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Lithologic and Petrophysical Characterization of the Upper Silurian Interlake Group, Nesson Anticline Area, North Dakota and Eastern Montana


The Silurian Upper Interlake of the central Williston Basin has produced over 65 million barrels of oil (10.3 million m^3) principally from over 240 wells on the northern Nesson anticline with minor Middle- and Lower-Interlake production in Montana along the Mondak trend and in Putnam and other fields. The Upper Interlake consists of stacked thin (2-20 ft, 0.5-6 m) depositional cycles deposited in high salinity, shallowing-upwards sequences beginning in subtidal, massive dolomite mudstones (0-25% Phi, 0.001-0.1 mD) that grade upward into intertidal rooted and desiccated dolomite mudstones (0-24% Phi, 0.01-0.5 mD) that are overlain by high intertidal fenestral dolomite algal boundstones (0-24% Phi, 0.01-1000 mD) and blackened dolomite caliche crusts (variable Phi, K) and capped by the unconformity surface at the top of the Tippecanoe sequence.

Dolomite mudstones, which make up most (i.e., ~80%) of the thickness of each cycle, contain high-porosity/low-permeability, micro-intercrystalline porosity with characteristic exceedingly low-resistivities and oil saturations <15% due to high entry capillary pressure. It is difficult to recognize on logs the thin-bedded oil-productive fenestral algal boundstones and caliche crusts (typically <0.5 ft, 0.2 m, thick) that are sandwiched within these.

The Middle - Lower Interlake poses a similar evaluation problem. In this interval, thin (1-2 ft, 0.3-0.6 m) high-porosity/high permeability fossil and oolite dolomite grainstones and packstones are separated by thicker, fractured low permeability/low porosity dolomite wackestone/packstone intervals that interconnect the thin reservoir units. Cuttings analysis and high-resolution logs provide important data for reservoir rock identification.

Interlake production has been defined by vertical well completions with production relying on lateral connectivity of the thin reservoir facies. Volumetric analysis indicates that wells drain 40-160 acres. Due to bypassed pay and incomplete drainage, potential may exist for horizontal multistage fracture well development in areas not fully exploited by vertical well development. Horizontal wells could establish connection to isolated thin beds which would provide increased oil production but would also result in associated high water production. The Interlake provides one analog for stacked thin pay within thick low-high porosity/low permeability, wet non-reservoir rock which is encountered in other Paleozoic-age rocks in many basins.