The Wattenberg Field north of Denver has become a big deal in the industry since its discovery in 1970 by Amoco Production Company.
It’s a twofer, kicking out both oil and natural gas.
To many, mention of the field conjures up thoughts of the upper Cretaceous Niobrara shale, which is a prolific producer there, along with other geologic formations.
The Codell sandstone, for example, is a major pay in this giant field. It occurs as a member of the Carlisle shale formation found directly beneath the Niobrara, and it overlies the Greenhorn shale, which also contains organic-rich source beds.
“The Codell is situated in a sort of ideal spot, being between organic-rich intervals,” said AAPG Honorary member Steve Sonnenberg, professor and Charles Boettcher distinguished chair in petroleum geology in the Department of Geology and Geological Engineering at the Colorado School of Mines. “That definitely contributes to the production.”
Early on, Amoco and other operators ignored the Codell, drilling right through it.
This changed in 1981 when significant shows grabbed their attention.
It quickly became a primary target for vertical wells, which were successfully completed using hydraulic fracturing. Some of these were co-completions with the Niobrara.
Advanced technology entered the picture a number of years later.
Horizontal drilling and multi-stage hydraulic fracturing were applied, and production soared.
Much of this action has been in the same locations as the earlier vertical holes.
“In the last two to three years, people have been drilling it horizontally in the same places where old vertical wells were drilled and increasing production,” Sonnenberg noted. “In some cases, production goes from 70 to 700 barrels of oil per day, which is a ten-fold increase owing to horizontal drilling and multi-stage fracturing.
“Together, the Codell and the (connecting) basal Niobrara, or what we call the Fort Hays, is a huge resource play in the Denver Basin,” he said.
Sonnenberg emphasized that technology is key to production, essentially giving new life to the resource.
Still, it always goes back to the geology, and he itemized a number of geologic and related factors that are fundamental to the production:
- Proximity to thermally mature source beds.
- Geothermal gradients.
- Pressure gradients.
- Fault-bounded reservoir compartments.
- Gas-oil ratios.
- Sufficient reservoir quality.
The Codell is a tight oil reservoir with low porosity, low permeability and abnormal pressure, according to Sonnenberg. He noted that it’s a low resistivity-low contrast pay (LRLC) for a variety of reasons, principally the clay content within the sandstone itself.
The sandstone is very fine- to fine-grained and bioturbated, with the depositional environment interpreted to be a shallow marine shelf setting.
“The fault-bounded reservoir compartments form mainly from a well-developed polygonal fault system,” he said. “The polygons are generally about one-and-a-half square miles in size.
“Orientation of the polygons is influenced by pre-existing basement fault systems,” he added.
When queried about current drilling activity, given the low oil price environment, Sonnenberg noted that while the rig count has dropped somewhat, a significant amount of the action continues to be economic.
“People are still drilling quite a few wells,” he said. “The rig count probably dropped less here because the quality of the production is really known, so people are not only getting after the Niobrara but also this Codell below.”