--> --> Abstract: Bakken Mudrocks of the Williston Basin, World Class Source Rocks, by Steve Sonnenberg, J Frederick Sarg, and Hui Jin; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Bakken Mudrocks of the Williston Basin, World Class Source Rocks

Steve Sonnenberg1; J Frederick Sarg1; Hui Jin1

(1) Colorado School of Mines, Golden, CO.

Bakken mudrocks are extremely important source rocks for the Bakken petroleum system of the Williston Basin. Published estimates of oil generated from the source rocks range from 10 to 400 billion barrels.

The mudrocks are variable in mineral composition and total organic carbon content. The dominant minerals present are quartz, feldspar, pyrite, dolomite, and illite. The quartz is both detrital and biogenic in origin. Recognizable biogenic components in the mudrocks include: conodonts, fish bones, radiolaria (recrystallized), large algal spores (tasmanite), small cephalopods, small brachiopods, small corals and ostracodes. Thin black limestones are locally present in the lower Bakken mudrock. A thin siltstone interval is also locally developed in the lower part of the lower Bakken mudrock.

TOC content appears to vary inversely with percentage of biogenic silica (radiolarian). The TOC content of the mudrocks averages 11 wt%. The type of kerogen present is largely Type I/II based on visual and pyrolysis analysis. The mudrocks are red to brown in color where immature and black where mature. The mudrocks appear to be mostly finely laminated but small ripple forms are also present indicating bottom current activity.

The mudrocks were deposited largely under anoxic conditions, probably due to density stratification of the ocean water column in a restricted basin setting. Some burrowing has been reported in the mudrocks suggesting times of more oxic conditions.

Knowledge of petrophysical properties, TOC content, organic facies, mineralogy, fracturing, and source rock maturity of the Bakken mudrocks are important to the understanding of hydrocarbon occurrence and distribution in the Bakken petroleum system.