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Stratigraphy, Mineralogy and Depositional Environment of the Sanish Member of the Upper Three Forks Formation, North Dakota Portion of the Williston Basin, USA

Berwick, Brian 1; Curtis, John B.2; Hendricks, Michael 3
1 Samson Resources, Denver, CO.
2 Geology & Geological Eng., Colorado School of Mines, Golden, CO.
3 Hendricks & Associates, Centennial, CO.

The Late Devonian Upper Three Forks and Lower Bakken Shale formations were described from 16 cores taken in the North Dakota portion of the Williston Basin, USA. Five lithofacies were identified in the Upper Three Forks and one lithofacies in the Lower Bakken Shale. Beds in the upper part of the Upper Three Forks Formation were informally termed the Sanish member, following earlier usage in the literature.

Locally, the base of the Upper Three Forks is red shale and siltstone (Facies A) deposited in a continental sabkha environment. In other portions of the study area, the base of the Upper Three Forks is dolomitic shale with rip-up clasts (Facies B) created by very shallow marine reworking. Both Facies A and B are recognized on wireline logs and are widespread in the study area.

The Sanish member of the Upper Three Forks consists of three additional facies, a highly deformed and brecciated silty dolomite (Facies C) deposited in tidal mud flat and sabkha environments; a silty dolomite, dolomitic siltstone and shale deposited in tidal flat and sabkha environments (Facies D); and, locally capping the Sanish member, a burrowed dolomitic and silty shale (Facies E) deposited in a shallow subtidal environment. Facies C and D comprise petroleum reservoirs in portions of the study area. A black, organic-rich shale deposited in an open-marine anoxic basin, the Lower Bakken Shale, unconformably overlies the Sanish member.

In order of abundance, Sanish detrital mineralogy consists of quartz, potassium feldspar, illite, dolomite, muscovite, and biotite. Authigenic minerals include potassium feldspar, dolomite, chlorite, quartz, biotite, calcite, and pyrite. The clay-sized fraction contains potassium feldspar, dolomite, illite, and chlorite. The Sanish member has fair to poor porosity. Bedding plane and cross-cutting microfractures enhance permeability.

Bathymetric shifts occurred during late Devonian and Early Mississippian time, creating flooding at the base of the Upper Three Forks, periodic exposure of Facies C and D of the Sanish, local flooding at the top of the Sanish (Facies E), and finally transgression of the Lower Bakken Sea across the Sanish member. The top of the Sanish member was subaerially exposed prior to inundation by the Lower Bakken Sea. The unconformity and transgression clearly separated the Three Forks and Sanish depositional systems from the Lower Bakken system.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009