Abstract

Detailed organic geochemistry has been performed on a large number of Mississippian-Devonian Bakken shale samples from the North Dakota portion of the Williston basin as well as on 28 oils mainly from Mississippian Madison Group sediments from different areas of the basin. Distinct organic metamorphic imprints were present in the sediments of the Williston basin due to extreme but variable paleo heat flow in the basin and from lateral migration of the crude oils in the basin from the source areas in the basin deep.

Due to the variable paleo heat flows in the Williston basin, the threshold of intense hydrocarbon generation occurred at different depths in different areas of the basin. In higher paleogeothermal gradient areas of the basin, this event occurred at a depth of 7,650 to 8,000 ft, and at a depth of 10,000 ft in lower paleogeothermal gradient areas of 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 Mississippian-Devonian Bakken shale. Thus, maturation indices that have been so useful in delineating the TIHG and mainstage hydrocarbon generation in other studies were of little or no use in this study. In this study, these events could clearly be identified only by pyrolysis data.

Primary petroleum migration from the Mississippian-Devonian Bakken shale is believed to have been initiated and maintained by gaseous solution, although the actual expulsion of oil from the rock, if observed, probably would have appeared to be bulk phase migration. Gaseous-solution bulk-phase primary petroleum migration was responsible for the selective leaching (or retention) of certain compound classes from (or in) the Mississippian-Devonian Bakken, shale, and thus was responsible for distinct compositional differences between the bitumen extracts from this shale and the Mississippian Madison Group oils. Primary petroleum migration has also left other distinct organic-geochemical imprints on the Mississippian-Devonian Bakken shale, which were observable by Soxhlet extraction analysis, and to a lesser extent by pyrolysis analysis.

The upper and lower shale members of the Mississippian-Devonian Bakken Formation were organic-geochemically indistinguishable at all maturation ranks. Compared to the total volume of these members in the Williston basin, only a small volume of the shale in restricted areas of the Williston basin was responsible for the oil found reservoired in Mississippian Madison Group sediments. However, this small shale volume has been responsible for a relatively large amount of crude oil.

R_o profiles in the Tertiary through Middle Jurassic sediments from wells in the Williston basin had steep, linear R_o versus depth gradients, with strong reversals of R_o values occurring in the Lower Jurassic sediments. The Lower Mesozoic through Paleozoic sediments of the Williston basin had strongly suppressed R_o values compared to the values in the Tertiary through Middle Jurassic sediments. This was especially true of the R_o values in the Mississippian-Devonian Bakken shale. This R_o suppression was due to a change in organic matter type from oxygen-rich terrestrially derived organic matter to a marine derived organic matter in the deeper, older sediments. The threshold of intense hydrocarbon generation occurred in Mississippian-Devonian Bakken shale samples from the high paleogeothermal gradient areas of the Williston basin at minimum R_o values of 1.62 to 1.73, as would be read in oxygen-rich, terrestrially derived organic matter. By contrast the R_o values read from the Mississippian-Devonian Bakken shale at this event were 0.34 to 0.38. Much higher burial temperatures (and consequently R_o values) than usually held to be the case, were required for both the threshold of intense hydrocarbon generation as well as mainstage hydrocarbon generation in the exinite-rich Mississippian-Devonian Bakken shale. These results are most probably directly applicable to exinite-rich, source rocks in general. The data of this study have major application to petroleum exploration as well as to petroleum resource assessment.