The Granite Wash play enticed operators into the eastern Texas Panhandle and far western Oklahoma, where they thought they were fully exploiting an attractive hydrocarbon accumulation.
The truth: They weren’t.
Cordillera Energy Partners LLC in Greenwood Village, Colo., found that producers had been overlooking promising pay zones across the 2.5-million-acre Wash.
AAPG member Ed LoCricchio, senior geologist for Cordillera, said bypassed pay started to become apparent when the company compared wells within a defined area.
The Granite Wash is the result of clastics shed from the Amarillo Uplift and Wichita Mountains from early Pennsylvanian through early Permian. The Desmoinesian Wash, which is the interval targeted by most operators today, attained gross thickness as much as 3,400 feet.
The Desmoinesian Wash can be subdivided into at least 11 productive benches, separated by regionally correlative shales that are also frac barriers.
“The problem is when one examines a well reported as a ‘Granite Wash’ producer, that vague term could refer to an individual zone, combination of zones or all 11 zones over 3,000 feet of section,” LoCricchio said.
That was a significant realization, LoCricchio said – and a key to his new understanding of the Wash’s history and potential.
Cordillera now estimates the total recoverable resource potential of the Granite Wash, which produces both natural gas and liquids, at more than 500 trillion cubic feet of gas equivalent.
LoCricchio presented an overview of the Granite Wash in the Discovery Thinking forum at the 2012 AAPG Annual Convention in Long Beach – and later told the EXPLORER how Cordillera had unraveled the mystery of Wash stratigraphy.
What’s In a Name?
Among the problems that confounded analysis of the play, LoCricchio listed:
- The perception that the Granite Wash contained only one or two reservoirs.
- A limited number of published studies.
- Analysis limited to subsurface study only, with no outcrops.
- Over 30,000 wells to correlate (which LoCricchio called “a blessing and a curse”).
- Petrophysical challenges, including the presence of radioactive minerals and variable clay content.
- No established stratigraphic framework.
When LoCricchio began looking at the local stratigraphy, the inconsistent nomenclature used by the play's multiple operators presented a major hurdle.
“The number one thing was the nomenclature problem. If you went back five years there were several established fields – Buffalo Wallow, Hemphill, Stiles Ranch – but everyone was calling the reservoirs different names,” he said.
Players extending exploration from the shelf side of the basin into the Granite Wash play compounded the problem, since they also tried to extend their existing shelf terminology and stratigraphic understanding into the Wash play.
LoCricchio described the Granite Wash area as bounded on the south by the Amarillo Uplift and Wichita Mountains, on the west by the Cimarron Arch and on the east by the uplift of the Nemaha Ridge, with a shelf to the north.
The presence of radioactive grains in the Granite Wash often made conventional gamma-ray logs ineffective as a tool for determining net pay, according to LoCricchio.
“I can't tell you how many log images I looked at where somebody took a ruler, you could tell, and tried to establish a gamma-ray cutoff. And they were missing hundreds of feet of pay,” LoCricchio noted.
“We also had variable matrix densities that affected the porosity logs,” he added.
In addition, the long history of the Granite Wash development had produced logs of all types that were difficult to correlate or compare. There were “multiple logging companies out there, using different tools, and you were comparing logs run all the way from the 1950s up to today,” he said.
Therefore, a rigorous log normalization process was required.
The Granite Wash is anomalously underpressured throughout almost the entire Texas portion of the play, LoCricchio noted.
“People were targeting deeper horizons in the past, primarily the Pennsylvanian Morrow and the Devonian Hunton,” he said. “Both of those reservoirs are very highly overpressured.
“People would want to drill with overbalanced systems,” he observed. “This masked a lot of the shows and what ultimately was proven productive in the Granite Wash.”
Analysis of the Granite Wash stratigraphy benefited from an excellent core study initially based on only 14 wells.
“Core Labs did a tremendous job,” he commented. “It became obvious that there were a bunch of individual stacked pays in there.”
Well control provided abundant information. The first Granite Wash well was drilled in 1920, LoCricchio noted, and the play contains tens of thousands of wells, from early vertical wells to numerous horizontals drilled more recently.
Cordillera had to develop another key resource itself. The variable matrix densities complicated density porosity calculations and frustrated attempts to make use of porosity logs.
“We saw that we could solve it if we had digital logs – which we did not have,” LoCricchio said. “So we had to do that on our own.”
He used IHS PETRA software to define zones or formations by surfaces, using identified tops when he could get them from operator reports in the Wash.
“A lot of people would just stop. They wouldn’t release any tops. Or when they reported to the state, they just reported the tops they could pick easily,” he noted.
As a result, Cordillera ended up picking many of those tops.
“We picked tops in thousands of wells, resulting in a database with over 200,000 proprietary tops,” he said.
Cordillera created very detailed structure maps for each one of the surfaces identified in the Granite Wash. LoCricchio said he projected those surfaces into every well in the basin.
“You can then define formations as zones based on those mathematical surfaces,” he said.
“Once you have that, you can extract meaningful perf, test and production data based on those surfaces, so all nomenclature issues disappear,” he explained.
Because Core Labs had taken an alphabetical approach to designating the Wash formations, LoCricchio used a similar method to label the Wash sections.
He divided the Desmoinesian-age portion of the Granite Wash into 11 zones, separated by regionally separated flooding surfaces. The base of the Granite Wash “F” and top of the “G” turned out to be something of a Rosetta Stone for correlating the stratigraphy, LoCricchio said.
“Once you find that shale bed in any Granite Wash well out there, you can flatten your cross section and begin picking out the shales above and below,” he said.
He had an unusual problem in identifying productive zones in the Wash. Instead of having to avoid including zones that weren’t productive, he had to keep adding in all the pay zones that were known producers.
“One of the toughest things to do was to make sure you’re honoring the pay where there’s production,” he said.
In the end, it became apparent that the Granite Wash didn’t contain only one or two productive reservoirs, but at least 15 distinct reservoirs, according to LoCricchio.
He said it would be theoretically possible to drill out 44 laterals in one square mile, with potentially four laterals drilled within each of the separate, productive pay zones.
“Once we were able to establish a stratigraphic framework,” he said, “we were one of the first to demonstrate the potential of each zone across the entire play area.”
Right now economics is driving development in the Granite Wash play, where a completed horizontal well with a 4,700-foot lateral can cost in excess of $6 million, according to LoCricchio.
“In the central part of the Wash, the upper zones are liquids-rich and that’s what people are going after,” he said.
Looking ahead, he sees the future of the Granite Wash in horizontal drilling and deeper horizons. Horizontal drilling revolutionized the Wash play and allowed it to be seen as a resource play, he said.
“It was a great example of new technology enabling development of a giant field in a very mature basin,” with decades of development to come, he observed.
“In summary I’d say, pick the thing apart,” LoCricchio advised.
“Every time we turn around, we find more and more to go after.”