The Waltman Shale Total Petroleum
System: Does it have a favorable future?
The 2005 USGS assessment of undiscovered petroleum resources
in the Wind River
includes an evaluation of Paleogene rocks in the Waltman Shale Total Petroleum System (TPS). Source rocks in
this TPS are organic-rich lacustrine shales that charged conventional accumulations in stratigraphic or structural/stratigraphic
traps within the Shotgun Member of the Fort Union Formation. To date, more than
2.5 million barrels of oil and 6 billion cubic feet of associated gas have been
produced from this petroleum system in the Fuller Reservoir, Haybarn, and Greater Madden fields.
Previous studies indicate that the Waltman Shale
Member of the Fort Union Formation contains a mix of Type-II and Type-III
organic matter; total organic carbon averages about 2.7 weight percent. Waltman oil is high gravity (>40˚API) with a high
paraffin content; associated gas is isotopically
lighter than gas produced from Upper Cretaceous and Tertiary reservoirs
underlying the Waltman Shale Member. Thermal maturity
of the Waltman in the deep Wind River Basin
generally ranges from 0.75–0.80 percent Ro, and approaches 1.10 percent Ro in
areas west of the Madden anticline.
Sandstone and conglomerate deposited in fluvial, shoreface, and deltaic systems prograding
from western and southern margins of Lake
and fan-delta complexes present along the northern lake-basin margin are
potential reservoirs for additional Waltman petroleum
accumulations. Future exploration might target areas where these contiguous
depositional systems are interbedded with thermally
mature Waltman source rocks, and areas where faults
or fractures provide pathways for petroleum migration to shallow reservoirs
from mature source rocks at depth.