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The AAPG/Datapages Combined Publications Database

Journal of Petroleum Geology

Abstract

Journal of Petroleum Geology, Vol.9, No.2, pp. 125-162, 1986

┬ęCopyright 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.


Abstract

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.

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