NOT Middle Bakken or Three Forks!

Rocks Led the Way to Sweet Pronghorn Spots

The regional Hydrogen Index (HI) of the upper Bakken Shale, based on public domain data accessible from the U.S. Geological Survey, supplemented by in-house data. Hot colors are low HI values indicative of greater levels of thermal maturity; cooler colors are higher HI values indicative of lower thermal maturity. Named project areas highlight Whiting’s Williston acreage holdings; the productive Pronghorn zone is the southern-most entity on the map.
The regional Hydrogen Index (HI) of the upper Bakken Shale, based on public domain data accessible from the U.S. Geological Survey, supplemented by in-house data. Hot colors are low HI values indicative of greater levels of thermal maturity; cooler colors are higher HI values indicative of lower thermal maturity. Named project areas highlight Whiting’s Williston acreage holdings; the productive Pronghorn zone is the southern-most entity on the map.

It’s a jubilant occasion, indeed, when one discovers a new sweet spot in a known play area.

A key to success: Keep an open mind.

“The discovery of various Bakken petroleum system sweet spots over the last 10 years typically advanced through application of preceding paradigms to new areas,” said AAPG member Orion Skinner, senior explorationist at veteran Bakken shale player Whiting Petroleum Corp. in Denver.

“By contrast, each new sweet spot represents a distinct combination of multiple play factors including source rock quality, maturity, reservoir quality, pressures, to name a few,” Skinner noted.

“Remaining open minded to a variety of sweet spot factors is fundamental to tight oil resource play exploration.”

The team members at Whiting practice what they preach.

A striking example of this approach to business is their discovery of the highly productive Pronghorn zone that lies beneath the lower Bakken shale and overlies the Three Forks formation in the Williston Basin in North Dakota.

“It’s an extra unit that was predicted by good core work and sequence stratigraphy,” said AAPG member Lyn Canter, technical adviser at Whiting, now the dominant producer in the Pronghorn interval in Stark and Billings counties.

“We recognized this new zone and got about a year’s jump on everybody else and got our lease position put together,” Canter said.

This Was No Accident

Whiting drilled the discovery well in the Pronghorn zone in the first half of 2010 and has drilled more than 30 wells in the field to date. The horizontal wells have a measured depth of approximately 20,000 feet, and some IPs have tallied as much as 3,100 bopd with very little water.

Photo a: A slabbed core photo (three inches across) showing the main Pronghorn pay facies is burrowed detrital dolomitic siltstone with finer grained rippled interbeds (light colored beds). 

Photo b: The SEM image of the main pay facies shows common zoned dolomite grains with rounded detrital dolomite cores. Dark gray grains are quartz.
Photo a: A slabbed core photo (three inches across) showing the main Pronghorn pay facies is burrowed detrital dolomitic siltstone with finer grained rippled interbeds (light colored beds). Photo b: The SEM image of the main pay facies shows common zoned dolomite grains with rounded detrital dolomite cores. Dark gray grains are quartz.

The main Pronghorn pay facies is burrowed detrital dolomitic siltstone with finer grained rippled interbeds, according to AAPG member Mark Sonnenfeld, vice president geoscience at Whiting.

At first glance, Whiting’s high profile operating presence and expertise in the basin – particularly in the Middle Bakken interval at the giant Sanish and Parshall fields in Mountrail County, N.D. – might suggest that the Pronghorn discovery was a lucky break.

Au contraire. Pronghorn was no accident.

“Detailed core and detailed log evaluation across the basin identified this extra package of rock that ultimately became this Pronghorn zone, which is not present at Sanish-Parshall,” Canter said.

“My view is it’s the very close team interaction and the integration between core work and the logs at a very detailed level,” Sonnenfeld emphasized. “That’s what really unlocked this.

“We ran across a core that was very obviously different than anything we had seen,” he said. “In trying to understand it and map it out, this area fell out of that effort.

“One of the elements of this story we’re trying to get across is that this was not an accidental discovery at all,” he noted. “It was very much something we converged on through our mapping, integrated with core data.

“For us, informally, the moral of this story is geology matters,” Sonnenfeld emphasized.

Similar, But Different

Canter noted they have studied nearly every Bakken-Three Forks core available in the basin.

“We go to the North Dakota Geological Survey in Grand Forks twice a year deliberately to see newly released cores,” she said. “We had evaluated more than 150 cores when we came across this new information, so we were able to recognize that it was different, because we had the context to put it into and to understand what it meant in terms of a new play.

“We have a dynamic look into the Bakken because of our dedication to evaluating as much rock data as possible,” Canter said. “Our focus is a basin-scope effort and not just small project areas.”

Whiting has an in-house laboratory that includes a large core layout area where the drilling engineers can visit to see the rock they’re drilling through, which accelerates the learning curve to get the horizontal drilled quickly and in the right zone, according to Canter.

The non-accidental nature of the Pronghorn discovery is highlighted when you consider that a lot of other plays have progressed by searching for a look-alike from a previous play.

“This works to some extent, but there are always important differences when you find the next one,” Skinner said. “This play is not at all a look-alike of any other Bakken system accumulations, so to some extent it’s a first principles discovery, other than we are operating within the petroleum system of Bakken shales.

“The reservoir of the Pronghorn is different, and that’s what sets it apart from the Middle Bakken fields and the Three Forks fields,” he said. “It has similar source rocks, but it’s a different strategic interval.”

Comments (0)

 

Discovery Thinking Forum

AAPG member Orion Skinner will present the paper “Discovery of Pronghorn and Lewis and Clark Fields: Sweet Spots Within The Bakken Petroleum System Producing From the Sanish-Pronghorn Member – NOT the Middle Bakken or Three Forks!” as part of the Discovery Thinking Forum at the AAPG Annual Convention and Exhibition in Long Beach, Calif.

The Discovery Thinking session will be held from 1:15-5:05 p.m. Monday, April 23. Skinner’s paper is slated to begin at 3:40 p.m.

Skinner is senior explorationist for Whiting Petroleum Corp., Denver.

Emphasis: Annual Convention