5.1 Devonian – Guysborough Group

The oldest potential source rock in the Sydney Basin is the McAdams Lake Formation. It is found in a narrow, elongate, northeast-southwest trending half graben formed at the fault boundary between two Avalon composite terranes – the Mira and Bras d’Or – at the southwestern end of the onshore Sydney Basin (White and Barr, 1998). The formation is unconformable overlain by alluvial fan conglomerates of the Tournasian Horton Group (Grantmire Formation), with a time gap of about 40 Ma spanning the unconformity. Plant and palynology date the formation as latest Emsian to Early Eifelian (latest Early to Middle Devonian).  Mapping and drilling of the succession reveals a thickness of about 1000 m, and post-deposition it underwent a period of compressive deformation with related folding.

The McAdams Lake Formation represents deposition in a rifted continental setting, with two sequences defined: a lower unit with siliciclastic deposition in a shallow lacustrine setting with proximal alluvial fan deltas, and an upper unit represented by prograding alluvial fan deltas and associated braided streams possible due to rift margin uplift.  Within the upper unit were periods of possible basin subsidence (and climatic changes?) whereby lakes expanded and transgressed over coarser grain marginal facies (White and Barr, 1998).

The depositional setting and lithofacies is similar to that of the younger Horton Group, with the middle Horton having well defined oil- and gas-prone source rocks elsewhere in the various onshore subbasins of the Maritimes Basin, and is the source for hydrocarbon production in southeastern New Brunswick (see Section 2). Within the McAdams Lake’s lower unit there are carbonaceous to coaly shale beds 0.6-3.0 m thick informally described as ‘oil shales’ (citations in Smith and Naylor, 1990). Limited mining and assessments were undertaken with liquid petroleum generated from the rocks, but with no commercial production.

In an assessment of potential oil shale resources in Nova Scotia, Smith and Naylor (1990, p. 11) quoted assessments completed by others that revealed the shales were “…thermally mature (within the oil window)”, and with Ro of 0.86-1.06. Total organic carbon (TOC) content ranged from 2.5 to 18.5%. Hydrocarbon indices were not quoted by these authors, though they stated that the organic carbon was derived from humic (Type III) rather than algal sources.

Mukhopadhyay (2004) analysed three samples from a borehole with results similar to those quoted by Smith and Naylor (1990) with shale samples thermally mature (Ro 0.74-0.87), and TOCs from 1.67-9.28 though with the Type III kerogens having quite low hydrogen indices (13.71 mg HC/g TOC). Fowler and Webb (2017) undertook analyses of outcrop and borehole samples. The former were had very low TOCs thought due to the weathered nature of the outcrop.  Better results were obtained from borehole samples. The shales were organic-rich with TOCs from 2.07-41% with low hydrogen indices (43-124 mg HC/g TOC), and revealed to be mature (Ro 0.92-1.00%). The authors suggest the observed low HIs could be the result of elevated maturity and migration of most hydrocarbons, therefore original HIs are estimated to originally been significantly higher with good potential for liquid hydrocarbon generation (see Table 5.1).

The McAdams lake Formation could have good potential as a source rock. Its presence in a half graben formed adjacent to a terrane boundary and lacustrine-alluvial stratigraphic succession mirror that of the younger Horton Group that has known excellent petroleum source rocks, occurrences, and hydrocarbon production. However, the McAdams Lake has limited areal extent though its presence in the basin beneath Mississippian and younger strata is unknown.  In its marginal setting, it is thermally mature, and in distal deeper parts of the basin would have higher maturation. Furthermore, following deposition it underwent moderate compressional deformation followed by a period of erosion with ca.40 Ma elapsed time before buried by earliest Mississippian Horton Group sediments. Over such an extended time, any existing traps and migration pathways for possible hydrocarbons generated could have been breached / eroded. Distal basin accumulations may have had a better chance of preservation but in turn deeply buried in the Carboniferous with commensurate elevated thermal maturity.