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
- 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
- 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
- lowermost sandstone lithofacies
- fine- to very fine-grained quartz sandstone
- difficult to identify with certainty on wireline logs
- 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
- thin, medium brown-grey mudstone to siltstone
- a medium grey, nodular-bedded limestone
- represents an open marine environment
- again, difficult to identify on wireline logs
- 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
- 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
- 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
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.