Journal of Petroleum Geology, Vol.9, No.2, pp. 125-162, 1986
©Copyright 2000 Scientific Press, Ltd.
ORGANIC METAMORPHISM IN THE LOWER MISSISSIPPIAN-UPPER
DEVONIAN BAKKEN SHALES PART 1: ROCK-EVAL PYROLYSIS AND VITRINITE REFLECTANCE *
Leigh C. Price, Ted Daws, and Mark Pawlewicz**
* Appreciation is extended to Joel Levanthal and Chuck
Spencer for reviewing the manuscript and to the North Dakota Geological Survey, especially
Sidney Anderson, for their cooperation.
** U.S. Geological Survey, P. O. Box 25046, Denver Federal
Center, Denver, CO 80225, USA.
Detailed organic geochemistry has been performed on a large number
of Lower Mississippian-Upper Devonian Bakken shales from the North Dakota portion of the
Williston Basin, and 28 oils mainly from Mississippian Madison Group rocks from different
basinal areas. Here we report results of Rock-Eval pyrolysis and vitrinite reflectance (Ro)
analyses. Variable paleoheat flows in the Williston Basin caused the threshold of intense
hydrocarbon generation to occur at different depths in different basinal areas. in higher
paleogeothermal gradient basinal areas, this event occurred at depths of 7,650-8,000 ft,
and at 10,000 ft or deeper in lower paleogeothermal gradient areas of the Basin. Distinct
organic metamorphic imprints in Williston Basin sediments were also caused by extreme, but
variable, paleoheat flows in the basin, as well as secondary migration of crude oils from
deep basinal source areas. The high paleoheat flows are postulated as being due to a Late
Cretaceous-Paleocene aborted rift event. Only a small volume of Bakken shales in
restricted areas of the Williston Basin was responsible for the oil found reservoired in
Mississippian Madison Group rocks. However, this small shale volume has been responsible
for a relatively large amount of crude oil.