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CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 35 (1987), No. 4. (December), Pages 469-487

Oil Shale Composition Related to Depositional Setting: A Case Study from the Albert Formation, New Brunswick, Canada

William D. Smith,, Martin R. Gibling

ABSTRACT

The lacustrine Albert Formation (Late Devonian to ?earliest Visean) of southeastern New Brunswick contains oil shales that show variability on the scale of a few centimetres. A detailed sedimentological study of three cores revealed eleven distinct lithotypes distinguished on the basis of colour, nature and thickness of lamination, specific gravity, grain size and composition.

A nearshore lacustrine assemblage is formed by the coarse- to fine-grained organic-poor lithotypes classified as breccia, sandstone, massive siltstone, laminated siltstone and massive mudstone. An offshore lacustrine assemblage is formed by the fine-grained organic-rich lithotypes classified as laminated mudstone, dolostone, oil shale types C, B and A, and massive oil shale. Markov Chain analysis supports this model statistically by showing that the lithotypes belong to a transitional series of increasing organic content and decreasing grain size. Sandstone and siltstone beds associated with massive oil shale that contains detrital organic material suggest density-flow sedimentation.

Several properties of the organic-rich lithotypes can be related to the depositional setting. Total organic carbon content correlates with megascopic lithotype, increasing through the lithologic series of laminated mudstone to oil shale A. Lamalginite and carbonate content, especially calcite, increase through the series, whereas liptodetrinite, silicate content and couplet thickness decrease progressively. The lithotypes show a general change in colour from grey to brown and a decrease in specific gravity through the series. Other noteworthy trends include the decreased contents of sporinite, analcite and clays other than illite in oil shales of high organic content. These statistically confirmed compositional changes reflect a nearshore to offshore transition that owes its origin to an abundant nearshore terrigenous supply accompanied by fragmentation of organic material. Offshore carbonate production and algal accumulation occurred under conditions of low energy. The abundance of liptodetrinite in the massive oil shale supports the hypothesis of density-flow activity.

LA COMPOSITION DES SCHISTES BITUMINEUX EN RELATION AVEC LE MILIEU SEDIMENTAIRE: UNE ETUDE DE LA FORMATION ALBERT, NOUVEAU-BRUNSWICK, CANADA

RESUME

La formation lacustre Albert (Devonien tardif a Viseen le plus tot[?]) du sud-est du Nouveau-Brunswick, renferme des schistes bitumineux montrant une variabilite a l'echelle de quelques centimetres. Une etude sedimentologique detaillee de trois carottes revele onze types lithologiques differents qu'on distingue a partir de la couleur, du caractere et de l'epaisseur des laminations, de la densite, la granulometrie et la composition.

Un assemblage littoral lacustre est compose de lithotypes dont la granulometrie varie de gros a fin, pauvres en matiere organique et classifies comme breche, gres, microgres massif, microgres lamellaire et argile massive. Un assemblage lacustre au large du rivage est compose de lithotypes a grain fin, riches en matiere organique et classifies comme argile lamellaire, dolomie, schiste bitumineux types C, B et A, et schiste bitumineux massif. L'analyse de chaines Markov soutient statistiquement ce modele en montrant que les lithotypes appartiennent a une serie de transition dont la teneur en matiere organique augmente et la granulometrie decroit. Des lits de gres et de microgres affilies a des schistes bitumineux massifs contenant de la matiere organique detritique suggerent une sedimentation par courant de densite.

Plusieurs proprietes des lithotypes riches en matiere organique peuvent etre reliees au milieu sedimentaire. La teneur en carbone organique total est correlatif au lithotype megascopique, augmentant de l'argile lamellaire au schiste bitumineux A. La teneur en lamalginite et en carbonate, speciallement la calcite, augmente a travers la serie, tandis que la liptodetrinite, la teneur en silicate et l'epaisseur des "couplets" decroit progressivement. Les lithotypes subissent un changement de couleur general, du gris au brun, et une diminution de leur densite a travers la serie. D'autres tendances qu'il convient de noter comprennent une diminution de la teneur en sporinite, analcime et argiles autre que l'illite dans les schistes bitumineux a teneur en matiere organique elevee. Ces changements de composition, confirmes par les statistiques, refletent une transition du littoral au large du rivage, qui doit son origine a une abondante provision de sediments terrigenes littoraux accompagnee de la fragmentation de la matiere organique. La production de carbonates et l'accumulation d'algues au large du rivage se deroula dans un milieu a faible energie. L'abondance de liptodetrinite dans le schiste bitumineux massif appuient l'hypothese de la sedimentation par courant de densite.

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