ORGANIC METAMORPHISM IN THE LOWER MISSISSIPPIAN‐UPPER DEVONIAN BAKKEN SHALES — II: SOXHLET EXTRACTION1

Abstract

In this paper, we report on Soxhlet extraction (and subsequent related analyses) of 39 Lower Mississippian‐Upper Devonian Bakken shales from the North Dakota portion of the Williston Basin, and analyses of 28 oils from the Basin. Because of the influence of primary petroleum migration, no increase in the relative or absolute concentrations of hydrocarbons or bitumen was observed at the threshold of intense hydrocarbon generation (TIHG), or during mainstage hydrocarbon generation in the Bakken shales. Thus, the maturation indices that have been so useful in delineating the TIHG and mainstage hydrocarbon generation in other studies were of no use in this study, where these events could clearly be identified only by Rock‐Eva1 pyrolysis data.The type of primary petroleum migration operative in the Bakken shales led to the selective concentration of certain compound types in the oils of the basin, as well as the selective concentration of certain compound types in the shale extracts. The compositional differences between the crude oils and shale extracts provide evidence of the primary migration mechanism operative in this case, which is believed to have been gaseous solution, although the actual expulsion of oil from the rock, if observed, would probably have appeared to be bulk phase migration. Gaseous‐solution bulk‐phase primary petroleum migration also left organic‐geochemical imprints on the Bakken shales, observable by Soxhlet extraction and to a lesser extent by pyrolysis. The data of this study demonstrate that primary petroleum migration is a very efficient process.Four distinctive classes of saturated hydrocarbon gas chromatograms from the Bakken shales arose from facies, maturation, and primary migration controls. As a consequence of maturation, the percentage of saturated hydrocarbons increased in the shale extract at the expense of decreases in the resins and asphaltenes. Measurements involving resins and asphaltenes appear to be excellent maturation indices in the Bakken shales. Two different and distinct organic facies were present in immature Bakken shales.Rock‐Eval pyrolysis analysis was a critical research tool in this study; however, combined use of pyrolysis and Soxhlet extraction allowed organic‐geochemical features to be distinguished which would not have been possible by using either analysis alone.