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July 2014

Vol. 19, No. 30 Week of July 27, 2014

The allure of the Alaska Peninsula

With intriguing petroleum geology in this remote, underexplored region, the state continues to offer land for oil and gas leasing

Alan Bailey

Petroleum News

With known oil and gas seeps and with geology broadly similar to that of the prolific Cook Inlet basin, the Alaska Peninsula has intrigued oil and gas explorers since the early days of Alaska hydrocarbon exploration. The peninsula lies along the southern side of the North Aleutian basin, a geologic basin with clear hydrocarbon potential in the southern part of the Bering Sea shelf.

Areawide lease sales

In the interests of encouraging exploration in extensive state lands along the peninsula, Alaska’s Division of Oil and Gas has been conducting areawide lease sales for the peninsula since 2005. Although, as these lease sales got under way, some companies purchased leases, the state did not sell any leases in sales held from 2008 to 2013. In the 2014 lease sale two companies purchased leases on a total of three land tracts.

Some people view the possibility of exploration of the North Aleutian basin, in offshore federal land, as providing an essential underpinning to exploration along the neighboring peninsula. But those federal waters were closed to exploration between 1989 and 2007, following the Exxon Valdez oil spill in Prince William Sound. And in 2010, following a brief re-opening of the area, the Obama administration withdrew the Bristol Bay area, including the North Aleutian basin, from future federal oil and gas leasing.

But what is the potential for a commercial oil or gas find in state land on the peninsula?

Sparsely explored

A total of 26 wells have been drilled in the Alaska Peninsula region at various times between 1900 and 1985. Of these wells, 19 encountered oil shows and 13 encountered gas shows, but no commercial discoveries have been made. Only one well has been drilled in the offshore North Aleutian basin, the remainder being onshore. Given the scale of the region, this intensity of drilling can be viewed as sparse in terms of oil and gas exploration.

As in the Cook Inlet basin, the peninsula has a thick succession of Tertiary-age sedimentary rocks, primarily formed on land, rather than under the sea, and containing coals seams. These Tertiary rocks lie in juxtaposition with a sequence of older Mesozoic rocks, formed in a marine environment - equivalent rocks lying under the Tertiary sequence of the Cook Inlet basin include the rocks that sourced the oil in the Cook Inlet oil fields.

DGGS research

In support of the program of state areawide lease sales, a team led by Alaska’s Division of Geological and Geophysical Services conducted a research program on the Alaska Peninsula between 2004 and 2007, doing fieldwork and placing data from earlier exploration programs into a modern petroleum geology context.

Sampling spectacular exposures of Mesozoic strata in cliffs along the southeast coast of the peninsula, around Puale Bay, the DGGS-led team confirmed the presence of excellent potential source rocks, both in the Kialagvik formation, equivalent to source rocks from the Cook Inlet basin, and in the older Kamishak formation, a rock unit equivalent to the prolific Shublik source rock of Alaska’s North Slope. The team determined that these source rocks have thermal maturities in the low end of the oil-generating thermal window, with a long-known surface oil seep in Mesozoic rocks at Oil Creek, about 30 miles southwest of Puale Bay, confirming the presence of an oil source in the region.

A chemical analysis of oil from the seep has confirmed a Mesozoic source for the oil.

The DGGS-led team also conducted a carbon isotope analysis of gas from a gas seep from Mesozoic rocks south of Port Moller. This analysis indicated that the gas originated from the thermal decomposition of organic material, rather than from bacterial action, almost certainly from a Mesozoic source rock. There are extensive surface exposures of Mesozoic rocks across the Alaska Peninsula southwest of Port Moller, near and around Herendeen Bay.

Tertiary sequence

The sequence of Tertiary rocks known to lie in the offshore North Aleutian basin can be observed onshore, in juxtaposition with the Mesozoic rocks near Port Moller. The Tertiary rocks seen onshore in this area represent the steep southern flank of the Tertiary basin, most of which lies offshore. However, Tertiary rocks forming the southeastern margin of the basin extend onshore northeast from the Port Moller area, up the western side of the peninsula.

Analyses of the Tertiary rocks by the DGGS-led team indicated that the majority of potential hydrocarbon sources in the Tertiary are more likely to have generated natural gas than to have generated oil, confirming a widely held view that the North Aleutian basin is likely to be gas prone rather than oil prone. However, one rock unit, the Tolstoi formation, contains material that could generate liquid hydrocarbons. And evidence from a stratigraphic test well, the COST No. 1 well, drilled offshore in the basin in 1982-83, indicates that in that part of the basin the Tolstoi had been buried to depths about 5,000 feet below the depth at which subsurface temperatures would have reached temperatures conducive to oil formation, the DGGS-led team reported.

As with the Tertiary rocks of the Cook Inlet basin, the Tertiary sequence along the northwestern side of the Alaska Peninsula contains a range of rock units with hydrocarbon reservoir potential. The Alaska Peninsula region also exhibits many geologic structures that could trap oil and gas. And the DGGS-led team identified rocks fine-grained enough to seal hydrocarbon traps, with some of these seal rocks extensive enough to capture a commercial-sized oil or gas pool.

Mesozoic underneath?

A key issue relating to the petroleum system of the Alaska Peninsula is the question of the extent to which the oil and gas prone Mesozoic rocks extend under the Tertiary rock sequence of the North Aleutian basin - in the Cook Inlet basin the Mesozoic sequence, including the regional oil sources, extends continuously under the Tertiary, even in the deepest parts of the basin.

To the immediate northeast of the Alaska Peninsula a major geologic fault, the Bruin Bay fault, separates metamorphic and igneous rocks, with no petroleum potential, to the northwest from the Mesozoic rocks, with petroleum potential, to the southeast. The fault runs through the northeastern end of the peninsula, but because of a lack of surface exposed rock, cannot be traced south of Becharof Lake, near Puale Bay. A continuation of the fault down the length of the peninsula would presumably place igneous and metamorphic rocks under the Tertiary of the North Aleutian basin, thus making it highly unlikely that Mesozoic oil would have flowed into Tertiary reservoirs, as in Cook Inlet. If, however, the fault veers to the west, heading out under Bristol Bay, those oil and gas prone Mesozoic sediments would be placed under the Tertiary of the basin.

The DGGS-led team identified a major structural dislocation trending southwest from the known southerly limit of the Bruin Bay fault. Should that dislocation represent the continuation of the fault, Mesozoic rocks are likely to be absent under the basin. But, with insufficient evidence to substantiate the fault’s route, no one knows for sure what lies under the deeply buried Tertiary strata of the basin. However, regardless of what does lie under the Tertiary, the Tertiary rocks themselves, with potential hydrocarbon sources, hydrocarbon seals and hydrocarbon traps, can form a complete petroleum system, the DGGS-led team has reported.






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