--> Petrography of the Bakken Black “Shales” in the Eastern Williston Basin of North Dakota

2014 Rocky Mountain Section AAPG Annual Meeting

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Petrography of the Bakken Black “Shales” in the Eastern Williston Basin of North Dakota


The Upper Devonian (Fammenian) and Lower Mississippian (Kinderhookian) Bakken Formation in the eastern Williston Basin of North Dakota is ∼25 to 30 m thick and consists of a lower black “shale,” a middle dolomitic siltstone, and an upper black “shale”. The black “shales,” which comprise about half the Bakken thickness, are better described as silty organic-rich black mudrocks, and are excellent hydrocarbon source rocks where thermally mature. The bulk of the 23 m.y. represented by Bakken deposition is reflected by accumulation of mud comprising the “shales” at a calculated average depositional rate of ∼0.7 mm/1000 yrs vs. ∼5 mm/1000 yrs for the middle-member. Petrographic study of the “shales” shows that they are silty, dolomitic non-fissile mudrocks which average 15 to 20 wt% TOC where thermally immature and about 10 wt% TOC where thermally mature with a vitrinite reflectance approaching 1.0%. These organic-rich mudrocks are pyritic and generally lack evidence of bioturbation and bottom currents except in the transitional meter or two at their base and top. Randomly distributed detrital silt grains composed of quartz, dolomite, and feldspar comprise 5 to 20% of both “shales.” Very early pyrite partly filled some algal cysts prior to compaction. Delicate fossil fragments derived from brachiopods, echinoderms, fish, algal cysts including Tasmanites, radiolarians, and conodonts occur sporadically in the “shales.” They represent pelagic organisms or rafted skeletal debris derived from well-oxygenated shallow surface waters in the Bakken Sea rather than transported lags or benthic organisms living on the anoxic, H2S-rich sea floor. Where silt laminae are present, they occur as somewhat discontinuous layers only a few silt grains thick with no evidence of traction bottom currents. These silty laminae are interpreted as fallout from dust storms that blew out over the Bakken Sea. Flattened fecal pellets sinking from shallow oxygenated waters have previously been misinterpreted as burrows. Previous claims that agglutinated foraminifers occur in the shales are also false in that these silicified organic remains are dominantly authigenic chert rather than detrital silt. In summary, most of the Bakken black “shales” represent deposition on an anoxic sea floor probably similar to that found in present-day Jellyfish Lake in Palau, but on a tremendously larger scale.