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Clues for oil and gas on peninsula DNR and DOG geologists overview the results of four years of intense research into the petroleum geology of the Alaska Peninsula Alan Bailey Petroleum News
Its geology seems tantalizingly similar to that of the oil and gas-endowed Cook Inlet; oil seeps attracted early Alaska oil and gas exploration to its huge geologic structures; but never a drop of commercial hydrocarbon has ever left its shores: the Alaska Peninsula, running along the southeast side of the North Aleutian basin in the southern Bering Sea, remains something of a petroleum enigma. Is there enough oil or gas for commercial development in this region, a region that is both remote and generally lacking in support infrastructure?
Following a 2003 decision by Alaska’s Division of Oil and Gas to include state nearshore waters and land along the peninsula in its annual program of areawide oil and gas lease sales, a team of geologists led by Alaska’s Division of Geological and Geophysical Surveys embarked on a research program, running from 2004 to 2007, to help better understand the region’s petroleum potential. The team, with funding and support from the state, the U.S. Department of Energy and Bristol Bay Native Corp., included a wide variety of experts, including field geologists, paleontologists and geochemists.
And, following the recent publication of a report on the research results, geologists Rocky Reifenstuhl from DGGS and Paul Decker from Alaska’s Division of Oil and Gas reviewed some of the findings from the research program at the May 21 meeting of the Alaska Geological Society.
Petroleum potential Reifenstuhl characterized the state lands on the Alaska Peninsula as having a complete petroleum system in an area four times the size of the 1002 area of the Alaska National Wildlife Refuge.
“All of the components of a petroleum system are present on the Alaska Peninsula,” Reifenstuhl said. “We have sources there. We have high quality oil-prone rocks. … We’ve got proven Tertiary reservoirs.”
Yet, despite this oil and gas potential, only 11 wells have been drilled in the region, he said.
The apparent similarities between the geology of the Alaska Peninsula and the nearby Cook Inlet are striking. A thick succession of strata, Tertiary in age and including sediments deposited by rivers on land, lie in down-warped basins in juxtaposition with a thick succession of older Mesozoic rocks deposited in a marine environment.
All of the Cook Inlet oil and gas fields have reservoirs in the Tertiary. And geologists have shown that the oil originated from Mesozoic rocks buried deep under the Tertiary, while much of the gas originated within the Tertiary sediments, the gas having been formed by bacterial decomposition of organic materials.
The Mesozoic succession on the Alaska Peninsula contains rocks equivalent to the main sources of Cook Inlet oil, and the Tertiary succession on the peninsula includes coal seams and other potential sources of natural gas. But do the Alaska Peninsula and the North Aleutian basin have the potential to become another Cook Inlet oil and gas province?
Source rocks By sampling from spectacular exposures of Mesozoic strata in cliffs along the southeast side of the peninsula, around Puale Bay, almost opposite the southern end of Kodiak Island, the DGGS-led team confirmed the excellent oil or gas source rock potential of the Kamishak formation, a rock unit equivalent to the prolific Shublik source rock of the North Slope, and the Kialagvik formation, the rock unit that is equivalent to the main Cook Inlet oil source.
And the team determined for the first time that these Mesozoic source rocks have thermal maturities encroaching the low end of the oil window, a finding that seems confirmed by the presence of a well-known oil seep from Mesozoic rocks at Oil Creek, near Wide Bay, some 30 miles to the southwest of Puale Bay.
A chemical analysis of oil from that seep indicates that the oil originates from a Mesozoic source, Decker said.
There are extensive exposures of Mesozoic strata across the Alaska Peninsula near and around Herendeen Bay, southwest of Port Moller. And from an analysis of carbon isotopes in a natural gas seep in Mesozoic rocks south of Port Moller, the team determined that this gas originates from the thermal decomposition of organic material, almost certainly within the Mesozoic, Decker said.
The sequence of Tertiary rocks that forms the North Aleutian basin is exposed onshore in the southern margin of the basin, near Port Moller, and has been sampled near the deepest part of the basin, offshore Port Moller, from the 1982-83 drilling of the COST No. 1 well. The Tertiary sequence contains potential source rocks, but analyses by the DGGS-led team indicates that the majority of possible Tertiary sources are more likely to generate natural gas than oil, thus confirming a widespread view that the North Aleutian basin is likely to be gas prone rather than oil prone.
Tertiary oil? One possible exception is the Tolstoi formation, a rock unit relatively low in the Tertiary section. The Tolstoi exhibits characteristics intermediate between oil prone and gas prone source rocks and, although a microscopic examination of the organic material in the Tolstoi suggests a marginal potential for the generation of liquid hydrocarbons, it is likely that Tertiary oil shows found in some wells in the region originated from this rock, Decker said.
And the COST well bottomed in the Tolstoi at a depth of 17,000 feet, about 5,000 feet below the top of the zone in which temperatures would have been high enough to cause oil to form, thus giving rise to speculation about whether oil may exist in the deeper parts of the North Aleutian basin.
From the perspective of reservoir rocks that might host oil and gas migrating out of the source rocks, the DGGS-led team identified a range of candidate rock units, with the Tertiary Bear Lake formation having the most promising reservoir characteristics. At the bottom end of the reservoir-quality scale sit tight gas sands, suitable for holding gas rather than oil and mostly found in the Mesozoic.
The Alaska Peninsula region exhibits many geologic structures that could trap oil and gas, and the DGGS-led team found rocks fine grained enough to seal the hydrocarbons in a rock reservoir — tests revealed that many potential seal rocks in the Bear Lake and Tolstoi formations would be capable of capturing a commercial-sized oil or gas pool, Decker said.
Regional structure And the team has made substantial progress in gaining an understanding of the major geologic structures in the region, an understanding that has been hampered by a lack of rock exposed at the surface in much of the peninsula and a shortage of seismic data that might shed light on the subsurface geology.
In particular, a major regional fault, the Bruin Bay fault, runs through the northeastern end of the peninsula but, because of a lack of exposed rock, cannot be traced south of Becharof Lake, near Puale Bay. The location of this fault is of crucial importance because it marks the boundary of metamorphic and igneous rocks, with no petroleum potential, to the northwest, and the Mesozoic strata with petroleum potential to the southeast.
Decker explained how, primarily using aeromagnetic data, it is possible to identify a major structural dislocation that the team has called the Becharof discontinuity, trending northwest from the known southern limit of the Bruin Bay fault. Southwest of the discontinuity lies a newly identified minor basin, termed the Ugashik sub-basin. And to the east of the Ugashik basin a system of faults known as the Ugashik Lakes fault system, runs southwest from the Bruin Bay fault down the peninsula to the southern boundary of the North Aleutian basin in the Port Moller area.
If the Ugashik Lakes fault system represents the southern continuation of the Bruin Bay fault, that might suggest that petroleum bearing Mesozoic rocks are absent under the deep part of the North Aleutian basin, thus making it unlikely that Mesozoic oil has migrated into the Tertiary, as in the Cook Inlet basin. However, there is as yet insufficient evidence to substantiate what happens to the Mesozoic at depth under the basin.
And the Tertiary rock sequence on the Alaska Peninsula and in North Aleutian basin by itself constitutes a complete petroleum system, Reifenstuhl and Decker said.
The Alaska Peninsula report is available on the DGGS Web site at www.dggs.dnr.state.ak.us/pubs/pubs?reqtype=citation&ID=17921.
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