ABSTRACT

Interest in organic-rich strata in the argillaceous Upper Cretaceous Niobrara and Favel Formations of the Williston Basin as a potential oil shale resource is indicated by leasing and test drilling in the outcrop area of western Manitoba. The rocks are typically dark brown to black, weakly indurated marlstones and claystones. In hand sample, they are structureless to thinly bedded with discontinuous, parallel stratification, and rare pyrite-filled burrows and fractures. Thin lenses of siltstone and bentonite are found locally. Thin section examination reveals very fine primary lamination. The sediments are moderately fossiliferous throughout and concentrations of foraminiferal and inoceramid debris are common. Inorganic components are mainly clay minerals, calcite, pyrite, and quartz. Clay minerals are dominantly illite and kaolinite with secondary abundances of various expandable lattice clays. Lesser amounts of gypsum and feldspar are also present but rarely make up more than 10 percent of the mineral suite.

The oil shales average about 6 percent total organic carbon with the richest sections giving oil yields of about 60 litres/tonne. The kerogen is immature to marginally mature as indicated by vitrinite reflectance and peak yield temperature data. Detailed petrographic examination and pyrolysis yield determinations suggest Type II (marine) and Type III (terrestrial) organic matter are intermixed, and show considerable stratigraphic and spatial variation in their ratios.