Defining the 'Extra Bakken': Sequence stratigraphy of the Sanish play in the Williston Basin

John Hohman

A third reservoir has been identified and pursued in the greater unconventional Bakken play of the Williston Basin joining the more established Middle Bakken and Three Forks reservoirs. This dolomitic, silty sandstone when first encountered between the Lower Bakken shale and the underlying Three Forks dolostone was informally described as the 'Extra Bakken' for its lithologic similarity to the Middle Bakken above. This sandstone proved oil-productive in the conventional Antelope Field where it was given the name Sanish. Subsequently, use of the term Sanish diverged from its original definition causing considerable stratigraphic confusion. Addressing the confusion, this study leverages the original definition of the term Sanish and uses sequence stratigraphy to define a stratigraphic framework within which to place the Sanish and adjacent Three Forks and Lower Bakken deposits. The key observation is that the Sanish represents a separate sequence. It is separated from both the underlying Three Forks and the overlying Lower Bakken by sequence boundaries each marked by distinctive lag deposits. The sequence boundary at the base of the Sanish Sequence is characterized by a coarse lag of Three Forks clasts produced by the erosion of the underlying Three Forks. In contrast, the sequence boundary at the top of the Sanish Sequence is characterized by phosphatic sandstone with localized sandstone and limestone clasts. Internally, the Sanish can be subdivided into a lower, transgressive unit characterized by the dolomitic, burrowed, silty marine sandstone with event (storm?) beds that represents the reservoir section. An upper, highstand unit characterized by limestone is also recognized. It is separated from the underlying transgressive unit by a marine flooding surface and consists of argillaceous, skeletal mudstone/wackestone that is essentially nonreservoir. It is the erosion of the transgressive Sanish sandstone and the highstand Sanish limestone that produces the clasts entrained in the phosphatic sandstone lag that marks the sequence boundary capping the Sanish Sequence. It is also important to note that the Sanish Sequence stratigraphic distinction separate from the overlying Lower Bakken includes the informally recognized Lower Bakken silt. This localized Lower or Basal Bakken siltstone/silty shale unit is not part of the Sanish Sequence but rather is part of the overlying Lower Bakken.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012