One of Chevron's breakthroughs in the
offshore Congo Basin came using the technology called very AVO (amplitude
versus offset) analysis, which allows interpreters to look at the
offsets in the seismic shooting gathers and measure the changes
in response of seismic wavelets on those offsets.
"This developing technology is certainly not a pure
risk reducer yet," said Tad Schirmer, Chevron's exploration manager
for Block 14, "but it is helping us understand how we need to calibrate
and model the sands."
Understanding the distribution of these complex channel
systems is critical to targeting an exploratory well and characterizing
reserves for development decisions. The manipulation of the 3-D
data is especially important -- and Chevron's Block 14 team has
found that visualization technology is another important element
to the process.
"In many ways a 3-D seismic cube is usually interpreted
in a pseudo 2-D sense -- along inline and crossline views -- and
that's very limiting," Schirmer said. "We find 3-D visualization
techniques allow us to see the entire cube volume. That allows the
interpreter to develop a 3-D model in their mind of what the distribution
of channels and structures are and from that framework build a more
detailed interpretation.
"In some ways that initial visualization can cut
right to the quick," he continued. "You see a flat spot or an amplitude
anomaly immediately and are able to focus in more quickly on those
anomalies."
He said that 2,000 square kilometers of 3-D data
would require years for a traditional line-by-line, trace-by-trace
interpretation. With visualization technology, however, they can
focus on the primary anomalies and structures and then get in and
work the details.
"We have one team working on building the more traditional
regional framework of surfaces and faults, while other specific
team members are focused on specific anomalies, getting those to
the forefront and into the prospect cue quicker," he said. "This
approach allows us to speed up the process of identifying prospects."
Running a Trap Play
In addition, Chevron scientists have written code
that allows them to link these visualization programs with traditional
interpretation programs, providing a seamless link between visualization
and traditional interpretation -- and integrating the work process.
Following the interpretation phase, all the data
goes into building 3-D models.
"We can run well-path planning ... do volumetric
assessments ... look at distributions of structures and channels
in three dimensions at depth," Schirmer said. "It's the integration
of all this information in three dimensions that really makes it
possible to understand the distribution of the channels, lower the
risk and get to the anomalies that look the most prospective.
"We think we now understand why those two wells in
deeper water were unsuccessful," Schirmer said, "and we have offsetting
prospects we think have significantly lower risk and higher probability
of success that we will be following up on."
Chevron, with numerous untested channels in the block,
is focusing its activity on the primary structural traps. Chevron
still has several significant structural features with multiple
channels that are completely untested.
"We've got quite a robust inventory, since our strategy
is to drill one primary well on a structural trap that can carry
a development with stand-alone economic thresholds," he said.
Offsetting this primary well are numerous opportunities,
Schirmer continued, some of which are quite large and some more
satellite type accumulations.
"At present, the economic threshold for a primary
well is about 250 million barrels," he said, "but once that well
is in place the economic threshold for satellite fields could be
in the 50 million range using tiebacks and subsea developments.
"We can really tap deeply into the portfolio once
we make a primary discovery."