The southwest Wyoming region has almost everything you could want in a Rocky Mountain hydrocarbon province:
- Abundant exploration opportunities.
- Multiple petroleum systems and producing horizons, with good (if sometimes ambiguous) sourcing.
- Excellent existing gas production, including the Pinedale field, one of the top three gas fields in the United States.
- An emerging unconventional resource play in the Baxter shale.
- And a history of geological study and drilling going back 80 years.
Production from the Cretaceous typifies this area of the state, according to Randi Martinsen, a senior lecturer in petroleum geology at the University of Wyoming who also serves as AAPG treasurer.
“The productive horizons were deposited during the time of the major Cretaceous seaway,” she said. “It was a foreland basin at that time.”
The area contains a thick interval of Cretaceous age marine shale and associated marine to transitional-marine sandstones that historically have been the major exploration targets.
Oil production in the region includes the Patrick Draw field, which dates back almost 50 years and is now on tertiary, CO2recovery.
“Patrick Draw is a major oil field that produces out of the Upper Cretaceous Almond Formation. It’s a structurally influenced strat trap,” she said.
More recently exploration has focused on thick successions (thousands of feet) of variable net-to-gross, sand-rich fluvial systems, she noted. Production is from numerous vertically stacked, relatively thin and highly discontinuous tight sandstone reservoirs.
“That’s why the spacing can be down to 10 acres” in the large gas fields, Martinsen said.
The Pinedale field is “a big, whopping anticline -- it’s a really big structure,” and the nearby Jonah gas field also is a more subtle fault-bounded structural trap, she said. Both produce from the Lance formation.
Interestingly, Jonah, in addition to being the state’s biggest gas producer in 2006, also was the state’s biggest oil producer. Pindale, when more fully developed, is expected to overtake Jonah in both gas and condensate production.
Continuous or basin-centered gas plays not obviously tied to structure or stratigraphic facies changes can be found in the Wyoming Cretaceous, but Martinsen said those plays still require study and analysis.
“In reality they are very subtle traps,” she explained. “There’s a lot of gas in the basin but it’s not a case of, ‘There’s gas everywhere.’ People still have to do the work. They have to do the geology.”
Good Results
One new unconventional play generating good results in southwest Wyoming targets the Cretaceous marine Baxter shale.
Questar Exploration and Production Co. of Denver, a wholly owned affiliate of Salt Lake City-based Questar Corp., recently announced a nine million cubic feet-a-day well producing from the Baxter and Frontier formations.
That’s far and away the best results from any of the company’s 18 wells in the play, and Questar Exploration president Charles Stanley said the unusually high production rate probably was related to naturally occurring fractures.
The Baxter shale play is located in the Vermillion sub-basin of the greater Green River Basin, about 75 miles south of Rock Springs, Wyo., said Vincent Rigatti, general manager of Questar Exploration’s legacy division.
“We’ve been active in the Vermillion since the 1920s,” he said, “and it’s been a pretty steady pac of activity through the decades.”
Questar and other operators had drilled into and tested deeper potential pay zones in the region, including the Frontier, Dakota and Nugget, Rigatti said.
“Through that drilling they had to drill through the Baxter, and that was difficult because it was so over-pressured,” he noted.
When operators completed those wells and tried to produce from the Baxter, they got a “real nice flow rate, for one or two days,” he added.
As the Barnett shale play unfolded, Questar decided to take another look at the Baxter shale, Rigatti said. The company started with re-entries in two existing wells.
“What we found out is that not all shale plays are the same,” he said, “so we tried to compare and get information from the other shales to help with our evaluation of the Baxter.”
A drilling program to define the play has proceeded methodically and somewhat slowly, “in part because we’ve been evaluating such a big area.
Questar has about 140,000 net acres in the Vermillion,” Rigatti said.
“We’re working on a new EIS for that whole area that covers 157,000 acres,” he added. “That’s been going on for about a year.”
The Baxter Learning Curve
Like any shale play, the Baxter involves a learning curve -- and Questar Exploration has just started moving up the curve, Rigatti said. The company recently spudded a horizontal well in the shale.
“This is our first horizontal well (in the Baxter play) so we’re keeping things fairly simple from a well plan and completion standpoint,” he noted.
“Once we get the well landed and tested, it will take at least six months of production before we can have a good understanding of the ultimate results,” he said.
Questar also is looking at results from fracturing its earlier Baxter wells.
“We started out with what you’d call a conventional frac. Now it’s more of a slickwater frac -- at least, that’s where we are right now,” he said. “In some of the wells we’ve done up to seven frac stages in the Baxter alone.”
To date, Questar has kept one or two rigs working in the play. With a drilling boom going on in Wyoming, “it’s always challenging to get qualified personnel, especially during these periods of high industry activity,” Rigatti said.
Several Questar wells outside of closure are productive in the Baxter, he noted. And results vary as evaluation drilling has moved southward in the Vermillion.
“We have one well that showed lower pressures. We’re starting to come out of the overpressured cell and we have seen some oil production,” Rigatti said.
Questar officials have estimated per-well ultimate recovery at two-to-four billion cubic feet equivalent in the Vermillion unconventional play area.
Successful shale plays seem to reach a tipping point when enough knowledge is accumulated or uncertainties resolved, followed by hundreds or even thousands of wells.
Questar’s impact statement for the area “evaluates the drilling of up to 4,000 wells.
“That doesn’t mean you’re going to drill 4,000 wells, Rigatti added. “I don’t know what the ultimate number will be.”
Mowry Shale Sweetspots
Wyoming could see another shale gas play extending more into the central part of the state, this one in the Mowry shale below the Frontier.
Ronald Surdam of the Wyoming State Geological Survey presented an analysis of Mowry shale potential at the recent AAPG Annual Convention in Long Beach.
Surdam said the Mowry is known as a significant source rock. Below 8,000 feet in Wyoming it has generated all its potential hydrocarbons but expelled only 20 percent of its gas, he noted.
He said the Mowry shows a regional seismic velocity inversion surface at 8,000-9,000 feet. Although the Mowry can reach a depth of 16,000 feet in the state, he focused on the 8,000-10,000-foot area for gas prospecting.
Regionally, seismic results can display a dramatic velocity slowdown in the Mowry. Total organic carbon in the shale would have to approach 25 percent to account for that slowing, according to Surdam.
Because analysis has found a maximum 2.5 percent organic content in the Mowry, the seismic indicates a rich gas content, probably associated with fracture porosity, he said.
Surdam concluded that seismic attributes can be used to detect gas-rich sweet spots in the Mowry shale.
A shallower Mowry oil play has developed at the far eastern edge of Wyoming. Brigham Exploration of Austin has drilled three vertical and two horizontal wells in that play with its partner, American Oil and Gas of Denver.
Boom Times?
The U.S. Geological Survey completed an undiscovered oil and gas assessment of the Southwestern Wyoming Province in 2002. At that time, the USGS estimated a mean undiscovered resource of 84.6 trillion cubic feet of gas, 2.6 billion barrels of NGL and 131 million barrels of oil in nine total petroleum systems.
Martinsen said the survey’s assessment of gas potential helped touch off the southwestern Wyoming drilling boom.
A six-fold increase in natural gas prices probably didn’t hurt, either.
Tom Finn, a USGS geologist in Denver, worked on the southwest Wyoming assessment and coauthored its section on the Hilliard-Baxter-Macos system, with a mean estimate of about 12 trillion cubic feet of undiscovered, unconventional gas.
That’s a remarkable call, given the lack of existing production or even well control in the Hilliard-Baxter-Macos at the time.
“I can count on a map five or six wells that produced from that unit at the time we were doing the assessment, so we had no idea what was going on, really,” Finn said.
In addition to Baxter shale, the USGS identified the Phosphoria, Mowry Composite, Niobrara, Mesaverde, Lewis, Mesaverde-Lance-Fort Union Composite, Lance-Fort Union Composite and Wasatch-Green River Composite petroleum systems in the southwest Wyoming province -- defined by sometimes overlapping sourcing.
“We attribute most of the Paleo oil to Phosphoria source rock. Then we have the classic Mowry oil, early-late Cretaceous,” Finn said.
“Then there’s evidence for Type 3 Kerogen sourcing in other shales, and of course there’s the coals,” he added.
In its assessment, the USGS used the terms “unconventional gas,” “continuous gas” and “basin-centered gas” interchangeably.
Almost.
“Some unconventional plays are not basin-centered -- for instance, coalbed methane or some of the shallow gas plays,” Finn noted.
Most of the undiscovered gas resource in the area was classified as unconventional and continuous, and Finn said coalbed methane was not a major factor.
“As I recall, when we did that assessment we didn’t give the coalbed methane as much as we should have,” he said.
With multiple exploration chances, southwest Wyoming should be active for the foreseeable future -- despite, or maybe because of, its long exploration history.
Said Rigatti:
“It’s interesting. The old adage that the best place to find oil and gas is around oil and gas fields -- that’s true.”