Low Fold 3-D Working in Montana
Cutting Costs Without Quality Cuts
After years of successful exploration with 2-D seismic coverage, low-fold 3-D seismic programs are helping to cut costs in the search for overlooked gas traps in some areas in Montana.
Specifically, placing shot and receiver lines farther apart and conducting fewer sweeps helped drive down costs of a 3-D seismic survey of the Eagle Formation gas play in northern Montana, according to Eric Johnson, of Johnson Geophysical in Billings, Mont.
Johnson discussed the low-cost, low-fold way of designing a seismic survey during the annual 3-D Seismic Symposium sponsored by the Rocky Mountain Association of Geologists and Denver Geophysical Society.
Johnson said that by reducing some acquisition parameters companies can shoot 3-D and get more data for the money.
“Seismic costs are escalating along with drilling costs, and everybody is trying to keep the costs as low as possible,” he said. “If saving money is your goal, the parameters you use for your 3-D survey can make a significant difference, while still acquiring useable data to evaluate your geological objectives.
“The number of sweeps affects the pricing,” he noted, “as does the number of shots and receivers. If you find you can get by with lower parameters, you do save money.”
Another way to cut costs, he suggested, is to join with other companies working in the same area to split mobilization expenses in bringing in a crew.
“That is possible if there are several companies exploring in the area,” he said.
“It’s a learning process,” he continued. “We don’t get as good data as higher fold 2-D data, but 2-D lines can’t image structure as accurately as 3-D. Better spatial imaging of structures and faults is a benefit of doing 3-D over 2-D.”
In the last three years, cost-effective low-fold 3-D seismic surveys have been used to exploit a prolific shallow gas area that surrounds the Bearpaw Mountains in north-central Montana.
Located in Montana’s Hill and Blaine counties, the area has produced more than 600 BCFG from the Cretaceous Eagle Sandstone, at depths ranging from about 500 to 1,400 feet deep.
“The strata containing the gas produce strong seismic reflections and the play is located below wheat and hay fields,” he said, “a perfect place, seismically and topographically, for low-fold seismic data.”
“This is an area with historically low exploration costs, where dry holes average about $50,000 and small operators have strived to keep seismic costs low, in line with drilling costs,” he said.
The 3-D programs were designed to reduce cost in order to compete with 2-D seismic at $6,000 per line mile and the low drilling cost. The low-fold 3-D seismic surveys were a way to provide affordable 3-D structural imaging.
“Small operators are willing to drill rather than shoot more seismic,” he recalled. “With 2-D seismic, we missed a lot of gas.”
Johnson said that 10 million cubic feet of gas will pay for a square mile of 3-D -- and gas traps much larger than that can be overlooked with a 2-D survey.
“The data are only 1- or 2-fold above 1,000 feet deep, but the complex fault trends and structural traps are imaged surprisingly well due to the contrasting reflectivity of the shallow strata,” he said.
Where the Lines Cross
About 100 miles west of this area, also in Montana, Johnson’s firm is conducting a 3-D survey in an area that has not been previously explored by seismic.
“We did a 3-D survey and the shallow data suffers somewhat,” he said. “It’s not perfect, but we’re accepting less perfection in resolution to get two times the data shot. Imaging a larger area for our limited budget has helped us to better understand the structural trends.”
Typically, 2-D seismic surveys require three or four line miles to evaluate a square mile, so they cost from $18,000 to $24,000 per square mile.
“The 3-D is twice as expensive, about $45,000 per square mile,” he said. “This includes about $3,000 for permitting, $5,000 for surveying, $35,000 for field acquisition and $2,000 for data processing.”
“The cost of the 3-D seismic compares favorably with drilling cost and with exploration risk,” he said. “The superior spatial imaging and data migration of the 3-D seismic over intermittent 2-D lines have clarified fault orientations and traps, resulting in more optimum well locations.”
Traps that had been overlooked or misinterpreted with 2-D lines spaced 1,500 to 2,000 feet apart were delineated and drilled, Johnson said.
“Careful record editing, velocity analysis and first-break muting are essential to optimize 1- and 2-fold data,” he said. “Of course, a lower field effort requires a more hands-on effort to process the data.”
He noted that processing represented less than 5 percent of the total project cost.
“This is not a place to cut corners,” Johnson said. “Don’t skimp on data processing, if you’re going to skimp on the field effort.”