Molecular genetic markers and maturity indices in intermontane lacustrine facies: Kishenehn formation, Montana

Molecular markers have considerable promise as thermal maturation indicators in source rocks having a well-defined depositional environment. This occurs in the Kishenehn Formation (northwest Montana and southeast British Columbia), an Oligocene unit comprised of fluvial and lacustrine shales. Up to 3 km of these non-marine sediments were deposited in a 900 sq.km elongated half graben atop the Precambrian rocks of the Lewis Thrust sheet. Specific outcrops of the Coal Creek Member of the Kishenehn Formation are known to contain excellent potential source rocks, including extensive oil shales. The source rock potential and molecular marker geochemistry of most of the major Kishenehn outcrops have now been investigated, and regional results are presented in this paper.source rock analysis of a regional suite of Kishenehn samples indicates excellent petroleum potential (immature Type I kerogen) throughout the basin. Several classes of distinctive diagenetic molecular markers occur within the Coal Creek Member of the Kishenehn Formation, including diasterenes, spirosterenes, methylspirosterenes, B-ring monoaromatic anthrasteroids, ββ-hopanes and moretanes. Molecular markers indicative of non-marine deposition include dehydroabietane (conifer input) and an onocerane recently identified in leaf fossils of Miocene lacustrine beds in adjacent Idaho. Triterpane ratios are significantly more sensitive than vitrinite reflectance methods in assessing thermal maturation in the Kishenehn, and are successfully used in this study to order outcrop locations according to thermal maturity level. The systematic variation of specific markers with increasing thermal maturation suggests that molecular correlation is feasible, and could provide a method of assessing stratigraphic continuity in the basin.