Composition and depositional environment of major eastern Canadian oil shales

• Published in: International journal of coal geology 1991-12, Vol.19 (1), p.385-438
• Main Authors: Smith, W.D., St. Peter, C.J., Naylor, R.D., Mukhopadhyay, P.K., Kalkreuth, W.D., Ball, F.D., Macauley, G.
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Hydrocarbons; Sedimentary rocks; Stratigraphy
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/0166-5162(91)90028-H

In Eastern Canada, following the Acadian Orogeny, two lithologically and chronologically distinct sequences of oil shale were deposited in the Late Devonian/Carboniferous Maritimes Basin. The oldest sequence occurs within the Moncton Subbasin, a transcurrent and asymmetric basin 200 km in length in southeastern New Brunswick. The oil shales are largely confined to the medial part of the 165–1800 m thick Early Carboniferous (Tournaisian) Albert Formation. The oil shales range from massive slightly organic-rich dolostone to carbonate-rich lamosite with an organic fraction of 1–30% TOC and comprising mainly alginite with locally abundant liptodetrinite and vitrinite. Rock-Eval and chromatography data indicate mostly Type I and Type II kerogens. Lamalginite fluorescent spectra indicate immature to mature sediments, in agreement with surface oil shows and the presence of a small oil and gas field sourced from Albert oil shales. The oil shale composition reflects deposition in a chemically stratified lacustrine setting characterized by slightly saline and alkaline waters and deltaic sedimentation. The younger sequence occurs within the Stellarton Basin, a 6×18 km pull-apart basin situated in northern Nova Scotia. The oil shales form part of the 2600 m thick Late Westphalian B-Early Westphalian D coal-bearing Stellarton Formation; a total of 60 oil shale beds (1–45 m thick), some transitional to coal, have been documented. The oil shale lithotypes are silicate-rich and include fissile shale, minor blocky (cannel) shale and lesser stellarite (boghead shale) with an organic fraction, depending upon lithotype, of 1–41% TOC comprising variable amounts of liptinitic groundmass material, telalginite and vitrinite. Rock-Eval, petrography and gas chromatography, data indicate mature Type II and locally Type I and Type III kerogens. Extract, chromatography and carbon isotope data indicate that the coals of the formation, rather than the oil shales, are the most probable source of the hydrocarbon shows within the Stellarton Formation. The composition of the oil shales and the transition of some oil shales to coal reflects deposition in a nearshore to open lacustrine environment bordered by peat swamps. The principal causes for compositional differences between the Albert and Stellarton oil shales are: (1) the influence of basin size and paleogeographic setting on sediment supply to the Albert and Stellarton lakes; (2) the hydrological conditions (closed versus open) of the