It
started last spring at the AAPG annual meeting in Salt Lake City:
a new Great Debate.
Three independent
Denver geologists gave a presentation challenging the conventional
wisdom of basin-centered gas accumulations, and on what controls
production from gas fields like the giant Jonah Field in Wyoming's
Greater Green River Basin.
The controversy
is more than academic. Potentially billions of cubic feet of gas
could be at stake depending on which theory is championed.
The debate
continued last fall at a Rocky Mountain Association of Geologists'
symposium on "Petroleum Systems and Reservoirs in Southwest Wyoming."
For years
the three authors have discussed the concept of basin-centered gas
— and have struggled with how it can be applied to exploration
models, risk assessment and resource assessment, according to Keith
Shanley, Littleton, Colo.
His co-authors
are John Robinson, with North Ranch Resources in Littleton, and
Robert Cluff, president of the Discovery Group in Denver.
"We just
didn't understand the mechanisms … so it became a topic of conversation
among us as to how petroleum systems function and how traps are
formed in areas of low permeability reservoirs," Shanley said. "What
were the controlling factors?"
Over several
years they developed a new concept for low-permeability reservoirs
like those in the Greater Green River Basin, and determined that
most fields are not part of a continuous-type gas accumulation or
a basin center gas system in which productivity is dependent on
the development of "sweet spots." Rather, most gas fields there
occur in low-permeability, poor-quality reservoir rocks in conventional
structural, stratigraphic or combination traps ("sweet spots").
They believe
the basin is neither regionally gas-saturated nor near irreducible
water saturation, and that water production is both common and widespread.
"Understanding
field occurrence as well as reservoir and well performance in these
low-permeability gas systems requires an understanding of multi-phase,
effective permeability to gas at varying degrees of water saturation
under conditions of overburden stress," they wrote. "Understanding
low-permeability gas systems such as those found in the Greater
Green River Basin does not require a paradigm shift in terms of
hydrocarbon systems.
"We conclude
that low-permeability gas systems should be evaluated in a manner
similar to and consistent with conventional hydrocarbon systems."
Successful
exploitation of resources within low permeability gas systems requires
a focused, deliberate effort that fully understands the unique petrophysical
nature of these reservoirs and is able to integrate that information
with all elements of petroleum systems analysis, particularly an
understanding of trap-related elements, they wrote.
The three
concluded that all of the larger fields in the Green River Basin
are controlled by conventional trapping mechanisms and produce down
dip water.
On the
Other Hand
Ben Law,
a retired U.S. Geological Survey geologist who was instrumental
in developing the basin-centered gas model, does not agree.
Law remembered
that a 1979 AAPG publication by John Masters on the Elmworth Field
in Canada (Memoir 38) coined the term "deep basin gas." At about
the same time the USGS, with funding from the Energy Research and
Development Administration, began studying western tight gas sands.
"Chuck
Spencer and myself at the USGS observed similar accumulations in
southwest Wyoming, but not on the regional scale Masters had discussed,"
Law recalled. "However, through additional study we determined these
accumulations were regional in extent.
"Over the
years these initial ideas have evolved," he added, "but the basic
premise of that early work has remained intact."
Law said
there are four criteria that define basin-centered gas accumulations,
including low permeability, abnormal pressure, gas saturated reservoirs
and no down dip water leg.
"In our
research we found, as did Masters, that there is no down dip water
leg in these accumulations, unlike conventional buoyancy driven
accumulations that always have a down dip water leg," he said. "Also,
we were able to determine that these accumulations without exception
are abnormally pressured — either over- or under-pressured — and
the pressure mechanism has to be hydrocarbon generated, not rapid
burial or other means.
"These
are the four defining elements," he said. "Without these four you
don't have a basin-centered gas accumulation."
Law said
nothing through the years has altered his opinion, although the
initial concepts have evolved somewhat.
"It is
quite possible — and appears more common than we used to think
— that within the gas saturated basin-centered accumulations there
are water bearing reservoirs as well as interbedded conventional
reservoirs," Law said. "This is something we did not emphasize in
the early literature, but it became clear early on that there were
interbedded conventional and unconventional reservoirs.
"I think
people have assumed without reading the literature carefully that
there are not any conventional structural or stratigraphic accumulations
within a basin-centered gas setting."
Challenging
the Tenet
According
to Shanley, Robinson and Cluff, this traditional view of basin-centered
reservoir behavior led people to think that the tight part of the
basin was gas saturated — that the water had somehow been driven
out and pore spaces filled with gas.
"Within
that framework, if you were to drill a well in these areas and encounter
a better than average rock or a series of fractures you would expect
to produce gas," Shanley said. "Resource extraction then is simply
a matter of drilling and completion technology as well as gas prices.
"We don't
believe that tenet is true," he continued, "although there may be
some minor gas present — much of the gas can not be produced under
virtually any economic scenario."
The critical
difference between the group's model and conventional wisdom is
that, although these are gas rich areas and a lot of gas indeed
has moved through the system, productive areas are distinguished
by having high gas saturation, resulting in high effective permeability
to gas.
The high
gas saturations reflect a buoyancy-driven system.
"The only
places we find that build up to sufficiently high gas saturations
are in traditional traps," Cluff said. "In every case it turns out
that the conventional paradigms of petroleum geology explain the
areas where gas has built up to moveable saturations."
The three
studied 44 fields in the Greater Green River Basin, each with over
50 billion cubic feet of estimated ultimately recoverable reserves,
and found that 100 percent of those fields were in conventional
traps, according to Shanley.
"We were
unable to find one significant field that we felt fit the standard
definition of basin-centered gas accumulations," Shanley said. "Importantly,
all the fields with greater than 50 billion cubic feet of estimated
ultimately recoverable reserves make up 92 percent of all the gas
in the basin, so it is a statistically significant number."
Cluff added,
"Everywhere we had sufficient data to draw the maps we found a structural
or stratigraphic trap."
Much Ado
About … ?
While reactions
to the new model have varied from both extremes, there are some
geologists who wonder what all the fuss is about.
"They seem
to be making the point that you can't just drill anywhere in the
center of a basin and get gas. We've known that for the last 20
years," said Larry McPeek, a geologist with Thomasson Partner Associates,
Denver.
"You need
some reason to have a sweet spot, and that sweet spot may be controlled
by structural and stratigraphic changes," he said. "The two views
don't have to be mutually exclusive.
"My only
concern is that some might take away from this discussion a negative
outlook on basin centers as hydrocarbon hunting grounds," he continued.
"That would be unfortunate, because there is a tremendous amount
of oil and gas in basin centers because it is the cooking pot, and
if you have any sort of trap it is apt to be filled."
Shanley
emphasized that the group is in no way detracting from the prospectivity
of these basins or basin centers.
"We want
to be perfectly clear i that we think there are substantial gas
resources in these basins," he said. "These are gas-charged, hydrocarbon-rich
basins that have a multitude of trap styles. They are complex, and
in that complexity lies opportunity — but it is not the low risk
hunting ground many believe it to be.
"We simply
cannot pray to the gods of fracture stimulation, drilling fluids
and strong prices to make gas come out of the ground," he added.
"So, we feel the industry needs to think in terms of the risk process
by evaluating source, reservoir, seal and trap, just as companies
do in other regions."
The group's
concepts have generated such a strong reaction because the resource
numbers are so large in these basins, according to Shanley.
"It is
part of the nation's energy trust, and the implications of challenging
the paradigm that underlies all that potential resource are significant,"
he said. "There is a great deal at stake for people at all different
parts of the value chain — explorers, investors and pipeline companies
just to name a few."