The Waltman Shale: An All Purpose Rock Unit

By

C.M. Hawkins

Hawkins Oil and Gas, Denver, CO

R.A. Baugh and G.A. Winter

Associated Energy Consultants, Casper, WY

D. Donoho

Resource Solutions, Denver, CO

The Waltman Shale member of the Fort Union Formation is the source rock for hydrocarbons produced in several fields in the Wind River Basin, is the source of at least one oil seep and oil saturated outcrops of adjacent units, and it also contains reservoir beds that produce. It acts as a partially effective seal for deeper accumulations that are geopressured and can itself be geopressured.

The Waltman was deposited in a Paleocene brackish to fresh water lake that covered the central and eastern portions of the Wind River Basin, Wyoming. The extent of the lake can be readily mapped except along its northern border where the Waltman has been generally removed along the Owl Creek thrust fault. The type locality of the Waltman is located in the hanging wall of this thrust southeast of Cave Gulch Field.

The presence of the low velocity Waltman has a major impact on seismic modeling and must be understood before underlying structures can be properly imaged. Because of its composition the Waltman must also be carefully addressed in drilling well design. Company files contain numerous examples of “problem” wells in the basin that failed to adequately address the Waltman either during data acquisition, prospect analysis, or well design. The Waltman Shale offers an excellent example of how the integration of geology, geophysics, and engineering can lead to a successful effort.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming