Oil and liquids-rich gas deposits are hot targets today among prospectors and operators – and the action in the self-sourced Niobrara shale play in the Rocky Mountain region is about as fired up as it can be.
There’s plenty of area to explore, given that the Niobrara is present all across New Mexico, Colorado, Wyoming, Kansas, Montana and North and South Dakota, according to Denver-based AAPG Honorary Member Randy Ray.
He noted the Niobrara is part of the Cretaceous seaway that covered the entire middle of the United States.
It’s a shale, but not a shale.
“The Niobrara petroleum system is a major petroleum system in the Rocky Mountain region, consisting of really rich source rocks with total organic content between 3 and 8 percent in areas, and the reservoir rock primarily being limestone or chalk intervals,” said AAPG past president and Honorary Member Steve Sonnenberg, professor and Boettcher Chair in petroleum geology in the Department of Geology and Geological Engineering at Colorado School of Mines, Denver.
“The formation demonstrates facies changes that range from limestone and chalk in the eastern end to calcareous shale in the middle and eventually transitioning to sandstone further west,” he said.“Depth and thickness are highly variable.”
Source bed plays are highly dependent on an array of technologies, including:
- Source rock evaluation.
- Normal surface and subsurface mapping.
- Resistivity mapping.
- Lineament discrimination.
- 3-D, 3-C seismic imaging.
- Borehole fracture mapping.
- Surface geochemistry (microseeps).
- Horizontal drilling.
- Multi-stage hydraulic fracturing.
Horizontal drilling itself is not new in the Niobrara, but going lateral in combo with multi-stage fracking has only recently become de rigueur – and as this technology becomes more commonplace, players will have much better insight into how the play will develop.
Plenty of Models
Probably the busiest of the busy areas today is the Denver (DJ) Basin in southeast Wyoming and northeast Colorado, where many wells are being drilled.
The basin extends into western Kansas and Nebraska.
“In the deeper parts of the Denver Basin, the Niobrara porosities and permeabilities are low, less than 10 percent and less than 0.1 md, respectively,” Sonnenberg said.
Natural fractures are important in enhancing the reservoir and have been related to multiple origins: wrench faulting, salt dissolution, local structures, pore pressure, listric faulting and regional stresses.
“Temperature gradients affect production type in the deep basin area,” he noted.“High temperature gradients are associated with gas and condensate production (e.g., Wattenberg field) and lower gradients with oil production with associated gas (e.g., Silo field).
“Resistivity mapping in the chalks has been used in the past as an indicator of hydrocarbons presence and source bed maturity,” he added.
Owing to the number of causes of fracturing, Sonnenberg cautioned to not get locked into one model.
“Understanding the regional stress field is most important, but local faults and folds have impact on fracture orientation also, so you need to understand the different kinds of models out there,” he emphasized.
“I think there will be multiple models to make the Niobrara work.”
Currently, there’s significant interest and activity in the Silo Field in extreme southeastern Wyoming in the Denver Basin’s northern area. It has produced since the 1980s, with the first horizontal well drilled in 1990.
A salt dissolution edge runs through the area trending north 70 degrees west, so the horizontal wells are drilled northeast-southwest to encounter the maximum number of open fracture systems trending to the northwest.
Depth of the Niobrara at Silo is 7,500 to 8,500 feet, and well costs are in the $5 to $6 million range, depending on the number of frack stages. Laterals generally are about 5,000 feet, but Sonnenberg said some operators are considering 10,000 feet – like the Bakken.
The B interval of the Niobrara is the favored target in the basin (the B2 in particular) as well as in the Powder River Basin. Sonnenberg noted that the Niobrara’s A, C and basal Fort Hays intervals also have potential, with the Fort Hays being less extensive aerially.
Silo has kicked out 10.4 million barrels of oil and considerable gas during its long productive history. The consensus is that horizontal wells using multi-stage fracking could greatly increase the production numbers.
Among the current activity, a number of new locations are showing significant production around the margins of the Silo field.
“This is normal for these unconventional systems that typically have diffuse, or fuzzy, field boundaries that can be enlarged,” Sonnenberg emphasized
It appears to be that there’s a new play everywhere you look these days.
“The Greenhorn under the Niobrara is another potential play,” Sonnenberg said. It’s a limestone-chalk unit sitting on top of the Graneros shale, which is organic rich. The Greenhorn is the reservoir but could be source rock also.
“Together, those should make an attractive target for horizontal drilling and multi-stage completion,” he commented.
It is significant that there are two large multi-client 3-D surveys being implemented in the Denver Basin:
- Global Geophysical Services is shooting an 800-square mile 3-D in greater southeastern Wyoming.
- To the south in northeastern Colorado, a partnership comprised of Geophysical Pursuit and Geokinetics is acquiring seismic data over a 1,000-square mile area, with plans to acquire more.
“Those are going to help unravel the structural history of the basin, the causes of the fractures, the orientation of the fracturing systems,” Sonnenberg said.“There will be lots of new learning from these large shoots.”
CGG reportedly has proposed a multi-client shoot, and several operators have implemented proprietary surveys.
“There’s significant potential in many Rocky Mountain basins, and we should see a lot of wells being drilled in the next three to 10 years,” Sonnenberg predicted.
“The good news about the Niobrara is the kerogen associated with it is Type II and will give both gas and oil,” he said.“When you get into the deeper part of the basins where it’s more thermal cracked, you still have a big liquids component, lots of condensate and it’s still very attractive.”