Steve Sonnenberg’s advice to Bakken operators?
“Don’t forget the shales.”
The illustrious geology professor from the Colorado School of Mines says there are plenty of untapped opportunities within the prolific Bakken petroleum system, where almost all drillers are concentrating on two tight reservoirs — the middle Bakken and upper Three Forks.
What’s being largely ignored by industry, Sonnenberg says, is the source of the oil in those reservoirs — the upper and lower Bakken, which are organic-rich shale formations adjacent to the producing reservoirs.
Although those reservoirs have made North Dakota the second largest oil producer in the U.S., they contain only a fraction of the light, sweet oil generated in the adjacent shale intervals, which he described as both source rock and reservoirs.
“We can go back and look at some of the people who are legends in the geochemistry area. Hunt says about 95 percent of the bitumen remains in the source rock,” Sonnenberg told attendees of the Bakken Tight Oil Congress in Denver May 31. “Momper said … typical efficiencies are in the 5-10 percent range.”
In other words, Sonnenberg said, the two experts are saying the same thing; 90 to 95 percent of the oil has not migrated from the shale source rock intervals.
“Most of you are only focused on the middle Bakken and the Three Forks,” he said. “But somewhere down the road you may want to also consider the old shales — that was the play in the late 1980s and early 1990s.”
One company doing it
That said, there is one company drilling into the upper Bakken shale in the Williston basin, and that is independent producer Slawson Exploration, which was the third company ever to drill a horizontal well in the Bakken formation. (See original story on page 1 of the May 20 edition of Petroleum News Bakken.)
Per Vice President Craig Slawson, the company is successfully producing “some oil” from the upper Bakken shale member in three fields — the South West Big Sky project in Montana’s Richland County; the Lambert project in Montana’s Richland and Dawson counties; and the Squaw Gap field in North Dakota’s McKenzie County, which dates back to the 1980s, possibly earlier, when vertical wells were the norm. Slawson purchased the field from Headington Oil in 2003.
The Montana wells were drilled in 2011-11; the first upper Bakken horizontals in the Squaw Gap field were drilled and or completed in 2007-08. Permits were issued for six new Squaw Gap wells in mid-April of this year.
Sonnenberg said drilling into the shale was one of Headington’s “big things early on with the development of Elm Coulee (in Richland County).” One company executive, he said, thought “people would be drilling horizontals in the upper shale and lower shale sometime in the future. People have kind of gotten away from that.”
Later in his presentation, however, and without naming a company, Sonnenberg said, “as we get over into the Elm Coulee area there’s lots of extensions of Elm Coulee going on, but some of those extensions are happening in the upper Bakken shale, just like the old play in the late 1980s, 1990s. That may or may not turn into a significant play.”
In May, Craig Slawson, reticent to be interviewed at length, said Slawson Exploration was “the only company” to date that was targeting the upper Bakken shale using horizontal wells.
His job to ‘tease’ listeners with ideas
When asked to do a presentation for the Bakken Tight Oil Congress, Sonnenberg agreed to talk about delineating the edges of the Bakken petroleum system — not a surprising topic for a professor with a doctorate in geology who holds the Charles Boettcher Distinguished Chair in Petroleum Geology at CSM and specializes in unconventional reservoirs, sequence stratigraphy, tectonic influence on sedimentation and petroleum geology.
As the last speaker in the two-day conference, he also said it was his “job” to “tease” attendees with ideas that they could use going forward.
The Bakken petroleum system, which includes the Three Forks, Lodgepole, Pronghorn and, of course, the Bakken itself, is just one of seven petroleum systems within the Williston basin, all of which might be prospective, Sonnenberg said, making the basin “a great place to be.” (The other six Williston petroleum systems are, from shallowest to deepest, the Tyler, Mission Canyon, (Bakken next), Duperow, Winnipegosis, Red River and Winnipeg Group.)
Sonnenberg encouraged explorers to push the boundaries of the Bakken system both “laterally and vertically.”
His first overhead slide was a structural contour map at the base of the Mississippian (see adjacent map) that showed the commonly agreed to current limits of the Bakken formation, marked by a pinkish line called the LOD, or line of death for the Bakken.
He pointed to both the Brockton Froid fault system on the map that “cuts through northeast Montana, northwest North Dakota and on up into Saskatchewan” and to “apparent indications of where the Bakken maturity lines are, too,” shown by the “a green dashed line.”
The Brockton Froid was essentially the LOD on the western edge of the Bakken, but Sonnenberg thinks it might have some potential: “I think it’s a fuzzy lineament system. It is definitely present at the surface. You’re a little harder pressed to map it out at the Bakken level, but is it really a dramatic line of death in the overall scheme of things? Well, we know the reservoir quality, like Mike (Hendricks) illustrated for you, does dramatically increase north of that blue line (map titled Structure Bakken FM, CI 250 feet). That’s where some of the better porosity and permeability is in the middle Bakken, but the Three Forks is still extremely tight there.”
“We know that when we look at a west-to-east cross section across the basin that the maturity lines are not flat but they’re actually tilted and so we have higher thermal maturities on the western part of the Williston basin than we do have in the deeper part, and that can play a role then in your future analysis of where perhaps the edges of the play are,” Sonnenberg said.
The primary shale members are the upper and lower Bakken, but he also mentioned the “shale package in the lower Lodgepole, the false Bakken. It’s also a very good source rock.”
He acknowledged companies were “moving into the middle Three Forks, and trying to see if that’s going to be as productive as the upper Three Forks.” Some, he noted, were looking at an even lower Three Forks bench.
“Back in 1953 we knew the Three Forks was going to be important for the Bakken, although that knowledge seemed to have disappeared with some of these oil and gas cycles, as happens with any depositional basin,” Sonnenberg said.
He noted that Bakken shales were very organic-rich. “They’re world-class source rocks — let’s not forget that. They average currently about 11 weight-percent total organic carbon content.”
Some source rock mature in Canada
And those topnotch mature source rocks go even farther north than Canadian geologists realize.
“We learned a long time ago that there are a lot of measurements of maturity within the Bakken,” Sonnenberg said. “… Part of the work that started in the Bakken in the late 1970s was by Fred Meissner. He illustrated that there seemed to be a magic temperature in the Williston basin of 165 degrees Fahrenheit (74 Celsius) where all of a sudden the resistivity in the shale almost went to infinity, and he envisioned that … was related to maturity of the source rocks, and going from something that’s water-wet to oil-wet at that magic temperature number.”
Using a Schmoker and Hester map that displayed bottomhole temperatures for the Bakken, Sonnenberg said “if we shade in the magic 165 degrees Fahrenheit and greater, we see that it goes up the international border, and some of those source rocks are probably mature in Canada.”
The Canadians, he said, do not claim to have thermally mature source rocks in their land.
“They always seem to claim that all the oil migrated out of the U.S. but we know that’s not entirely the case. Some of that oil is self-generated up there,” he said.
Activity headed back to Montana
Sonnenberg noted that the modern Bakken play got its start in Montana in 2000, and that’s where he thinks “the play is going to eventually head back.”
“The good news is there are plays everywhere within this Williston basin area, but the new plays, perhaps in northeast Montana,” (see map adjacent to this article named “Structure Bakken CI: 50 ft”) are possibly the most significant, he said.
The new plays include the following, as described by Sonnenberg: “Elm Coulee extensions, they are already happening; the middle Bakken, the north central area; the upper and middle Three Forks in the north area; the middle Bakken and Three Forks in the Wolf Creek Nose area; the Poplar Dome area; the False Bakken source rocks, south of Elm Coulee and so on may turn out to be important in that carbonate interval within the Lodgepole right below the False Bakken. The Scallion could be an important target there in that area.”
Sonnenberg said he “looked at one core from Poplar. It’s entirely oil-saturated, hydrocarbon-saturated through the middle Bakken and the Three Forks.”
There is “lots of activity that’s happening in the northeast-central part of northeast Montana with lots of operators drilling wells; and then just a few wells being drilled to the northwest of the Brockton Froid system. Time will tell if that is indeed a very important lineament system. It definitely shows up on most people’s diagrams.”
Back to the shales
Sonnenberg came back to the Bakken system’s shales more than once in his presentation, noting at one point that “some of the work Price and LeFever did and published back in 1992” contained a diagram “plotting some of the S-1 data, hydrogen index data, porolysis data in this … transformation ratio data,” the importance of which was “to note the shales do read differently than the Lodgepole, the middle Bakken and the Three Forks. But the shales themselves are the S1, and that would be an indication of the bitumen present,” he said.
Sonnenberg suggested “looking at cores that have actually analyzed the shales,” providing a Price and LeFever core diagram from the Elm Coulee area. “Just highlighting the upper Bakken shale,” he said, “note its extremely high residual oil saturation.”
‘The future is bright’
“The future is bright” and will likely include the following, Sonnenberg said:
• Middle Bakken extensions and infills;
• Upper and middle Three Forks;
• Lower Lodgepole;
• Upper and lower Bakken shales;
• Lateral and vertical migration;
• Secondary or tertiary recovery.
