Technology + Price = Giant Tight Gas Field
'Goldilocks Rocks' Pay Off for Vernon
A field that can be touted as an industry showpiece for what can be accomplished with the right technology in combo with the right commodity price sometimes attains its lofty status only after years of expensive trial and error -- and frustration -- on the part of the operators.
Vernon Field in north Louisiana is a prime example.
The field, which is located in Jackson Parish, was initially discovered in 1967 when a well drilled into pay in the Upper Cotton Valley Cadeville formation -- the Cadeville is the first sand below the tight Knowles limestone, which is a regional top seal for the over-pressured Cotton Valley, according to Andrew Mehlhop, manager of G&G for the eastern Gulf Coast at Anadarko Petroleum, which has operated Vernon for the past six years.
It wasn't until 1980 that the vast potential of the Lower Cotton Valley sands in the field was recognized, when a non-commercial well drilled and tested the formation.
Even though close to 20 additional wells were drilled following this discovery, Vernon languished as a kind of ho-hum little tight gas field for almost two decades as a succession of owners tried unsuccessfully to turn it into a winner. Low commodity prices and lack of appropriate technology were prominent hindrances.
Anadarko's success in developing the Bossier Sand in the East Texas Basin tweaked the company's interest in evaluating Vernon, and it purchased the field in late 1999.
As a result of Anadarko's ensuing development activity, Vernon has expanded from 7,400 acres in 1999 to 25,000 HBP acres today, with more than 300 wells on 40-acre spacing.
Over 400 Bcf of gas have been produced, and estimated ultimate recoveries per well range from 1.43 Bcf up to 27 Bcf, according to Mehlhop, who noted the gas is sweet with no significant H2S or CO2.
Today, Vernon ranks as one of the nation's giant tight gas fields, boasting more than four Tcf of original-gas-in-place with estimated recoverable gas pegged at 1.8 Tcf. The field produces from the Lower Cotton Valley at depths of 12,000 to 15,000 feet along a growth-faulted anticlinal trap.
The Sweet Spot
In what smacks of an oil patch version of musical chairs, a new-old operator -- EXCO -- is in the process of purchasing the field from Anadarko.
EXCO initially bought the field from Apache for $28 million, Mehlhop noted. Anadarko later purchased the field from EXCO for $40 million and is selling it back to EXCO today for $1.6 billion.
This big ol' tight gas field is sourced by the Bossier shale, which actually is a prolific oil-prone source rock of unknown thickness locally in the Vernon area, according to Mehlhop.
"Original expulsion from our basin modeling studies shows there was probably peak oil expulsion in the Lower Cretaceous," he said. "There was a structure and a trap in place and a reservoir in the Lower Cotton Valley. Conditions were right to migrate the oil into that structural trap, and the implications are Vernon Field was likely a nice oil field during the Lower Cretaceous, with high porosity and permeability.
"Then another 100 million years of time went by, and the field was buried another 10,000 feet or more," Mehlhop said. "The temperature was hotter and all of that source rock went into the gas window, as well as all the oil previously generated and trapped in the field.
"That's why Vernon is a nice sweet spot in north Louisiana," he noted. "It probably retained hydrocarbons in the trap for a long time, which preserved the porosity when everything was getting compacted and buried and affected by water and diagenesis.
"The sweet spot at Vernon was protected because it had oil in early that stayed there through its history until it cracked to gas," Mehlhop said. "Early oil emplacement preserves the porosity -- average porosity today in the Lower Cotton Valley at Vernon is 7 percent, with streaks of 10-12 percent locally."
The field may be loaded with gas, but there are myriad factors that enter in to making a play like this work, according to Steve Blanke, exploration manager for the West Texas/Mid-Continent exploration team at Anadarko.
For instance, an understanding of clay content and type figure heavily in getting the gas; this knowledge is best acquired via thin sections of the rock.
"One of my colleagues coined the term 'Goldilocks rocks,'" Blanke said. "The amount of clay in the sand has to be just right.
"Too much clay and you get ductile deformation of the clays around the sand grains, and it won't fracture easily," he noted. "This also occludes porosity and permeability.
"Too little clay and you have very clean sands," Blanke said, "that because of the depth the sands were buried to, you get a lot of quartz overgrowth and destructive cementation that basically makes the rock unproduceable."
Blanke attributes the field-wide overpressure at Vernon to the long-ago conversion of oil to gas. The ensuing increased volume had no place to go, resulting in pressure build-up.
Looking for 'Smoking Guns'
Advances in technology were key to unlocking the trove of natural gas at Vernon.
"High Pressure High Temperature (HPHT) drilling played a key role in this thing going gangbusters," Mehlhop said. "We couldn't have developed the field before the industry started breaking through with the right technologies for drilling and completing tight gas sands.
"Much of the challenge is temperature-related," Mehlhop added. "Most of the tools historically would fail above 300 degrees Fahrenheit."
He likened it to the perfect storm, where the technology and other requirements all came together, including advances in 3-D seismic data, which now covers the entire field and provided a highly improved structural and stratigraphic picture over the earlier 2-D seismic.
A return to respectable commodity prices following the downturn of the late '90s also figured heavily in the field's extensive development.
In fact, Mehlhop noted the field reached its peak production rate of 350 MMcfd at the end of 2004 and into 2005, when gas prices skyrocketed up to the $14/mcf range at one point -- another bit of serendipity.
So, you ask, why sell something that appears to be a cash cow?
It's strictly business, Mehlhop emphasized -- just part of the process Anadarko is going through to pay down debt and fine-tune its focus following a couple of big acquisitions.
"We love the tight gas play, and Anadarko is full-blown in the play," Mehlhop said.
He noted Vernon is just one of many 'string-of-pearls,' similar-type fields the company has been discovering, including the East Texas Bossier Field prior to Vernon. The company believes the Lower Cotton Valley tight gas play extends eastward into Mississippi and is exploring in that direction.
Blanke noted there are fields and show wells scattered about this whole area.
"I call the old wells drilled in the late '70s and early '80s 'smoking guns,'" Blanke said. "There would be kind of an artificial end to a field's life because prices couldn't support the stimulation they had to do.
"We've seen more of these 'smoking guns' along trend into south Mississippi," he noted. "They're usually data sparse, maybe one well and older suites of logs and likely no core data."