--> Abstract: Facies of the Middle Bakken, Mountrail County, North Dakota, by Lyn Canter and Mark D. Sonnenfeld; #90089 (2009)

Datapages, Inc.Print this page

Facies of the Middle Bakken, Mountrail County, North Dakota

Lyn Canter1 and Mark D. Sonnenfeld2
1Orion Skinner
2Whiting Oil and Gas Corp., Denver, CO

We recognize 5 facies within Middle Bakken cores from the Sanish and Parshall fields of Mountrail County, North Dakota.

Facies E is a basal unit abruptly overlying the Lower Bakken shale. This thin, 1-4’ facies is characterized by muddy, intraclastic-skeletal lime wackestone sometimes exhibiting a “patterned” texture suggestive of (temporary) near-hypersaline conditions. It exhibits distinctively cleaner GR than overlying Facies D.

Facies D is a bioturbated, aargillaceous, calcareous, poorly sorted, very fine grained sandstone/siltstone with common Helminthopsis/Sclarituba burrow traces. This thickest Middle Bakken unit is interpreted as offshore deposition below storm wavebase. Both intergranular pores and open horizontal discontinuous microfractures are rare in this poor reservoir quality facies which is subject to progressively increasing bedded to nodular calcite cementation to the south.

Facies C is composed of rhythmic, varve-like, mm- to cm-laminated, well-sorted, very fine grained sandstone and siltstone with considerable calcite cement. Dominantly parallel laminae and subordinate very low angle, hummocky cross-stratification grade upward to increased wave-ripple lamination. This facies is interpreted as a combination of distal storm deposits and distal prodelta hyperpycnal gravity flows. Visible porosity is limited to rare intergranular, clay intercrystalline, and minor open discontinuous horizontal microfractures. Facies C is confined to middle Bakken paleolow areas.

When present, Facies B forms the Middle Bakken’s “clean bench” characterized by the middle Bakken’s cleanest GR. Facies B varies from 0 to >20’ with two sub-facies. Basal Facies B2, a muddy calcareous sandy/silty “disturbed” facies, has common syn-sedimentary micro-faults, microfractures, and slumps representing soft-sediment deformation concurrent with inferred structural movement on Nesson Anticline. Facies B1 is the highest energy, coarsest grained unit in the Middle Bakken, represented by alternating units of cross-bedded bioclast-rich, very fine to fine-grained sandstone and sandy skeletal lime grainstone deposited subtidally in shoals and/or channels above storm wavebase. True trough or planar tabular cross-stratification is rare; most inclined bedding is either swaley or disturbed. Pore types include rare to common intergranular and minor clay intercrystalline. Syntaxial calcite cement in crinoidal lime grainstone B1 occludes all primary pores. Primary porosity retention in Facies B1 improves with greater allochem diversity and/or quartz abundance. To an even greater extent than Facies C, Facies B1 and especially B2 are largely confined to and sandier in paleolow areas.

Facies A contains four sub-facies (from bottom to top): 1) A-GR is a thin organic-rich mudstone forming a widespread Gamma Ray log marker at base of A; 2) A2 is a thin-bedded dolomite mud/wackestone that is significantly more dolomitic than any other middle Bakken facies; 3) A1 is a calcitic, whole fossil, dolo-to lime-wackestone with fossil-rich storm beds that constitute “guide beds” while drilling; and 4) A0 is a “patterned” pyritic dolomudstone immediately below the contact with the Upper Bakken Shale. Rare visible porosity in thin sections is limited to secondary pores and rare open discontinuous microfractures. Despite the paucity of visible porosity in core and thin sections, Facies A has weak yellow UV fluorescence emanating from micropores in grainy beds.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009