5.3 Pennsylvanian – Riversdale, Morien & Pictou Groups

Riversdale Group

Within the onshore Carbon Capture & Storage Nova Scotia No. 1 well, Weston et al. (2017d) discovered an approximately 80 m thick sequence of thick fluvial sandstones and minor shales dated as Westphalian A (Langesettian). It was bounded above by the Westphalian B/A unconformity, and below by the major mid Carboniferous (Westphalian-Namurian) regional unconformity. Lithological correlations extended it into the nearly (6 km) Birch Grove No. 1 well. The lithologies are similar to the Silver Mine Formation defined by Boehner and Prime (1993) and are also of Westphalian A age. It is not observed in any of the offshore wells.

A similar aged and unconformity-bounded succession is present on the Nova Scotia mainland originally designated as the Riversdale Group. Boehner and Giles (2008) note that the revisions in Carboniferous stratigraphy had the Riversdale had long been incorporated into the underlying Mabou Group (Belt, 1964; 1965). For sake of convenience, these rocks are placed under the old Riversdale Group nomenclature. Regarding their source rock potential, the unit’s dominantly sandstone lithologies and thin nature negate its potential, at least in the region surrounding the well. Its existence, facies development, and source potential elsewhere in the basin is unknown.

Morien Group

A major unconformity divides the Mississippian and Pennsylvanian successions in Atlantic Canada region. It was a precursor to the subsequent Alleghenian Orogeny, and representing a period of regional transpression and erosion, also coinciding with global sea level lowering related to Gondwanan glaciation (Gibling et al., 2008). Basin-bounding faults were re-activated with associated normal and thrust faulting, and structural inversion of some basins or parts thereof. Though variable depending on location and basin, the maximum amount of erosion encapsulates sediments of late Namurian A to early Westphalian B (approximately late Serpukhovian – late Bashkirian). Older strata exhibit several degrees of angular discordance though locally near active faults could be significant. This is the case over the Pennsylvanian in the Sydney Basin, with unconformities / faunal breaks recorded in the early Westphalian B/A, and late Westphalian C/B.

The Morien Group defines a period of alluvial, fluvial and lacustrine sedimentation with the succession fining upward over time. It represents long-lived regional drainage system sourced from the southeast that filled the Maritimes Basin that was tectonically stable and undergoing gentle thermal subsidence (Rust et al., 1987; Gibling et al., 1992). The succession in the Sydney Basin is up to 1.8 km thick onshore and at least 2.5 km thick offshore (Boehner and Giles, 2008). Sediment loading in the region contributed to salt diapirism of the lower Windsor Group.

The basal South Bar Formation is composed of thick, stacked fining upward cycles of grey sandstones with minor shales and rare though significant coal beds deposited by braided river systems over a broad, sandy braidplain. The upper Sydney Mines Formation reflects the transition to a muddy alluvial floodplain with large meandering rivers, lakes and coal swamps, with some minor marine influences (Rust et al., 1987). Its strata are composed of repeating cycles of grey to red mudrocks, fine grain grey to brown sandstones and thick coals.  Between the South Bar and Sydney Mines is the Waddens Cove Formation. It is a transitional unit between the two formations and present only on the eastern side of the basin (Boehner and Giles, 2008).

The two onshore wells in the basin were spudded in and penetrated a near-complete Morien Group section. No hydrocarbon shows were recorded and no geochemical analysis completed. Offshore, the North Sydney P-05 and F-24 wells penetrated the entire Morien succession, though it is thinner since here it overlies an uplifted horst block of possibly Horton Group sediments (this report). In both wells significant gas shows were present in the Morien Group, and are believed sourced from associated coal seems (this report). Several analyses were done for these wells but limited to a handful of samples from each formation (Tables 5.6 to 5.8). Overall, they indicated the selected shale samples to have variable TOCs (<1.0-18, average ~ 1.5%) with Type III kerogens.  They were generally all within the oil window, and commensurately mature based on the stratigraphic position, but had low hydrogen indices.

Fowler and Webb (2017) reviewed these data from previous workers studying onshore exposures, boreholes and mine workings, and grouped the analyses of Sydney Mines samples based on lithology: coals, coaly shales and shales. They are summarized in Table 5.1. Within coal seams are thin beds – ‘dull coals’ – that are organic rich high in lipitinites (Type II kerogens) with high HIs though are cumulatively thin.  Likewise, some shales (oil shales) associated with coals have very high TOCs and related properties (high HIs). These could have significant oil potential, though volumetrically are low, at least as expressed in onshore exposures. The best potential sources are lacustrine shales. These have a greater cumulative thickness and are composed of Type III terrestrially-derived kerogens with low TOCs and moderate HIs. They are the probable sources of gas encountered in wells and mine workings. These potential source rocks likely have significant areal extent and probably greater thicknesses in the basin depocentre, and could be a significant source of gas.

Pictou Group

Sediments of the Stephanian to Middle(?) Permian Pictou Group conformably overlie the Morien from which it is transitional. Its upper age and original thickness are unknown due to significant erosion. It is made up of alluvial and eolian redbeds and found only offshore in the Sydney Basin being penetrated by the two North Sydney wells. It crops out onshore in northern Nova Scotia, Prince Edward Island and from seismic data estimated to be about 6 km in the offshore Magdalen Basin.  It is essentially barren of organic material though its sandstones are porous with good reservoir potential.