Jonah Field in the northern Greater Green River Basin, one of the
largest gas discoveries in the last decade and a poster child for
basin centered gas.
Standard Draw-Echo Springs Field in the Washakie Basin, which was
found in the 1970s and was one of the original fields determined
to be a large, basin centered gas accumulation.
"We determined that the Standard Draw-Echo Springs Field is
simply a linear sand trend draped across an anticlinal nose and
the gas is trapped in the high part of the sand where it goes up
and over the top of the structural arch," John Robinson said.
"The reservoir waters out both north and south down dip from
the top of the arch."
To the west of the main "bar" production has been established
in isolated fluvial sandstones that comprise small individual pools.
Robinson said he thought Jonah was a basin-centered gas accumulation
until a 3-D seismic survey in 1996 identified two faults that bound
"They join up dip like the prow of a boat — it is a classic
structural trap," he said. "After the 3-D seismic was
shot very few wells were drilled outside the faults because every
time somebody did drill outside those limits they got an uneconomic
well or a dry hole."
Also, he said, most people initially believed there was no water
at Jonah, but 10 years of state production records indicate that
substantial amounts of water are produced from the down dip wells
Law, however, has a different view of the Jonah Field: He said
the top of a regionally pervasive gas system that covers the entire
northern part of the Greater Green River Basin is found at about
10,500 to 11,000 feet, but in the Jonah Field the top of the gas-saturated
interval occurs at about 7,500 to 8,000 feet, making it much easier
to fracture, stimulate and produce.
"Outside the field where the depths are 10,500 to 11,000 feet
you can still fracture the rocks," he said, "but it is
not as effective and therefore often not commercial."
Law said those challenging the conventional wisdom have determined
the accumulations are not regionally pervasive because they are
not regionally commercial.
"We’ve always known that — it’s not a big revelation,"
he said. "But just because it is not regionally commercial
does not mean it is not regionally pervasive.
"In a basin centered gas system the reservoirs render some
gas," he said. "It just may not be in commercial amounts."