For years people had been seeing a limestone/lime mudstone unit between the lower Bakken and the upper Three Forks underlying parts of Stark, Dunn, McKenzie, Billings and Golden Valley counties in western North Dakota, but that limestone/mudstone unit had never been well defined or well described. That was until three years ago when Anschutz Petroleum cut a long core in a well it drilled in far southern Dunn County that fully transcended what for years had been commonly known as the Sanish sand, a unit that has been considered part of the Three Forks formation in the larger Bakken petroleum system. With the information gained from the Anschutz core, the North Dakota Geological Survey now formally considers this limestone/mudstone unit to be part of the Bakken unit within the same petroleum system, and have redefined it as the Pronghorn member.
A little history
In 1954, NDGS described an “unconformity sand” lying between the lower Bakken shale and the Three Forks formation, and referred to it as the Sanish sand, and included it as part of the Bakken formation. The section was called the Sanish sand based on Stanolind Oil Co.’s #1 Woodrow Starr well, which produced from that member. NDGS considered the Sanish to be part of the Bakken formation.
The unit was later described by C.A. Sandberg and C.R. Hammond as a fine-grained sandstone and coarse siltstone ranging from five to 15 feet in thickness. Sandberg and Hammond considered the unit as part of the Three Forks formation.
Over the years the unit appeared and disappeared in texts as an understanding of the Bakken and Three Forks formations evolved from the 1950s to 2009, but where it was referenced, the unit was commonly referred to as the Sanish sand unit and was generally considered to be part of the Three Forks formation.
Then in 2009, Anschutz drilled its No. 24-14H Sadowsky well in far southern Dunn County and cut a core 88 feet in length from the eight feet above the bottom of the Lodgepole down to nine feet into the top of the Three Forks formation. From that core, NDGS Core Library Director Julie LeFever, along with Richard LeFever of the University of North Dakota and Stephan Nordeng of NDGS, was able to finally and definitively describe the Pronghorn member.
The Pronghorn identified in that core was 43 feet thick, extended from 10,491.5 to 10,534.5 feet, and unconformably overlies the Three Forks and is unconformably overlain by the lower Bakken, they said.
In a paper published by the Rocky Mountain Association of Geologists in 2011, LeFever and her colleagues formally argued for redefining the Sanish unit as the Pronghorn member, and to also formally include the Pronghorn as a member of the Bakken formation.
In that paper, LeFever and her colleagues said, “We consider the Pronghorn member to be part of the Bakken formation for two reasons. First, where present, the member lies above a recognizable unconformity, which probably has regional extent. Second, the succession from the Pronghorn member upward to the lower Bakken shale strongly resembles the succession from the middle member to the upper Bakken shale,” from which a large majority of Bakken oil is produced.
In an interview with Petroleum News Bakken, LeFever reiterated the similarity of the Pronghorn with the middle Bakken, and that the argument for including it in the Bakken formation is the fact that it sits on a major unconformity between the Three Forks and the Bakken.
“It’s that placement of that unconformity at the top of the Three Forks that I think really pushes it into the Bakken,” LeFever said.
LeFever also said that where the Pronghorn is producing, the lower Bakken shales are gone and the Pronghorn is overlain by the upper shales in most areas. So where one generally sees the Pronghorn, she said, the lower shale is gone.
According to LeFever, the Pronghorn member is predominant in Stark, southern Dunn, southern McKenzie, and northern Billings counties, as well as touching a small part of the east side of Golden Valley County.
Why is term ‘Sanish’ a problem?
According to LeFever and her colleagues, the term Sanish, while “well embedded in the literature,” as describing a sandstone bed at the base of the Bakken formation, has since been adopted as a regulatory pool name, and is also the name of a field that produces from both the Three Forks and the Bakken formations. Therefore, LeFever and her colleagues recognized the need for a formal clarification and nomenclature change.
A definite pay zone for Whiting
Regardless of what it is called, the Pronghorn unit has proven to be very productive.
For example, in announcing its third quarter 2012 financial and operating results, Whiting Petroleum said net production from its Lewis & Clark/Pronghorn prospects that focus on the Pronghorn unit, averaged 12,190 barrels of equivalent per day in the third quarter of 2012, an increase of 19 percent over the 10,275 barrels of equivalent per day in the second quarter of 2012.
Whiting said a typical Pronghorn well is its Solberg 14-11PH in Stark County, which flowed at an initial rate of 1,825 barrels of equivalent per day from the Pronghorn Sand on Sept. 17, 2012.
Whiting also cited results of two other wells completed in the Pronghorn unit and tested in late September; its Buckman 34-9PH well flowed at an initial rate of 1,964 barrels of equivalent per day, and its Buckman 44-9PH was completed flowing 1,545 barrels of equivalent per day.
Will the name stick?
Whether or not the new nomenclature will be universally adopted is yet to be seen.
LeFever told Petroleum News Bakken a speaker at a recent conference argued for continued use of the term Sanish.
“We proposed this,” LeFever said, “and now it’s open to discussion,” but went on to say that generally she believes it has been accepted.
LeFever said she is currently mapping the Pronghorn unit. Completion of the project, she said, depended on the availability of adequate well data.
