3.4. St. Paul P-91

The Petro-Canada et al. St. Paul P-91 well (1983) was the third (and last) well drilled in the offshore Sydney Basin. Technically, it is located at the basin’s western margin adjacent to St. Paul Island within the Cabot Strait, the narrowest point between the islands of Newfoundland and Cape Breton (Nova Scotia). Geologically, the strait is underlain by a series of northeast-southwest trending transverse and normal faults that separate the Carboniferous age Sydney and Magdalen basins with the well on the western flank of this fault zone. Reflecting this, Bibby and Shimeld (2000) identified the strata penetrated with unit names associated with the Magdalen Basin, though here both names for equivalent formations are given (Table A).

The St. Paul prospect was defined through several seismic, gravity and magnetic surveys, first by Murphy Oil Company (1971a, 1971b) and later by Petro-Canada (1981, 1983, and 1984a). It is located within a major transverse fault zone observed onshore Newfoundland and Cape Breton Island. The prospect is a northeast-southwest trending narrow elongate closure bounded on the southeast by a rhombic-shaped structural high that includes basement exposed on the island with several cross-cutting transtensional faults.  Mapping of the interpreted base Pennsylvanian horizon (i.e., top Canso Gp.; now termed the Mabou Gp.) defined approximately 9.3 km² of areal and 1000 metres of vertical closure. Other mapped horizons were the Permian-Carboniferous unconformity, base Windsor Gp., and basement. The structure’s stratigraphic successions were steeply dipping and reliant on both normal and transverse fault seals and simple closure, with the well located on the features' southwestern flank. A small salt diapir ridge was interpreted within the closure's northern portion, with other similar and larger features several kilometers to the west in the Magdalen Basin. The age of the structure was believed to be Permian with younger potential reservoirs being eroded. 

The operator postulated several reservoir intervals based on onshore exposures, well data, and shows, discoveries and production onshore. The anticipated primary reservoirs were lacustrine, alluvial and fluvial sandstones of the latest Devonian-earliest Mississippian Horton Group. Secondary reservoirs were subareally exposed / diagenetically altered shallow marine carbonates of the Early Mississippian Windsor Group, and fluvial sandstones of the Late Pennsylvanian Pictou Group. Some stratigraphic trap potential should exist in all intervals based on the Cabot fault zone being a structurally high region separating the two basins throughout their history and thus a source of coarse-grained clastics.

Vertical seals were expected within the Windsor Group (evaporates, limestones and shales), and fluvial and deltaic shales of the younger Mabou, Morien and Pictou groups. Fault offsets and fracturing could compromise lateral seals juxtaposed against porous zones. Regionally known source rocks are present in the region with the potential to generate oil (Horton group lacustrine oil shales and bituminous Windsor limestones) and gas (Pennsylvanian deltaic shales and coals). Due to their depth of burial (Horton) and source rock / kerogen types (Pictou), the majority of the predicted hydrocarbons in the structure was gas.

The P-91 well was located about 5.5 km southwest of St. Paul Island and spudded in 174 m water. About 400 m of Recent and older sediments were penetrated prior to setting the 508 mm surface casing.  Bibby and Shimeld (2000) suggest that some Morien Group sediments and the Mississippian-Pennsylvanian unconformity may exist behind casing from about 550 m. The logged section began at about 610 metres and was composed of 382 m of red siltstones and sandstones, and green and red shales later identified as Mabou Group’s Pomquet (Point Edward) Formation of early Namurian age. Based on the palyomorphs, Weston et al. (2017) interpreted a possible intra-Namurian A unconformity at 784 m defining the base of the Pomquet. Underlying this contact was about 265 m of alternating red calcareous siltstones and red shales with rare marlstones later identified as the Hastings (Cape Dauphin) Formation representing deposition in a continental playa-lacustrine setting (Weston et al., 2017). Near its base, dip logs and seismic profiles identify a thrust or reverse fault between 1160 and 1185 m.

The well next entered an approximately 195 m thick interval of grey shale, trace brown limestones and marlstones and increasing amounts of fine grain grey, calcareous sandstones. These are dated as Late Viséan and equate with the top part of the Windsor’s Woodbine Road Formation (Weston et al., 2017). This interval was followed by about 645 m of brown-red, very fine to medium grain, calcareous and siliceous thin-bedded sandstones, and occasional siltstones, minor red shales and rare brown limestones. The sandstones form a number of 50-75 m thick coarsening upward cycle ranging from very fine to medium grain with poor to fair porosity in the coarsest fraction.

At 2097-2098 m is a high gamma dark grey shale beneath which follows a 82 m thick interval composed of roughly equal amounts of thin-bedded brown sandstone, grey calcareous siltstones and shale, and brown silty limestones.  This is followed by 131 m of massive anhydrite with limestone, dolomite and salt interbeds, and then 276 m of red, very fine to fine grain tight, calcareous, dolomitic and anhydritic sandstones with minor limestones, dolomites, siltstones and anhydrites. The basal 27 m (2587.5 – 2614.5) marks the base of the formation and composed of several thin 2-5 m thick grey siliceous pebbly fine to very fine grain sandstones with a 5 m thick siliceous quartz-pebble conglomerate at its base.

Recent biostratigraphy analyses by Weston et al. (2017) determined a disconformable contact between the Woodbine Road and underlying Meadows Road formations. From 2590-2883 m the Meadows Road interval was composed of alternating thin-bedded red to light red very fine to fine grain dolomitic sandstones with subordinate amounts of red, sandy dolomitic siltstones and rare red to green shales dated as mid-Viséan (Weston et al., 2017).

A conventional core was cut at the base of the well with 2.2 m cut and 1.7 m recovered, and was composed of highly inclined (60°), indurated, tight pebbly sandstone. Following an extensive logging program, a number of sidewall cores were taken for biostratigraphic and petrographic study. No shows of any sort were present and the well was terminated at 2883.0 m total depth with no testing and abandoned.

Throughout drilling of the well, the increasingly higher dip of the strata with depth created deviation issues with keeping the well trajectory in the vertical plane. As a result, measured dips in the well (west-directed) ranged from 25° at the top of the borehole to 70°+ with depth. The stratigraphic succession was thus significantly attenuated in the borehole and the penetrated succession’s apparent thickness was far greater than prognosed. In hindsight, this no doubt influenced the operator’s initial identification of the units drilled.

The identity of the stratigraphic successions penetrated and genesis of the structure changed soon after the well was completed. Based on the well’s diplog, a fault (thrust) was interpreted within the 1160-1185 m interval. Biostratigraphy (Utting, 1984) supports this interpretation. The upper (hanging wall) section was originally identified as latest Viséan to earliest Namurian representing the boundary interval between the Windsor and Mabou groups respectively (Pomquet Formation). The Windsor group interval was correctly identified based on lithologies and biostratigraphy, but the inferred basal Horton Group could not be confirmed. The recovered spores and pollen suggested an age comparable to Windsor Group sediments that were coarser-grained, siliciclastic basin margin facies that are lateral equivalents to carbonate and evaporitic facies. This was reconfirmed by the recent work of Weston et al. (2017). Thus, the prognosed Horton Group primary reservoir was not penetrated, and the Pictou Group secondary reservoir was eroded and not present.

Langdon and Hall (1994) undertook a comprehensive study of the sedimentation and tectonics of this region.  They interpreted a complex Late Carboniferous transpressional fault system (“wrench borderland”) composed of a series of local depocentres and horsts. Based on their mapping, drilled structure was an inverted depocentre and the trap a compressional restraining bend anticline with associated thrust faults, all part of a positive flower structure bounded by the major Hollow and Cabot faults. Tectonism was most probably Permian in age associated with the Alleghenian Orogeny, though older episodes affecting the entire region (e.g. early Pennsylvanian and early Mississippian) likely affected deeper successions. Further evidence of uplift was manifested in the high thermal maturity of the succession based on recovered pollen and spores, with Utting (1984) interpreting its TAI values ranging from -3 to 4 (i.e. highly mature to over mature). Recent mapping (this report) offers a much different interpretation of this structure.