5.2 Mississippian – Horton, Windsor & Mabou Groups

Horton Group

The Tournasian Horton Group is found in all four Atlantic Canadian provinces, and is the first post-Acadian Orogeny sedimentary succession extending across all Appalachian terranes in the region. It was deposited in an inter-montane setting created through complex transtensional tectonism within which were rapidly subsiding grabens and half grabens. Though there are local variations, in a broad sense, the group displays a tripartite subdivision with a basal alluvial-fluvial succession, followed by underfilled deep lake systems, and capped by fluvial strata. 

On Cape Breton Island, the Horton is subdivided in ascending order into the Craignish, Strathlorne and Ainslie formations respectively. However, in the Sydney Basin, Horton sediments are limited to the basin margin conglomeratic alluvial fan facies of the Grantmire Formation (Boehner and Giles, 2008). The Grantmire is most probably a diachronous succession, and laterally equivalent to the three basinal units. These probably exist in the subsurface there, though the paucity of onshore seismic lines has yet to confirm this.

As summarized in Table 5.1,organic-rich Horton (Strathlorne) lacustrine successions on the Island may have a cumulative maximum thickness of up to 100 m thick. The formation is composed mostly of grey to dark grey mudstones with lesser amounts of fine grain sandstones and rare thin carbonates.  Beds within the mudstones can be very rich, with TOC ranges from 2-20% with Types I and II kerogens. Hydrogen indices can be high where it remains immature.

Horton sediments are the proven and most prolific source interval in the Maritimes Basin (Dietrich et al., 2011; Fowler and Webb, 2017). In southeastern New Brunswick’s Moncton Subbasin it is the source for the McCully field’s gas production, and current and historic oil and gas production at the Stoney Creek field (see Section 2: Exploration History). In various locations throughout the region, especially rich intervals are recognized as oil shales and were mined for petroleum extraction. In the Lake Ainslie area of west-central cape Breton, oil shows from Horton sediments are known from oil seeps, well bores and outcrops (Bell, 1958; McMahon et al., 1986).

Liquid petroleum is associated with several base metal deposits in Nova Scotia preserved in inclusions and porous rocks of the basal Windsor Group carbonate, the Macumber Formation (Sangster et al., 1998). For example, at the Walton Ba-Pb-Zn-Cu-Ag deposit on the mainland, its Horton source is confirmed through geochemical analyses and was formed by thermal maturation of proximal Horton organic rich source rocks by hot mineralizing fluids (Kontak and Sangster, 1995). A similar situation is recorded at the Jubilee Pb-Zn deposit in the Bras d’Or Lake region of central Cape Breton (Rogers and Savard, 2002).

Fowler and Webb (2017) sampled and analysed Strathlorne shale lithologies from several locations at the northern tip of Cape Breton Island in the Cape St. Lawrence area and compared them with earlier assessments (references therein). The rocks were found to have TOCs mostly greater than 1.00% (~0.27-6%) but variable HIs (<100-354 mg HC/g TOC) and S2 values. Those with elevated Tmax values (~450-485°C), low S2 and HI values indicated thermally maturity to overmaturity (Meat Cove and Salmon River). Conversely, samples from another site (e.g. Bay Road Quarry) were less mature with higher parameter values. Biomarkers confirmed Type II and some Type I kerogens from algal-rich lacustrine facies.

Though not penetrated in the on- and offshore parts of the Sydney Basin, Horton Group sediments most probably exist in the basin.  Development of the Middle Horton’s Strathlorne Formation lacustrine facies (or its equivalent) is uncertain, though if present would be an excellent source rock prone to generating liquid hydrocarbons, and gas with increasing maturity. Its distribution is confined to faulted depositional lows that through time may have become interconnected. However, consideration must be given to the fact that the Horton has undergone several tectonic events since its deposition: burial, faulting, uplift, faulting / folding, erosion, burial. All would have an impact on its burial history and timing of maturation and migration into coeval / adjacent or overlying reservoirs. Early inverted Horton basins adjacent to local and/or major faults would offer the best potential for retention of organic matter and attenuated maturation over time.

It should also be noted that the Horton outcrops described above are located in small depositional troughs within the Cabot Fault System.  This terrane-bounding fault system extends across the Cabot Strait to Newfoundland and separates the Sydney and Magdalen basins.  Its positive expression within the Strait was the target of the Petro-Canada St. Paul P-91 well (see Section 2: Exploration History). It is thus difficult to determine which basin these outcrops are associated with. Biostratigraphy has confirmed that the well did not penetrate any Horton or younger Windsor group sediments (Weston et al., 2017).

Windsor Group

Strata of the Viséan Windsor Group conformably to unconformably overly the Horton Group, with the latter having in some cases near active faults undergone modest deformation. Its areal extent is greater that the Horton and displays an onlapping relationship with underlying and marginal rocks. The Windsor records the first – and only – significant marine incursion into the Maritimes Basin with sediments recording deposition in deep water to peritidal settings. 

The Windsor Group is divided into five major depositional cycles and grouped into a lower (2) and upper (3) successions, and like the Horton shows some lateral variability in the different subbasins of the region.  The lower Windsor records rapid filling of the (likely) sub-sea level basins and transgression of the basal carbonate – the Macumber and Gays River formations – reflecting deposition in deep water slope to below wave-base outer shelf environments respectively (Lavoie and Sami, 1998). This was followed by deposition of thick anhydrites and later salt, both under deep water conditions with the basin rapidly shallowing due to evaporative filling (Lavoie, 1995).  The Macumber has not yet been discovered in the Sydney Basin though the four remaining depositional cycles are present with a maximum thickness of 1500 m (Boehner and Giles, 2008). The Upper Windsor represents deposition in the now shallow water and gently subsiding basins, with three cycles defined represented by a series of thin cyclic, transgressive-regressive carbonate-siliciclastic (and occasionally evaporitic) successions. Their formation is probably due to orbital forcing and related Gondwanan glaciation (Giles, 2009). Contact with the overlying Morien Group is generally transitional and conformable.

Potential source rocks are present within the entire Maritimes Basin Windsor succession though are volumetrically small with minor organic shales present in the upper Windsor with very good potential (Fowler & Webb, 2017). In such cases, the beds’ areal extent is not known and could have limited local expression. In the onshore Sydney Basin, a review of previous analyses and new analyses from borehole samples by Mukhopadhyay (2004) inferred such rocks contained Types II-IV kerogens (mostly III-IV) with low TOC values. Fowler and Webb’s review (ibid) generally concurred with this assessment though their analysis of several samples showed better potential. New sampling and analyses of borehole cores and outcrops of upper Windsor rocks by Fowler and Webb (ibid) found that they generally composed of Type III kerogens with low-moderate TOCs (1.0-2.5%), low to fair HIs (100-200 ppm) and variable maturities.

Only the St. Paul P-91 well penetrated about 700 m of the Windsor that was limited to the Upper Windsor (equivalents to the Woodbine Road and Kempt Head formations). Analysis of recovered lithologies by Kendall and Altebaeumer (1984) indicated that all successions drilled had reached a very high level of thermal maturity with little to no generative potential for hydrocarbons (Tables 5.2 and 5.3)

The better source rocks are present in its lower part: the Macumber and overlying evaporite succession. The Macumber contains thin-bedded limestones and dolomites with organic-rich shale beds. It has a regional distribution throughout the Maritimes Basin though is relatively thin with variable thicknesses (3-30 m; Lavoie and Sami, 1998). However, in the undrilled deepest parts of the Maritimes and Sydney basins it could be thicker. As shown in Table 5.1, they are composed of Types I and II kerogens (in varying proportions), with have high average TOC (5%) and hydrogen indices (450 ppm). Thin (cm) organic-rich dolomite and shale beds are common within the overlying thick anhydrite and salt interval and observed in outcrops, quarries and mines (salt) with in some cases liquid petroleum. Their contribution to known petroleum systems is modest though should be greater in the centre of the Maritimes Basin salt province offshore western Cape Breton Island.

Oil staining is common in Windsor group onshore Cape Breton Island, and there are a significant number of documented oil shows with a selected examples described in Section 2: Exploration History (see also McMahon et al., 1986; Mukhopadhyay, 2004). Numerous seeps and shows have been reported from outcrops, water wells, and mineral exploration borehole cores (Bell, 1958, McMahon et al., ibid). The latter are associated with Pb-Zn (plus other base metals) hydrothermal mineral deposits in the region hosted within the basal carbonate Macumber Member. In central Cape Breton, the Jubilee deposit is the best documented (Rogers and Savard, 2002). Indeed, the majority of such oil discoveries were inadvertent during exploration for these mineral deposits.

Analysis of oils and staining found in presumably Macumber carbonates from borehole cores from a number of locations in Cape Breton (e.g. Lake Ainslie, Jubilee Pb-Zn deposit, Malagawatch) was done by Mukhopadhyay (2004) and Fowler and Webb (2017).  They reached broadly similar conclusions that these hydrocarbons had a terrestrial lacustrine source, with some biomarker evidence to suggest a contribution from a potential hyspersaline source (i.e., lower Windsor evaporites). Their character is very similar to those from the Horton (Albert Formation) in New Brunswick – the source of oils for the Stoney Creek oil field – having Types I II (and III) kerogens with high TOCs and HIs. Fowler and Webb (2017) also reviewed analyses of interpreted Windsor age source intervals in the offshore Sydney Basin wells. However, recent work by Weston et al. (2017a, b, and c) places these units in the younger Mabou Group, thus confirming the wells did not penetrate Windsor Group sediments (see below).

As confirmed through analyses, petroleum shows in Windsor Group rocks are sourced from lacustrine sediments in the underlying Horton Group, with modest contributions from the lower Windsor (Macumber and evaporites). Most are the result of normal burial diagenesis of organic-rich source rocks. Those oils associated with base metal deposits were created through heating of hot mineralizing (basin- or basement-derived?) fluids migration through and/or adjacent to organic rich lacustrine shale beds were undisturbed in deeper basin settings, Windsor source rocks could be thicker with higher organic content and greater petroleum generative potential.  However, in such a setting very deep burial over time would exhaust its ability to create oils, and then gas.  Attenuating its maturation through inversion of the rock column along basin-bounding faults could retain its potential and that on the underlying Horton.

Mabou Group

The Windsor Group is transitional into and conformably overlain by the Mabou Group.  This is a transitional succession recording the Maritimes Basin’s change from a marine to terrestrial depocentre. The late Viséan to Early Namurian Mabou is present throughout Cape Breton’s Carboniferous basins and in the Sydney Basin and between 1 and 2 km thick deposited in a slowly subsiding basin with minor faulting along its margins.  It is subdivided into two principle formations: the Cape Dauphin and Point Edward. The former is composed of shales, evaporites and minor carbonates recording the transition from a mixed marine-lacustrine to saline depositional environment in a continental setting (Boehner & Giles, 2008). The latter formation is transitional with interbedded varicoloured shales, siltstones, sandstones and minor conglomerates deposited in a subaerial lacustrine-fluvial setting with fluvial facies more common in its upper part.

The Mabou is present in all five wells in the Sydney Basins and outcrops onshore. Analyses of a limited number of samples from the three offshore wells indicates a low potential with Type III kerogens dominating, and though mature, the rocks appear to have had low original hydrogen indices (Tables 5.4 and 5.5). Mukhopadhyay (2004) considered the succession to have good source rock potential.  A review by Fowler and Webb (2017) suggests that the Mabou has little petroleum potential due to low TOCs and HIs dominated by Type II gas-prone kerogens. The authors make an interesting observation regarding the possible potential in the lower Cape Dauphin. Similar age rocks in the Deer Lake Basin of southwestern Newfoundland are recorded to have very organic-rich and oil-prone lacustrine shales. Likewise across the Atlantic in Scotland a similar age unit contains oil shales composed of Types I and II kerogens with impressive TOCs approaching 15% with some hydrogen indices  HI>900 mg HC/g TOC. The onshore Deer Lake Basin is on the northeastern margin of the Sydney Basin. Fowler and Webb (2017) speculated that if a similar depositional setting existed in the deeper parts of the basin, and if mature, it would be a significant oil-prone source rock.