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: https://www.dmr.nd.gov/ndgs/documents/newsletter/2017Summer/Ichnology%20Applied%20to%20the%20Pronghorn%20Member%20Bakken%20Formation.pdf

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

History:

• 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

Location:

• 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