ABSTRACT

The transgressive deposits of Albian to Turonian age that lie between the top of the Viking Formation and the top of the Second White Speckled Shale in southwestern Alberta comprise a diverse sequence of mudstones and argillaceous siltstones up to 175 m thick. A combination of Rock-Eval pyrolysis, stable isotope geochemistry, organic petrology and iron-sulphur studies has been used to characterize the organic constituents and to examine the relationship between depositional conditions and organic facies of these important petroleum source rocks. The vertical (stratigraphic) and lateral organic facies variations are interpreted in terms of both the paleo-oceanography and paleogeography. The kerogen, which exists at maximum concentrations in excess of 4 per cent, is dominantly Type II. A good correlation is generally exhibited between total organic carbon (TOC) and hydrogen index (HI), typical of marine source rocks. The rocks within the lower portion of the study interval were deposited in a shallower and episodically higher-energy environment than those in the upper portion. The lower zone is relatively deficient in organic matter and the kerogen that is present is hydrogen-poor. The upper portion of the interval contains the richest source rocks. Within this upper zone, sub-intervals occur characterized by common, coarser interlaminae of both quartz silt and bioclastic limestone. TOC is low within the coarse laminae, however the organic matter within the adjacent mudstones is more abundant and more hydrogen-rich than that present in mudstones of more homogeneous sub-intervals.

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