Facies and Mechanical Stratigraphy of the Middle Bakken, Mountrail County, North Dakota

Lyn Canter, Orion Skinner, and Mark D. Sonnenfeld
Whiting 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 thin basal unit sharply overlying the Lower Bakken shale. This facies is characterized by muddy, intraclastic-skeletal lime wackestone sometimes exhibiting a “patterned” texture, suggestive of (temporary) near hypersaline conditions. Facies D is a bioturbated, muddy, calcareous, poorly sorted, very fine grained sandstone/siltstone with common Helminthopsis 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 shows some improvement upwards. Facies C is composed of parallel millimeter-laminated to low angle, hummocky cross-stratified, dominantly calcite-cemented, well-sorted very fine grained sandstone and siltstone interpreted as amalgamated storm deposits. Visible porosity is limited to rare intergranular, clay intercrystalline, and minor open discontinuous horizontal microfractures. When present, Facies B forms the Middle Bakken’s “clean bench”. Facies B varies from 0 to >20’ with 2 sub-facies. 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 in subtidal shoals and/or channels above storm wave-base. 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 and UV fluorescence improves in B1 with greater allochem diversity and/or quartz abundance. Facies A contains four sub-facies (bottom to top): 1) A-GR is a thin organic-rich mudstone forming a Gamma Ray log marker at base of A; 2) A2 is a thin-bedded dolomite mud/wackestone that is more dolomitic than B, C, or D; 3) A1 is a calcitic whole fossil dolo-to lime-wackestone serving as guide beds while drilling; and 4) A0 is patterned carbonate (pyritic dolomudstone with enterolithic-structure) 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.

Facies successions and event stratification yield a mechanical stratigraphy thought to impact fracture height and spacing. Facies D is a single, massive mechanical unit; as such it has the greatest fracture spacing. Facies A, B, and C are composed of thin mechanical beds, with dolomitic, centimeter- to decimenter-bedded Facies A being the thinnest bedded and most fracture-prone. Linking core facies to MWD GR, drilling time, and mud gas enhances real-time tracking of horizontal wellbore trajectories. Facies A, B, and C remain the principal horizontal target due to greater porosity, fracture susceptibility and therefore permeability.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado