Burial history, thermal maturity, and oil and gas generation history of petroleum systems in the Wind River Basin Province, central Wyoming: Chapter 6 in Petroleum systems and geologic assessment of oil and gas resources in the Wind River Basin Provinc

Burial history, thermal maturity, and timing of oil and gas
generation were modeled for eight key source rock units at
nine well locations throughout the Wind River Basin Province.
Petroleum source rocks include the Permian Phosphoria
Formation, the Cretaceous Mowry Shale, Cody Shale, and
Mesaverde, Meeteetse, and Lance Formations, and the Tertiary
(Paleocene) Fort Union Formation, including the Waltman
Shale Member. Within the province boundary, the Phosphoria
is thin and only locally rich in organic carbon. Phosphoria oil
produced from reservoirs in the province is thought to have
migrated from the Wyoming and Idaho thrust belt.
Locations (wells) selected for burial history
reconstructions include three in the deepest parts of the
province (Adams OAB-17, Bighorn 1-5, and Coastal Owl
Creek); three at intermediate depths (Hells Half Acre, Shell
33X-10, and West Poison Spider); and three at relatively
shallow locations (Young Ranch, Amoco Unit 100, and
Conoco-Coal Bank). The thermal maturity of source rocks is
greatest in the deep northern and central parts of the province
and decreases to the south and east toward the basin margins.
The results of the modeling indicate that, in the deepest areas,
(1) peak petroleum generation from Cretaceous rocks occurred
from Late Cretaceous through middle Eocene time, and (2)
onset of oil generation from the Waltman Shale Member
occurred from late Eocene to early Miocene time.
Based on modeling results, gas generation from the
cracking of Phosphoria oil reservoired in the Park City
Formation reached a peak in the late Paleocene/early Eocene
(58 to 55 Ma) only in the deepest parts of the province. The
Mowry Shale and Cody Shale (in the eastern half of the basin)
contain a mix of Type-II and Type-III kerogens. Oil generation
from predominantly Type-II source rocks of these units in the
deepest parts of the province reached peak rates during the
latest Cretaceous to early Eocene (65 to 55 Ma). Only in these
areas of the basin did these units reach peak gas generation
from the cracking of oil, which occurred in the early to middle
Eocene (55 to 42 Ma).
Gas-prone source rocks of the Mowry and Cody Shales
(predominantly Type-III kerogen), and the Mesaverde,
Meeteetse, Lance, and Fort Union Formations (Type –III
kerogen) reached peak gas generation in the latest Cretaceous
to late Eocene (67 to 38 Ma) in the deepest parts of the
province. Gas generation from the Mesaverde source rocks
started at all of the modeled locations but reached peak
generation at only the deepest locations and at the Hells Half
Acre location in the middle Paleocene to early Eocene (59 to
48 Ma). Also at the deepest locations, peak gas generation
occurred from the late Paleocene to the early Eocene (57 to
49 Ma) for the Meeteetse Formation, and during the Eocene
for the Lance Formation (55 to 48 Ma) and the Fort Union
Formation (44 to 38 Ma).
The Waltman Shale Member of the Fort Union Formation
contains Type-II kerogen. The base of the Waltman reached a
level of thermal maturity to generate oil only at the deep-basin
locations (Adams OAB-17 and Bighorn 1-5 locations) in the
middle Eocene to early Miocene (36 to 20 Ma).