3.2 North Sydney P-05

The Murphy et al. North Sydney P-05 well (1974) was the first of two wells drilled in the offshore Sydney Basin to test a very large fault-bounded structural closure incorporating presumed Early Permian to earliest Mississippian age sediments. Two regional geophysical programs were completed that included seismic, gravity and magnetic surveys in southeastern part of the basin and in the Cabot Strait area (Murphy, 1971a and 1971b). The operator noted that the hard water bottom affected the seismic response and thus mapping and identification of observed seismic horizons. In addition, the broad survey grids limited delineation of structural elements and closures. Nevertheless, at least three seismic horizons were mapped defining several prospects.

The shallowest interval – Horizon A – was interpreted as the top Carboniferous and revealed a broad shallow regional closure projecting into the basin, and coincident with the geology and structure observed onshore in the Glace Bay area. A second deeper reflector – Horizon B – was identified as an intra-Pennsylvanian reflector within the Canso (Mabou) – Windsor groups. It defined a very large (~75 km²), narrow (~3 km) and elongate (~25 km), northeast-southwest trending high that plunged to the northeast into the basin with the southeast flank with a steeper slope than the northwest. Three smaller, roughly circular, low relief closures were also mapped southeast of the feature though trending along an orientated northwest-southeast axis.

The succession beneath Horizon B was one of conformable reflections for over 900 metres with its base defined by an unconformity (base Canso / Mabou). This was followed by about 1500 m of segmented discontinuous reflections considered as the Middle Mississippian Windsor Group shallow marine series as well as possibly Early Mississippian Horton group fluvial-lacustrine successions. This reflection roughly tied to the onshore Birch Grove No.1 well thus supporting the interpretation. A third reflector – Horizon C – was considered to be a deep structural configuration of unknown age (base Horton Gp.?) though revealed similar structural trends and elements as Horizon B.

Targeted reservoirs were postulated to be present within two successions: Early Mississippian Windsor Group porous shallow marine carbonates, and Horton Group latest Devonian-Early Mississippian lacustrine and fluvial sandstones. These targets were based on regional onshore exposures and well / borehole data, oil and gas shows, and historic production from these rocks in southeast New Brunswick. For the Windsor Group, seals were expected to be intra-formational and overlying shales. The basal Windsor Macumber Formation would be sealed by thick, younger Windsor salts and anhydrites that would also be effective seals for deeper Horton reservoirs.

The P-05 well was located about 50 km equidistant from the southwestern and northwestern coastlines of Cape Breton Island, and spudded in 62.7 m water. It was positioned on the apex of the structure off-centre towards its northeastern terminus as defined by the Horizon B time structure map. It first penetrated a thin interval of Recent and Quaternary sediments followed by at least 250 m of undivided Permo-Carboniferous rocks over the surface casing interval. Details on formations encountered are tabulated in Table A, with their tops determined in conjunction with the listed sources.

From the casing shoe at 290 m, the well drilled through a 475 m succession of red-brown calcareous shales with minor red siltstones and tight, very fine to fine grain red sandstones, followed by 375 m of varicoloured shales and minor coals. These fluvial and related sediments are of latest Pennsylvanian to Early Permian age (Williams et al., 1990; Weston et al., 2017). They comprise the thick, regionally extensive undivided Pictou Group (Ryan et al., 1991) encountered almost exclusively in offshore wells in the Sydney and Magdalen basins and have few onshore exposures (Boehner and Giles, 2008).

This section was followed conformably by 368 m of green to occasionally red and grey shales with interbeds of siltstones, and light grey to green fine to medium grain, tight, siliceous fluvial sandstones. Coals become more common towards the base of the unit. Williams et al. (1990) dated these rocks as Stephanian to Westphalian D and equate to the Morien Group’s top unit, the Sydney Mines Formation. About 130 meters of the transitional Waddens Cove Formation sediments were next drilled composed of grey very fine to fine grain, thick (10-25 m), siliceous, tight fluvial and channel sandstones, and thin grey shales and coals. Conformably underlying these rocks were about 347 m of stacked, white to light grey, siliceous, calcareous, fine to coarse grain to pebbly sandstones with rare, thin grey shales and siltstones of the South Bar Formation. The interval is a succession of stacked, multi-storied fining-upwards fluvial channel sandstone displaying an overall upward fining of the sediments. Scattered, fair to good porosity is present and is related to the coarsest facies in the lower part of the unit.

Near the base of the South Bar Formation (1359.3 m), a low angle unconformity is noted on the dip log with a dip change from 3° S to 3° NW. This is not manifested biostratigraphically (Weston et al., 2017) though these authors observed it in the nearby F-24 well and identify it as the Westphalian C/B unconformity. This is followed by another at 1392.9 m (i.e. 33.6 m deeper) where the dips flip to 2° SE. Clowser and Lentin (1974) placed the top of the late Tournasian-early Namurian Windsor Group at 1388.2 m. Weston et al. (2017) reassessed the available data and determined that the underlying strata was not Viséan but Late Namurian.  Along with the lithological information (Boehner and Giles, 2008), this interval was identified as the Mabou Group’s top unit, the Point Edward Formation, with its top at 1392.9 equating to the Westphalian-Namurian Unconformity. Williams et al. (1990) also recognized this discordance though a bit higher at 1374.6 m and the missing Westphalian B, A, and most of the Namurian section. This unconformity represents the regional Mississippian-Pennsylvanian major transpressional and erosional event that separates the younger Morien Group from the older Mabou Group.

The underlying Point Edward Formation sediments contrast with the overlying coarser grain rocks. The well drilled about 65 m of an interbedded sequence of fine to very fine grain siliceous sandstones (<5%) porosity) and varicoloured (brown, red, grey to green) calcareous siltstones and shales. This was followed by 55 m of interbedded grey-green to red siltstones and shales with an increasing amount of medium to coarse grain, siliceous, pebbly sandstones.

A low angle (18 degree) angular discordance is noted on the diplog at 1511.7 m. From this point to the end of the well the lithology is a red pebble to cobble size polymictic conglomerate with no porosity and several beds of brown to cream silty, dolomitic limestone and minor siltstones. Recent biostratigraphic work by Weston et al. (2017) has determined this 149 m succession down to well TD is still of mid-Namurian age and must represent a marginal alluvial fan facies of the Point Edward.  The operator (Murphy, 1974, p.24-27) considered the possibility that this succession was of earliest Cambrian age based on lithological characteristics (and lack of feldspars) that appeared similar to rocks of the Morrison River Formation (Williams et al., 1985). If this alternate interpretation is correct, then the well bottomed in petroleum basement and not younger Mississippian age lacustrine sediments. A single conventional core was attempted near the base of the well and recovered about 1.3 m of conglomerate.  It was re-entered and drilled another 21 m of similar lithologies whereupon it was terminated at a total depth of 1660.8 m.

Numerous mud-gas shows were present in the well throughout the Mabou and Morien group sediments, with about 43 mud-gas peaks >50 TGU encountered over an approximately 750 m interval, of which nine > 150 TGU, five > 200 TGU and three > 300 TGU.  The gas was overwhelmingly methane and the shows increased with depth with the largest and most significant within the stacked sandstone sequences of the Morien Group’s South Bar Formation.  Several other large shows were related to coals and coaly intervals in the shallower units of the Morien Group and gas pay was indicated on well logs. Nevertheless, none of the intervals were tested and the well was subsequently plugged and abandoned.

Seismic mapping showed the North Sydney structure to have relief and probable closure as defined by the mapped horizons. Though some porous reservoir sands were present with associated gas shows, their quality was only fair with gas sourced from interlayered coals. The targeted fluvial-lacustrine Horton Group sediments was not penetrated as the well TD was based on incorrectly identified seismic horizons, so therefore the play concept remains untested.