3.5. CCSNS No. 1

The Carbon Capture & Storage Nova Scotia (CCSNS) No. 1 well (2014) was drilled under the direction of Carbon Capture & Storage Nova Scotia - an organization consisting of the Government of Nova Scotia, Dalhousie University and Nova Scotia Power Inc. – as part of its research program investigating the potential for CCS onshore and offshore Nova Scotia. The well was designed to assess and characterize the storage potential of geological formations in the Sydney region where two coal-burning power plants are located. Furthermore, the Sydney Basin was deemed attractive given detailed knowledge of its structural and stratigraphic components through many decades of historical geological studies of onshore exposures, bore holes and some vintage (1960s) 2D seismic data as well as data generated though industrial activities such as coal and base metal mining, limestone and gypsum quarrying, etc.

CCSNS No.1 was planned to drill a thick, complete stratigraphic succession of early Mississippian to late Pennsylvanian megasequences within a local unfaulted depocentre (CCSNS, 2015a). The objective was to “characterize and access the potential geological storage unit (Horton Group) and upper seal unit (Windsor Group)”; i.e. Units 1 and 2 of Pascucci et al. (2000). The shallower Unit 3 (Pictou and Morien groups) was not considered suitable for CO₂ storage.

Two new 2D onshore seismic lines were acquired in 2013 to assist in selecting the well location within the offshore Marconi Half-Graben of Pascucci et al. (2000): Line 1 (NW-SE dip line) and Line 2 (NE-SW strike line). Seismic data confirmed the presence of the depocentre and bounded by faults with the strata of Units 1 and 2 show evidence of NW-SE lateral compression and positive structural elements adjacent to the faults. Additional geological control was provided by detailed local and regional geological data and the Murphy et al. Birch Grove No.1 well (1968) located 6 km east-southeast of the well and at the eastern end of Line 1.

Four potential well locations were identified. Ultimately, the preferred Location A – almost at the intersection of the two lines – was determined to be too close to local airport approaches. It was moved about 1.5 km northeast of Line 1, though was still near to the graben’s syncline axis.

With these data, the well was prognosed (based on the original Site A assessment) to penetrate about 1000 m of Pictou and Morien groups fluvial and alluvial sandstones, and fluvial lacustrine sediments of the Mabou Group. Following this was an estimated 150 m of Windsor Group interbedded carbonates, evaporates and fine grain siliciclastics – the potential seal. Underling the Windsor were Horton Group fluvial sandstones estimated to be between 75 and 280 m thick.  One seismic interpretation had the underlying Horton reservoir strata resting directly on older igneous/metamorphic basement rocks. An alternative postulated the Horton underlain by an older synrift succession - “Lower Horton”. These Middle Devonian interbedded non-marine arkoses, conglomerates, shales and felsic volcanics of the McAdams Lake Formation were considered to also have reservoir potential, and are known for several outcrop occurrences of oil shales.

The CCS-NS No.1 well spudded in siliciclastics of the Morien Group (Table A). It penetrated 1020 m of an overall fining-upwards succession consisting of cycles of paleovalley fill channel sandstones, meandering alluvial sandstones and mudstones, and coastal plain minor coals (Sydney Mines Formation), and thick, stacked multi-story braidplain fluvial channel sandstones and conglomerates (South Bar Formation). The sandstones over this interval were variable porous with good to very good intergranular porosities in the Sydney Mines and Waddens Cove formations. They were noticeable poorer in the thick, coarse grain, fining-upwards sandstones of the South Bar formation due to pervasive silica cementation.

Though present in the Birch Grove No. 1 well 6 km to the east, the Waddens Cove Formation is absent here.  As defined by Boehner and Giles (2008), this formation is recognized as a transitional facies between the South Bar and Sydney Mines formations.  Furthermore, it is restricted to the eastern part of the basin with its interpreted distribution shown in Map 86-1 of Boehner and Giles’ report.

The base of the South Bar Formation was defined at 1035 m based on a noticeable change lithology from thick massive sandstones to grey-green to red shales, siltstones and minor sandstones. Weston et al. (2017) dated this 79 m thick interval as Westphalian A (Langesettian) thus is younger than the Westphalian / Namurian regional unconformity.  The lithologies are similar to the Silver Mine Formation defined by Boehner and Prime (1993) and are also of Westphalian A age.

Weston et al. (2017) defined the Westphalian-Namurian regional unconformity at 1114 m.  It also corresponds with a lithological change to dominantly dark grey to occasional red-brown shales and minor brown-grey siltstones and rare sandstones equating with the Namurian Mabou Group’s Point Edward Formation. The formation here was thin (62 m) and mostly eroded. A lithological break and possible intra-Namurian A unconformity was interpreted by Weston et al. (2017) at 1176.0 m. The Point Edward sediments beneath this unconformity were composed of 157 m of grey carbonaceous shales, coal, white, tight fine grain sandstones, and grey siltstones. This was followed by 39.8 m of interbedded red-brown to grey shales and siltstones, minor grey to white very fine to fine grain glauconitic sandstones, and common thin interbeds of marlstones and white limestones with rare anhydrite beds. The limestone is more common in the basal 30 m and described as having a cryptocrystalline texture originally interpreted as the basal Windsor Macumber Formation (CCSNS, 2015b) though is considered to be the lower portion of the Point Edward (Weston et al., 2017).

From 1372.8 m the well penetrated a 154 m interval of presumed felsic volcanic and volcaniclastic igneous rocks identified as rhyolites and dacites.  These were described as pink to red to red-brown, aphanitic to cryptocrystalline, calcite fractures fill veinlets, with the minor presence of metasediments (hard micaceous, pyritic shales and sandstones). The well reached total depth in this succession.

Though coaly and carbonaceous shale beds were present in the well to varying degrees, no significant hydrocarbon shows were encountered. Two conventional cores were recovered from the well; one within rocks of the Windsor Group and the other from near the base of the well in the presumed late Neoproterozoic of the Coxheath Group (Table B). After conventional logging and a single sidewall core run, the well was abandoned at 1527 m in the volcanic rock interval.

Subsequent assessment of the well confirmed that it met its first object that the Windsor Group seal was present. Log and measured (sidewall core) permeabilities <<1 mD confirmed good seal potential (Carbon Capture & Storage Nova Scotia, 2015). However, the Horton Group reservoir was not present nor the postulated “Lower Horton” (McAdams Lake Fm.). Instead, the well penetrated a deeper succession of about 154 m of slightly metamorphosed felsic volcanic and volcaniclastic basement rocks of presumed late Neoproterozoic age. The interpreted depositional low was compromised by the presence of a basement high or ridge composed of much older rocks upon which the Windsor Group onlapped.  These (internally layered?) basement features were not recognized on the existing seismic datasets as positive features. This knowledge gap compromises estimation of the Horton Group’s distribution in this part of the basin, and hence its reservoir potential and storage capacity.