Three different approaches to interpreting
3-D seismic data on Flat Rock Field in Utah helped build a more
comprehensive picture of the targets in that field.
The approaches were presented to members of the Rocky
Mountain Association of Geologists and Denver Geophysical Society
at the annual 3-D Seismic Symposium in February in Denver.
"By the application of three different approaches,
there was a more complete understanding of the exploration and development
targets in these areas," said Paul J. Harrison, owner of Fall-Line
Exploration Inc. in Silverthorne, Colo.
Although the seismic work was done more than a year
ago, it was not disclosed publicly until the symposium, so, in Harrison's
words, "there was a lot of high interest in this."
Harrison and two other speakers discussed a survey
in the Flat Rock Field Area that covered about 27 square miles on
the south flank of Uinta Basin in Utah. Flat Rock Field is operated
by Del-Rio Resources.
The survey was designed to delineate features in the
subsurface related to the Hill Creek anticline, seen on the surface
geology, and to determine if 3-D data could be used for exploitation
work in the Flat Rock field.
The results of a classical approach showed that the
area was not on a structure, but was on a flank of a structure.
"The structural style was different than what you
would anticipate based on surface geology," Harrison said.
Another presenter reviewed the stratigraphic seismic
work done at the site and determined that there was a relationship
between the Dakota sand thicknesses and isopach maps from the seismic
data.
"Actually, a discovery was made in the Jurassic section
based on seismic amplitude work," Harrison said. "His conclusion
was that there were even more detailed attributes."
The classical interpretation was made on a surface
within the Wasatch and was a very painstaking effort, he said.
"Based on that interpretation, he did a modern attributes
analysis, spectral decomposition," he said. "Utilizing that, he
imaged a system within the Wasatch that revealed channels and lobes
and other stratigraphic elements that are exploration targets.
"There were discoveries made in the Wasatch prior
to this analysis," he added, "and this analysis validated the interpretation
of channels within that system."
The attributes analysis demonstrated the existence
of more of these features, he said.
Getting Good Images
New technologies allow attribute analysis to identify
Wasatch gas sands, said Bill Keach, of Magic Earth and formerly
director of worldwide market development for Landmark Graphics in
Denver.
Keach told the symposium attendees that a very detailed
interpretation of surfaces within the Wasatch was completed — and
based on that interpretation, two wells were drilled and successfully
completed, producing oil and gas from thin channel sands.
"This success spurred a detailed analysis of several
attributes in windows bracketing the surfaces picked in the volume,"
Keach said. "Typically, the gas-charged sands are characterized
by low seismic amplitudes."
Stratigraphic features like channels and deltas have
been clearly imaged using spectral decomposition, and several wells
in the Flat Rock Field have been completed in some of these anomalies.
Keach said the new technology allowed geologists to
find shallow, low amplitude gas sands in the Wasatch.
"There was a lake and it got erosion," he said. "It
showed how thin the walls were."
To find the channels, he started with a very good
interpretation and worked in meticulous fashion: "We brought the
channels into 3-D," he said.
"The 3-D seismic spectral decomposition lets you break
the window down into all different frequency slices," he said. "At
high frequencies, we have no channel."
Harrison presented an overview of the structural features
and interpreted tectonic history for the area, saying that a high
angle reverse fault and anticline were very well imaged in the deeper
section.
A well has been drilled in a closure on the anticline
and a second well is currently drilling.
"An almost vertical fault can be seen cutting through
the Mancos section," Harrison said. "Several normal listric faults
cut the Mesaverde, Wasatch and younger section."
Although these Wasatch faults don’t appear to
have deep roots, they are probably related to the older faulting
as an extensional "relaxation phase" during the Wasatch, he said.
He noted that the surface geology does not directly
reflect the faulting in the deeper section.
Stratigraphic Interpretations
A third speaker, Mike Pentilla, owner of Target Surveys
in Denver, covered stratigraphic interpretations in the Dakota and
Jurassic sections.
Within the Flat Rock Field, seismic isopachs are useful
in predicting Dakota net sand thickness, Pentilla said.
An amplitude anomaly in the Jurassic was drilled resulting
in a new zone discovery in the Flat Rock.
Based on subsequent seismic modeling and interpretation,
the new production is believed to be coming from eolian sands, he
said.
Del-Rio Resources has drilled 10 wells in the Flat
Rock, Pentilla added, and 3-D seismic and subsurface well data have
been integrated there.
"We defined a substantial pile of Jurassic sand,"
he said. "On the synthetic model, you can see polarity from a very
strong peak. We attribute that to an acoustic impedance."
The Hill Creek 3-D survey was shot by Western Geophysical
for two small independent companies, Wind River Resources and Del-Rio
Resources. The survey covered a portion of the Uintah and Ouray
Indian Reservation.
Additional support for shooting the survey was provided
by the U.S. Department of Energy.
Bill Barrett Corp. committed to earn an interest in
the area by drilling several wells. The company is currently drilling
its second well.