Abstract

Whitehorse trough is a frontier basin in south-central Yukon that is thought to contain gas and possibly oil. It consists of up to 7000 m of sedimentary and volcanic rocks referred to as the Lewes River Group (Triassic), Laberge Group (Jurassic), and Tantalus Formation (Jura-Cretaceous). The Lewes River Group is informally subdivided into the Povoas and Aksala formations, and the Laberge Group is informally subdivided into the Richthofen, Nordenskiold and Tanglefoot formations. To evaluate the petroleum source-rock potential of this basin, over 600 samples from the Aksala, Richthofen, Tanglefoot and Tantalus formations from throughout the trough have been analyzed by Rock-Eval programmed pyrolysis and combustion and by study of thermal alteration indicies (TAI) of palynomorphs, conodont alteration indicies (CAI), and vitrinite reflectance. The Povoas and Nordenskiold formations were not sampled because they consist of volcanic and volcaniclastic rocks, respectively, and have no source rock potential. The analyses indicate that the Aksala formation is a poor source rock that is postmature with no hydrocarbon potential. The Richthofen formation is a poor source rock that is postmature with minor gas potential. The Tanglefoot formation is a good source rock that is immature to early mature and gas-prone. The Tantalus Formation is also a good source rock that is immature to early mature and gas-prone. The Tanglefoot and Tantalus formations contain petroleum fluid inclusions that were probably generated from within these units. The inclusions reveal homogenization temperatures of mostly between 75–115°C and reveal several distinct, but minor pulses of liquid petroleum (mainly 19–21°API, 23°–35°API and 40–46° API). Potential petroleum (gas) generative intervals occur at surface and in the shallow subsurface in deltaic mudstones (Tanglefoot formation) and fluvial mudstones (Tantalus Formation). The lateral extent of these intervals is unknown due to poor outcrop exposure and limited shallow drilling. Both the Tanglefoot and Tantalus formations are interpreted as potential source rocks and possibly effective source rocks. The most prospective areas for petroleum exploration are Five Finger Rapids, Division Mountain and Tantalus Butte and in the northern portion of Whitehorse trough.