Saturday, July 29, 2017

The Pronghorn Member, Bakken Formation (MIssissippian-Devonian)

In the geology portion of file reports, the geologist will occasionally mention the various lithofacies of the Pronghorn Member of the Bakken Formation. This is a bit of background. A huge "thank you" to Don for sending me the link.

The Pronghorn, from GEO News, July 20, 2017. Ichnology Applied to the Pronghorn Member, Bakken Formation (Mississippian-Devonian). This link will open a PDF on your desktop:

I did not note the ichnology; if interested, go to the source, linked above.

  • formally proposed by Julie A LeFever and others in 2011
  • defined as the strata underlying the Lower Member and overlying the Three Forks Formation
  • had previously been included in the Three Forks and referred to as the "Sanish sand"
  • significant unconformity at the top of the Three Forks section
  • lithologies within the Pronghorn section are mappable and consistent with those observed within the Middle Member
  • located throughout the Williston Basin
  • divides easily into proximal and distal bed
  • proximal bed primarily in the southwestern portion of the basin
  • distal portion is more centrally located
  • proximal beds are associated with production
The geology:
  • Three Forks Formation
  • an alternating sequence of apple green and tan dolomudstones and claystones
  • Pronghorn Member
  • Bakken Formation
  • can be divided into five (5) lithofacies, in ascending order from A-E
  • focus of this paper: middle section B of this member
A. Basal Sandstone
  • lowermost sandstone lithofacies
  • fine- to very fine-grained quartz sandstone
  • difficult to identify with certainty on wireline logs
B. Dolomitic Mudstones with Storm Beds
  • upper portion of the interval: dolomitic mudstone with thicker storm beds of very fine- to fine-grained quartz
  • the upper portion of the interval becomes significant when examining production
  • although the porosities are similar throughout the interval, the upper portion of the lithofacies appears to have better permeability than the underlying portion
C. Lime Mudstone
  • thin, medium brown-grey mudstone to siltstone
D. Limestone
  • a medium grey, nodular-bedded limestone
  • represents an open marine environment
  • again, difficult to identify on wireline logs
E. Silt
  • the uppermost lithofacies has been previously referred to as the "Bakken silt" and is possibly the distal equivalent to the proximal lithofacies -- the sandstone, storm beds, lime mudstone, and limestone
  • organic values (TOCs) are significantly less than the overlying Lower Member of the Bakken formation
Lower Member of the Bakken
  • a dark brown to black organic-rich shale
  • the Lower Member was deposited in a restricted marine setting allowing for the stratification of the water column and the development of anoxic bottom waters
Relationship of Facies to Production
  • minor changes in the depositional environment of the "B" beds appear to have an effect on production; although porosity over the section is consistent, the permeability decreases from the upper section to the lower section
  • the silt interval of the Pronghorn should negatively affect production
  • however, the silt portion performs better as a reservoir because of the higher percentage of oil saturation to total fluids and therefore produces less water

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