Every interpreter has experienced the benefits of "shelving" his
seismic interpretation for a couple of weeks — or even a few months
— and revisiting it with fresh eyes.
Almost
20 years have passed, however, since most interpreters shelved their
seismic interpretations of the Mackenzie Delta and the adjacent
Beaufort Sea.
So what's
new?
Recent
breakthroughs in geological understanding — based upon new geochemistry
and biostratigraphy analyses, plus the acquisition of 3-D seismic
data — have transformed Canada's Arctic region from a source-limited
basin to one that is rich in mature source rock.
Proximity
to newly defined source rock beds has opened up a host of new play
concepts.
Opening
the Window
Situated
due east of Alaska's North Slope, the Mackenzie River forms the
second largest drainage system in North America. When the Mackenzie
River meets the shallow Beaufort Sea, it fans out to form the Mackenzie
Delta, an area of 13,500 square kilometers — an area where the
distinctions between land and water blur.
Underlying
this modern delta complex is the Beaufort Mackenzie Basin (BMB),
which consists of 12 to 16 kilometers of Tertiary age progradational
sediments.
The Geological
Survey of Canada (GSC), in conjunction with oil and gas industry
partners, has undertaken a reinterpretation of well data in the
BMB. Using a multitude of diagnostic tools, the GSC is performing
organic geochemistry and petrology studies, including:
- Vitrinite
reflectance.
- RockEval
(Total Organic Carbon).
- Biostratigraphy.
- Foraminifera
colouration indexes.
- Apatite
fission track analyses.
- Fluid
inclusion homogenization temperatures.
Historically,
it was believed that source rock was limited to the Tertiary age
Richards Formation, but recent work indicates that three other Tertiary
age formations have been expelling hydrocarbons into the basin since
mid-Tertiary time: the Aklak, Taglu and Kugmallit formations.
"In order
to attract industry, we had to have some proprietary time frame,"
said AAPG member Dale Issler, a GSC research scientist and project
leader of the GSC-industry consortium. "Our project is outcomes
driven, and we're strongly aligned to industry's needs."
The selling
point for the consortium, Issler said, was a password-protected,
Web-accessible database with real-time delivery for the exploration
decision-making process. The consortium's work is held for two years
before being released in open files.
"In many
cases, the oil window is below drill depths in the offshore, so
you need to rely on thermal modeling to figure out what's going
on," Issler said.
According
to Issler, the oil window shallows in the onshore regions of the
Mackenzie Delta. The existence of five stacked deltaic complexes
in the Tertiary, he said, leads to "extensive recycling of sediments,
fossils and organic matter.
"Every
tool helps when working in a deltaic environment," he said. "There's
very little published in the BMB, in terms of thermal history."
New Potential
A lot of
the structures that were drilled in the 1970s and 1980s weren't
filled to spill point, explained AAPG member Peter Graham, an exploration
geologist with Devon Canada, one of the consortium's industry partners.
"Some people
attributed this to a poor source rock basin," he said, describing
this problem as a historical "fill to spill" issue.
"The primary
risk that we now concern ourselves with is seal integrity," he added.
"The majority
of the big structures have been drilled already," said AAPG member
Chris Bergquist, also an exploration geologist with Devon Canada.
"But the 3-D seismic data have basically opened our eyes to the
stratigraphic and structural/stratigraphic plays on the flanks of
these structures.
"There's
a whole new area of potential that was mostly ignored," Bergquist
added, "and the new geochemistry work expands the area of potential
for hydrocarbon-charged traps."
Devon has
identified several new and exciting Tertiary plays in the offshore:
multiple unconformity subcrops and channel fills, turbidites (base-of-slope
and basin floor fans) and wrench-related structures.
According
to Graham, the biostratigraphy markers described by the GSC can
be correlated — and tested — on the new 3-D seismic data. He likened
Devon's turbidite play to producing analogs in the Gulf of Mexico
and offshore Angola.
"Now we
can make out the internal components in the turbidites," Graham
said, thanks to he new, high-resolution 3-D seismic data, "and figure
out where the reservoirs are."