Amid
concerns over aging fields and declining production, new technology
and some surprising geological discoveries are combining to reawaken
Alaska’s petroleum promise, experts on the region say.
AAPG member
Dave Houseknecht has worked on Alaska North Slope basin analysis
and petroleum resource assessment for the U.S. Geological Survey
since 1995. Tim Ryherd is a geologist with Alaska’s Department of
Natural Resources Oil and Gas Division.
Both agree
that oil exploration strategies are changing, and that economic
factors are coming together to create a new Alaska boom in natural
gas.
Historically,
exploration strategy on Alaska’s North Slope was to look for Prudhoe
Bay-type traps — pure structural traps or combination traps involving
very large structures, Houseknecht said.
Along the
way, companies sometimes chanced upon some pure stratigraphic traps
— usually too small to be commercially viable unless they were
located near existing infrastructure.
In the
past six to eight years, two developments altered the traditional
approach, he said. They are:
- Advanced geologic and engineering technology allowed companies
to consider smaller targets.
"Specifically,
3-D seismic in exploration mode, applied sequence stratigraphic
concepts and directional drilling in exploratory and especially
in developmental wells really reduced the footprint and expense
of developing smaller accumulations," Houseknecht said.
"In the 1990s,
suddenly instead of 400 million barrels as a minimum viable field
size, we saw companies stepping out on 100 million or even smaller
fields," he said.
"Technology allowed
them to to start looking at stratigraphic traps as primary objectives,"
he said.
- A "serendipitous" discovery by Arco Alaska in 1994 —
announced in 1996 — "revolutionized North Slope exploration
strategy and kicked off the hottest play in Alaska today, usually
called the Alpine play," Houseknecht said.
The primary target
— a lower Cretaceous turbidite sand — proved unsuccessful. The
company took the project deeper because older wells in the general
neighborhood had tested non-commercial amounts from upper Jurassic
strata, Houseknecht said.
The resulting discovery
in the Alpine field, located on the Colville River delta on the
eastern northern boundary of the NPRA, had an announced size of
429 million barrels.
"But since it
went on line, the rate has been well ahead of predictions, so
we expect the Alpine will go 500 million or more," Houseknecht
said.
"This is a pure
stratigraphic trap," he added. "The significance is
they found a reservoir that was previously unknown, and a trap
type previously unknown on the North Slope."
It exposed a petroleum
system that had not been encountered in the region before, he
said.
The Alpine
Play
Houseknecht
works for the USGS out of Reston, Va. He joined the Survey in 1992,
serving as energy program manager until 1998. He has been working
on Alaska North Slope basin analysis and petroleum resource assessment
since 1995, and has represented the USGS scientific perspective
of ANWR, NPRA and other Alaska oil and gas issues to Congress and
the administration.
Most
North Slope oil, he said, is expelled from super rich source rock
in the Shublik Formation. It usually has a low API gravity, typically
25-30 degrees, and tends to have 1 percent or so sulfur content.
Oil tested
from previous Colville wells apparently was a mixture from the Shublik
and Kingac Shale, long known to be an oil prone source rock.
None ever
had been sourced exclusively from the Kingac, Houseknecht said.
"Because
the Jurassic reservoir was fine grained sandstones, it was difficult
to produce oil from fine grained rocks," he said.
But the
Alpine produced a 40 degree API gravity oil exclusively from Kingac
Shale.
"Suddenly,
we had a play that had stratigraphic traps of significant size and
a fine grained reservoir that had in it a very light, low-sulfur
oil that could be produced effectively," Houseknecht said.
The result
has been a couple of federal lease sales in the last four years
and about a dozen exploratory wells and a number of announced discoveries.
"Companies
are actively pursuing the Alpine play and similar stratigraphic
plays that were previously unknown or considered sub-economic,"
he said.
"In
the work we (USGS) have done, the potential for Alpine-type petroleum
systems exists across all of the northern NRPA — a broad fairway
200-250 miles westward across NPRA."
The USGS
in 2002 released a new evaluation of undiscovered gas in NPRA, he
added, "and basically documented the very broad extent of the
play."
Plenty of
Gas, But …
Also in
Alaska, activity continues on the Barrow Arch, Prudhoe Bay’s main
structure, and companies are developing satellites close to existing
infrastructures at Prudhoe and to the west.
Some offshore
lease offerings from the state and Minerals Management Service have
been picked up close to shore, "but no one seems interested
in stepping very far out into the outer continental shelf, "
Houseknecht said. Some OCS finds in the early 1990s were not viable.
Ryherd
said the offshore promises "huge opportunities. The north side
of the Barrow Arch is underexplored, to say the least," he
said.
"Gas
is the exciting thing for Alaska’s future," he said. "If
gas is part of the mix, then the whole state has potential. If you
have an oil target with a fallback as a gas play, other areas become
much more attractive."
"Right
now, there is no way to market gas from the North Slope," Houseknecht
said. "The Trans-Alaska Pipeline is oil only. Gas resources
found to date have been a by-product of oil exploration — there
has been no significant gas exploration so far."
Of some
35 trillion cubic feet of reserves known on the North Slope, most
— about 25 trillion cubic feet — is in the Prudhoe gas cap.
Gas that
is produced is reinjected.
Another
very large gas and condensate field just outside ANWR and east of
Prudhoe is the Point Thomson Field.
The field’s
major operator, ExxonMobil, has not released reserve estimates,
but others — including the state — say the potential is for 200-400
million barrels of liquids, oil and condensate, and five-six trillion
cubic feet of gas.
Significant
overpressuring has been a major engineering impediment to producing
any of the liquids, he said. Reinjecting any gas would be difficult.
Discussion
of building a gas pipeline from the North Slope has gotten more
serious in the last two or three years, prodding some companies
to acquire leases in the Brooks Range foothills, an area generally
more gas than oil prone.
U.S. and
Canadian companies have staked "very aggressive acreage positions
in the foothills of the North Slope, expecting a gas market will
exist in the relatively near future," he added.
They are
conducting expensive seismic surveys, active field programs, and
"a lot of outcrop work to constrain the subsurface geology
in expectation of exploratory wells.
"If
we see an agreement that allows a gas line to get started, everyone
expects a very active gas exploration program within the next couple
of years," he said.
And then,
"you’re going to see everybody coming out of the woodwork,"
Ryherd said.
While the
North Slope and adjacent offshore areas are seen as having the greatest
potential in the near term, interior basins in central Alaska are
gas prone.
Interest
in those areas is driven mainly by the need for energy in rural
Alaskan villages, Houseknecht said. There are some attempts to evaluate
possible export potential if a gas line is built.
There is
oil potential to feed local refineries, he said.
"Oil
production from Prudhoe Bay is declining at a rate of about 10 percent
a year. There is concern about the future viability of the Trans-Alaska
pipeline system. The state and industry are seeking new reserves
to keep the pipeline full and/or viable," Houseknecht said.
"It
takes years to bring discoveries on line," he said, "so
there is starting to be — I don’t want to say 'desperation’ —
'anticipation’ about finding reserves soon to keep the pipeline
flowing."