The use of 3-D seismic and subsurface geology applications in Wyoming's Jonah Field should result in enormous savings in drilling costs, a consulting geologist recently told members of the Rocky Mountain Association of Geologists.
"Subsurface geology was really of value here," said Edward M. Warner, a consulting geologist in Denver who said he doesn't anticipate drilling any dry holes now in the Jonah Field.
"The 3-D seismic program showed it was a curved fault with a single plane," he said. "Jonah's a big fat pressure compartment."
Because of 3-D, "we are drilling right now right along these boundaries. We're putting wells right on the fault," he said. "We can get extremely close to the edge of the field without drilling dry holes."
Warner was one of several geologists making presentations in Denver at a one-day 3-D seismic symposium sponsored by RMAG and the Denver Geophysical Society.
The Jonah Field, located in the northwestern arm of Green River Basin in Sublette County, Wyo., produces gas-condensate. The field was initially believed to be a basin-centered gas accumulation.
But in the last two years, evidence from 3-D seismic and wellbore data have confirmed that the Jonah Field is a complicated wrench fault bounded structural trap, Warner said.
The bounding fault acts as lateral seals to a large pressure compartment, he said.
A northeast-southwest trending crossfault provides the updip seal to the west and a lateral seal to the north, while a northeast-southwest wrench fault provides the lateral seal to the south, he said.
"It's an area of compression that gives rise to structural elements," he said.
A 51-square-mile 3-D survey shot in December 1996 by Amoco Production confirmed wrench faults in Jonah that form the boundaries of a large pressure compartment.
"The faults extend from the basement to the near surface," Warner said. "Splays, minor reverse and normal faults internal to the field boundaries are also visible in this survey."
Throwing Darts
Warner's exploration in the Jonah Field area started with success from the very first well -- in fact, when drilling began in 1993, the first three wells were successful, producing about 3 to 4 million cubic feet of gas per day.
But the fourth hole that was drilled came up dry, even though it was located only half a mile south of a very good producer, he said.
"This well had silica cement from the top to the bottom," he said. "From my training, I knew that silica cements means faulting. I proposed there was a thrust fault in the near vicinity."
At this point it began to look like there was structure inside Jonah, he said. "I started drawing faults into the system," he said.
When Amoco was about to drill a well in the field in 1996, "we decided Jonah had to be a wrench fault structure," Warner said. "It could be a normal or reverse fault."
Amoco later abandoned that well site -- and Warner and his partners then demonstrated there is a pressure compartment inside Jonah.
Warner said 3-D seismic has enabled operators to drill within several hundred feet of the pressure compartment boundary and remain within the field.
"Dry holes should almost be eliminated during field development due to the accurate mapping of the field boundaries by 3-D seismic," he said.
The 3-D seismic conducted in early 1997 confirmed the field boundaries. The fault on the south side is a wrench fault that is near-vertical and extends from the basement to the surface.
"Segments appear to be imbricated and sinusoidal rather than merely vertical," he said.
Currently there are four wells south of the field, four wells west of the field, two wells north of the field and 110 wells within the field that confirm that Jonah is a large wrench fault bounded structure with a separate pressure compartment inside.
Inside the field, pressures start at 7,500 feet, while outside the field pressures start at 4,500 feet.
To the north of Jonah, all the wrench faculty is northeast-southwest. But the change in principal stress direction is to the north-south, he said.
"That's what is responsible for the north-south direction of the boundary of Jonah," he said. "It cuts off the line of death fault."
Jonah Geology
The Jonah Field covers more than 37-square-miles, under which there is an average of 400 net feet of productive sandstone. Gas in place exceeds 9 TCF.
Currently, there are 110 producing wells and 150 proven and probable undeveloped locations on 80-acre drillsites, Warner said. The preliminary ultimate recovery per well appears to be about 5 to 6 bcf.
Jonah Field is the second largest commercial gas accumulation in the Northwestern Green River Basin after the Pinedale Anticline.
Warner said that gas production at Jonah comes primarily from non-marine sandstones of the upper and middle Lance members of the uppermost Cretaceous Lance Formation of Maestrichtean age.
The upper Lance interval at Jonah is thermally immature but abnormally pressured, he said.
Outside the field these same rocks at the same depth are water-bearing or water and gas-bearing and normally pressured, he added.
Outside of Jonah Field, the top of overpressures coincides with thermal maturity at depths ranging from 11,000 feet to deeper than 12,000 feet. Inside of Jonah, from updip to downdip the top of overpressuring varies from 7,500 feet to 9,800 feet -- for the most part conforming to the field's structural dip.