Abstract

The Waltman Shale Member of the Fort Union Formation was deposited in Waltman Lake within the Wind River Basin. The lake formed in response to Laramide tectonics. Deposition occurred during the late Paleocene. Two hundred twenty-six cuttings samples from 13 wells within the Waltman Shale depocenter were analyzed to examine lateral and vertical lithologic and geochemical variations in the Waltman Shale Member.

Bulk x-ray analyses of the Waltman Shale indicate general lithologic uniformity, with clay minerals accounting for greater than 50% of the unit, and with smectite and mixed layer clays dominating. The unit displays a limited range of organic enrichment (0.9 to 6.2 wt. % organic carbon) and hydrocarbon generation potential (0.30 to 24.02 mg HC/g rock). Although the kerogen displays some variability, much of the material appears to be a mixture of organic matter types, displaying a mixed character intermediate between type II and type III. In part, this is a result of the presence of recycled organic matter. Pyrolysis-gas chromatography indicates that the generated product would be a high wax crude oil.

The generative portion of the Waltman Shale is areally limited within the basin. Observational data suggests that the main phase of hydrocarbon generation and release is obtained at current depths as shallow as 2100 meters (6900 feet). Generation is largely limited to the north-central portion of the basin and is closely associated with a seismic velocity anomaly and over-pressuring.

The effectiveness of the source system appears to be largely limited by its ability to expel hydrocarbons. This is part of the explanation for the regional velocity anomaly. The lack of sands and silts within the Waltman Shale has prevented effective expulsion of hydrocarbons from much of the generative portion of the basin. Consequently, reserves which can be attributed to the source are limited compared to the overall volume of generated hydrocarbons.

The general exploration strategy for Waltman Shale-derived oils is directed toward the identification of sandstone bodies which are in communication with primary carrier systems permitting the effective expulsion of hydrocarbons.