past century, the occurrence of oil shales, solid bitumens, and oil and/or gas shows from wells have suggested that these rocks have petroleum potential. The Deer Lake Subbasin originated in Late Devonian-Tournaisian time as a distensive, east-dipping half-graben (Anguille Group deposition), which was subjected to late Visean-early Namurian broad pull-apart along a developing transtensional fault system (Deer Lake Group deposition). Later transpressional shear in Namurian-Westphalian time created a narrow, central, uplifted flower structure complex and "lateral basins" 3-4 km deep. The Rocky Brook Formation can be divided into five informal subunits, representing separate depositional systems, which are laterally persistent in the subsurface over the area studied. The succession is interpreted to represent a deepening-upward, fluvial-to-lacustrine trend, related to increasing dominance of subsidence rate over sediment supply. Forty-four subsurface core and five outcrop samples of dark grey mudstone, from four of the five subunits, were subjected to palynological and geochemical analysis. In addition, five bitumen and one gas sample were obtained and analysed geochemically. Organic matter present in the oil shale samples is a mixture primarily of Types I and II, and includes coagulated masses of amorphous material (fecal pellets) and specimens of the alga Botryococcus sp. Less organic-rich intervals contain greater amounts of Types III and IV organic matter. Deposition occurred in a relatively arid climate, in an oxic lacustrine setting, with periods of anoxia. The presence of stratigraphically significant elements of the Grandispora spinosa-Ibrahimispores magnificus Zone throughout the five subunits establishes that the entire Rocky Brook Formation is of Late Visean (V 3 ) age and correlative with the Hastings Formation of the Mabou Group (Cape Breton Island). The Lower Grey Beds and Upper Grey Beds, both interpreted to represent deposition in relatively deeper, quiet lacustrine settings, have potential as hydrocarbon source rocks. Black, laminated, organic-rich (commonly 1-2 per cent, ranging up to 15 per cent Total Organic Carbon), intermittently-anoxic lacustrine mudstones occur in intervals up to 2 m, but commonly 10-50 cm, thick. These immature to marginally mature source rocks at or near surface may have provided the source for oil, gas and bitumen recovered from wells, diamond drillcores and outcrops. More deeply buried equivalents, present under large portions of the area, suggest significant hydrocarbon potential in Deer Lake Subbasin. Likewise, Late Visean correlatives elsewhere in Atlantic Canada and adjacent offshore areas may also have significant, but currently unconfirmed, hydrocarbon potential.

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