3.1. Birch Grove No. 1

The Murphy et al. Birch Grove No. 1 well (1968) was the first petroleum exploration well drilled in the Sydney Basin. It was designed to test an elongate (3 x 14 km / 52 km²) northeast-southwest trending anticline that was first defined through surface mapping. Over the 1963-1965 period, Pacific Petroleum Limited acquired 53 kilometres of vibroseis and dynamite sourced seismic data that provided additional information on the structure’s attributes and stratigraphic successions. The well was located on the crest of the anticline off centre towards its southwestern nose.  Subsequent interpretations suggested that the feature had a basement fault component with movement associated with the Westphalian age regional transpressional and erosional event.

It is important to note that the geological tops designated at that time were at the group level.  Since then, Boehner and Giles (2008) subdivided these groups and described the related formations and members. This new information is presented in the following narrative and tables below. Note that except for a cursory assessment of the upper 550 m of the well by Barss (1968; South Bar formation), no comprehensive biostratigraphic analyses have been done on this well. Thus, unit identities and elevations are based on comparison of lithological characteristics and log character with the nearby (5.9 km to the west) Carbon Capture & Storage Nova Scotia (CCS-NS) No. 1 well (Table A).

Birch Grove No.1 spudded in alluvial siltstones, shales, sandstones and occasional coals of the Morien Group’s Sydney Mines Formation. About 316 m of this succession was drilled with the basal half of the unit dominated by several 40-60 m thick sequences of stacked grey to green, fine to medium grain sandstones. The base of the formation was picked with the first occurrence of thick stacked packages of fine to coarse grain fluvial sandstones of the South Bar Formation. In addition to this change, the well’s dip log shows a corresponding change in dip angle and direction from 2-3° SE (Sydney Mines) to 2-5° NE (South Bar) representing the Westphalian C/B unconformity.

After drilling the 525 meters of South Bar Formation sandstones, the well encountered a succession dominated by grey-green to red shales, siltstones and sandstones with the latter becoming more abundant with depth. Biostratigraphic analysis of a similar interval in the adjacent CCS-NS No. 1 well (Weston et al., 2017) indicated a Westphalian A (Langesettian) age with the top defined by an unconformity. This 218m interval is thus younger than the Westphalian A/B-Namurian regional unconformity with lithologies and age similar to the Silver Mine Formation further inland to the southwest (Boehner and Prime, 1993).

At 1061.9 metres a noticeable change in lithology occurred that is equated with sediments of the Mabou Group’s Point Edward Formation.  Equivalent strata in the CCS-NS No. 1 well is dated as Namurian and represents a lacustrine and marginal marine(?) depositional setting. This interval is also marked with a change in regional dips from a strong, tight and consistent 4° NE trend in the overlying Silver Mine Formation to scattered 3-15° N-directed dips. The change in structural dip and age at this elevation thus defines the Westphalian A/B-Namurian regional unconformity.

The first 58 m of the Point Edward’s 281.6 m of sediments was made up of a succession of calcareous grey shales and thin-bedded brown limestones.  Next was about 72 m of mostly grey to black, calcareous and carbonaceous dolomitic shale interbedded with anhydrite and fine to coarse grain sandstones. From 1192.0 to 1343.5 m the well penetrated 151.5 m of pink, red to maroon fine to coarse grain, angular, micaceous, quartzitic sandstones and possible granular conglomerates with minor maroon and black shale interbeds. This latter interval probably represents the basin margin alluvial fan facies of the Point Edward. Dips range from 2-4° with a strong NE trend though at 1307.5 m swing to the SE with no associated lithological change. A short conventional core was taken near the middle on the interval (Table B).

Well TD was called at 1343.5 m (4408 ft.), and prior to termination a 9.2 metre interval at the base of the well was selected for open hole testing (Table C). The zone was within the presumed Woodbine Road Formation and composed of fine to coarse grain siliceous sandstones. After completion of the test, about 3 metres of drilling mud was recovered.  The well was subsequently logged but the quality of the curves was poor due to excessive wash-out.

In late 1972, the operator further defined the feature’s structural picture though acquisition of a ten-line seismic program using a dynamite source, but no further drilling was done. Later surface mapping revealed that a NE-SW trending normal fault existed at the structure’s southwestern nose. 

The Birch Grove well revealed that Late Carboniferous formations at depth did not have effective porosity, and that the targeted Horton Group lacustrine source-reservoir succession was not penetrated. Indeed, it remains uncertain if it underlies the Windsor strata at all with older basement rocks present instead.