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The AAPG/Datapages Combined Publications Database

Rocky Mountain Association of Geologists

Abstract


The Bakken-Three Forks Petroleum System in the Williston Basin, 2011
Pages 127-145

Chapter 5: Clastic Injectites in the Devonian (Famennian) Lower Bakken Shale, Williston Basin, North Dakota

William J. Clark, Amy L. Maldonado

Abstract

Clastic injectites have been observed in the Devonian lower Bakken shale in twenty three cored wells in the Williston Basin, North Dakota. The injectites are stratigraphically restricted to the lower Bakken shale (LBS) member of the Bakken Formation. Though the size and lateral extent of the injectite network is difficult to determine due to the limited viewing scope of core data, heights observed in cores are up to 6 ft (1.8 m) and widths range from 1/16 to 3 inches (2 mm-7.6 cm). Most of the injectites are vertical or near vertical, although a few are sub-horizontal. The clastic injectites are enigmatic and intriguing for several reasons. First, the injectites are present in many, but not all, wells in a northwest-southeast trending football (US) shaped area that includes the northern Nesson Anticline (northeastern Divide County) and its northeastern flank extending through and to the southeast of Parshall Field in southeastern Mountrail County. Their occurrence in this northeastern area of the Williston Basin in North Dakota is not correlative to any known structural or stratigraphic feature. Second, injectites are comprised primarily of very fine to fine-grained, with occasional coarse-grained quartz sand, mostly dolomitized bioclasts, intraclasts, and ooids, minor phosphatic clasts, replacive pyrite, calcite and dolomite cements, and rare iron enriched (siderite) cement. Possible source candidates for the clastic material include a sandstone underlying the LBS and/or the overlying middle Bakken mixed sandstone and carbonate interval. Third, timing of emplacement is not entirely clear. Many injectites are contorted due to compaction of the LBS, which indicates emplacement occurred prior to or during compaction. One injectite is not compacted indicating either post-compaction emplacement, or that it has a different origin. The injectites are believed to have been emplaced after relatively shallow burial and well before peak hydrocarbon generation during the latest Cretaceous. There is a thus possibility that these clastic filled features were pathways for oil migration. Their greater concentrations in the LBS in Parshall and Ross field areas suggest a relationship between injectites and presence of oil in the overlying middle Bakken reservoir interval.


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