When
once-rampant drilling activity in a region begins declining and
the majors begin losing interest, "it's all drilled up" becomes
the common refrain.
A close
look at the North Sea, however, suggests the oil finders may be
looking for the wrong thing in many instances.
"A key
thing in my mind is that 88 percent of the fields discovered are
structural traps and only about 5 percent are stratigraphic traps,"
said Rod Christensen, chief geologist at Canadian-based Oilexco.
"It's no
surprise that most companies are looking for structures," he said,
"so the majors, when most big bumps are drilled, look for other
places with big bumps not drilled, or big anomalies you can see
on seismic."
According
to Irene Haas, senior vice-president institutional research at Sanders
Morris Harris, future discoveries, at least in the UK offshore,
are more likely to be the under-explored stratigraphic plays, or
combination structural-stratigraphic plays.
She emphasized
new fields will be subtle and require better data and more rigorous
analysis, which ideally will yield innovative concepts leading to
new discoveries.
"When you
say there are more stratigraphic traps to be found in the North
Sea," Christensen said, "we're not on the downside of the exploration
parabola showing just small things to be found."
He pointed
to the Buzzard discovery in 2001, which is an accumulation of maybe
400 million barrels of recoverable resources. Then there's Oilexco's
recent find at the Brenda field with perhaps 200 million barrels
recoverable.
"I think
if you go in and do AVO-type seismic work looking for stratigraphic
traps rather than just using seismic to look for four-way closure
on structures," Christensen said, "you're likely to find a lot more
stratigraphic traps. It's like in the Gulf of Mexico where structures
were drilled first and then new seismic techniques found a lot of
stratigraphic accumulations."
Reinforcing
this opinion, the Geological Society of London recently sponsored
a conference on what was called "the deliberate search for the stratigraphic
trap."
"Everyone's
realizing we must start looking for these deliberately," Christensen
said, "and not find them like we used to, either by mistake or for
the wrong reason, such as accidentally while drilling structures."
Tools of
the Trade
The Brenda
hydrocarbon accumulation and trapping mechanism initially was a
head-scratcher given the differing oil/water contacts in neighboring
wells that were originally thought to be the same accumulation.
Figure
1 and Figure 2
To get
a handle on things, Oilexco used the London-office arm of Houston-based
GX Technology (GXT) — an approach that's in line with its practice
of employing locally-sited companies in the North Sea region, where
possible. In turn, GXT brought UK-based rock-physics experts IKON
Science on board for the project.
A multi-disciplinary
effort ensued to identify potential drilling targets, using both
3-D seismic and data from nearby producing and non-producing wells.
"The focus
was on pre-processing of the seismic data, model building and pre-stack
depth migration," said Ian Jones, geophysical adviser at GXT.
"We didn't
know in advance we were dealing with a stratigraphic occurrence,"
Jones said, "but one of the relevant details in using the technology
we used is that it preserves the amplitude information correctly
so we're not destroying the stratigraphic information in processing."
Among the
tools used was gridded tomography, where velocities were estimated
continuously throughout the volume. Gridded tomography technology
employs an auto-picker so the process is fully automated and the
manpower can be focused on the Quality Control of what was picked.
Because
the process is automated, huge volumes of data are run quickly on
a supercomputer cluster.
A pre-processing
technique known as SRME (surface-related multiple elimination) was
used for multiple suppression.
"A distinguishing
feature here is you don't need knowledge of the velocity field to
use it," Jones said, "whereas other de-multiple techniques require
you to have a good guess at the velocity."
Careful
pre-processing to remove noise and multiple contamination, followed
up by high fidelity 3-D pre-stack depth-migrated imaging resulted
in a data volume suitable for accurate AVO and EI analysis, according
to Jones.
"We happened
to map a big blob of this AVO anomaly that couldn't be attributed
to a structural reason to be there," Christensen said. "It had to
be stratigraphic, so based on that and core work we thought that's
what it was, but we still had to drill the first well to be sure.
"We also
reappraised the structural interpretation based on the stratigraphic
evidence," he said, "leading to a correct re-classification of the
stratigraphic ties. It was intriguing to see the mis-positioning
of some of the old wells based on old-tech imaging, and then to
see we were spot-on with the new images."