ABSTRACT

Fine grained strata of Early to Middle Triassic age occur at outcrop and in the subsurface of the Peace River Arch region of Alberta and British Columbia. The Montney and Doig formations, and equivalent outcrop units in northeastern British Columbia, are commonly rich in organic carbon. Samples of these units were examined as potential sources of hydrocarbons using Rock-Eval/TOC, gas chromatography (GC), gas chromatography/mass spectrometry (GC/MS) and organic petrographic (reflectance and UV fluorescence) techniques. Analysis of over 50 outcrop samples showed variable maturity south of Pine River (Whistler and Vega-Phroso members, Sulphur Mountain Formation), whereas Triassic rocks between the Pine and Peace rivers (Toad and Grayling formations) are overmature. These units contain Type II/III organic matter, and TOC contents range up to 15 per cent.

In the subsurface, the Montney and lower Doig formations were sampled from core. Thermal maturity increases in a southwesterly direction, from mature (Tmax values of about 440°C) close to the Triassic subcrop edge, to overmature (Tmax > 460°C) close to the Foothills Belt. The Montney Formation yielded TOC values up to 4.7 per cent and Hydrogen Indices (HI) rarely over 300. The lower beds of the Doig Formation are commonly highly radioactive (high gamma-ray log response) and phosphatic. Rock-Eval/TOC analyses indicate moderate to high maturity, with TOC contents commonly greater than 5.0 per cent, and HI/OI values suggesting a predominance of Type II organic matter. This is consistent with optical analyses of moderately mature samples, which show an abundance of yellow to orange fluorescing algal material. Biomarker analyses (GC, GC/MS) of bitumen extracts from the Doig and Montney formations and their equivalent surface strata were compared to biomarker signatures of oils recovered from Triassic reservoirs. The lower Doig is the source for oils in the Halfway and Charlie Lake formations.

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