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Looking off the beaten path for gas DGGS director reviews some of the less explored Alaska basins where natural gas resources might be found at some time in the future Alan Bailey Petroleum News
Although oil and gas interest in Alaska understandably tends to focus on geologic basins in the proven regions of the North Slope and Cook Inlet, there are numerous other Alaska basins with the potential for the discovery of at least natural gas, and in some cases of oil.
On March 24 in a meeting of the Alaska House Special Committee on Energy, Robert Swenson, director of Alaska’s Division of Geological and Geophysical Surveys, gave an overview of some of these less well known basins and their resource potential.
Swenson explained that Alaska is traversed by several major geologic faults, such as the well known Denali fault, and that the relative movement of rocks on either side of these faults has thrown up mountains in some areas, while causing other areas to sink into low-lying basins. Erosion of the mountains has caused sand and gravel to flow as sediment into the basins. And, in some cases, where a basin has continued to sink over an extensive length of time, tens of thousands of feet of sediment have accumulated.
Mountains and basins The basins formed in this way are Tertiary in age and generally contain non-marine sediments, sediments consisting of sands, gravels and shales laid down from ancient rivers and lakes. Coal seams interspersed with these sediments have formed from rotting and compressed vegetation. And bacteria feeding on that rotting organic material have created methane, the primary component of natural gas, with that gas becoming adsorbed onto the coal. If stresses in the Earth’s crust cause folding and uplift of the coal seams, the resulting drop in pressure in the coal can release the gas into sandstone reservoir rocks to form gas fields, Swenson said.
The Cook Inlet basin, with its prolific gas fields, is a Tertiary basin of this type.
For oil and gas to be formed from the cooking of organic material, as distinct from the formation of gas from bacterial action, the organic material needs to be heated to an appropriate temperature. And, given the rate at which the subsurface temperature increases with depth in Alaska, the organic material would need to be buried to depths of 18,000 to 20,000 feet for the material to become hot enough for oil and gas to be generated, Swenson said.
Since there are relatively few places in the Alaska Tertiary basins where these depths are attained, geologists consider the basins to be generally gas-prone, with bacteria-generated gas.
There are oil fields in the Cook Inlet basin, but the Cook Inlet oil has flowed into Tertiary sandstone reservoirs from older Mesozoic source rocks underneath the Tertiary strata.
Nenana basin The 8,500-square-mile Nenana basin, about 50 miles southwest of Fairbanks, is a classic Tertiary basin in Interior Alaska. Thick coal seams and sandstone strata exposed at the surface near Healy, on the north side of the Alaska Range, demonstrate the potential for finding gas resources in the basin.
“This is exactly what you want to see from an exploration standpoint,” Swenson said. “The coals are what are going to be generating the gas. The sands are what are going to be actually reservoiring that gas.”
The basin attains depths of around 18,000 feet in its deepest parts, thus possibly putting some of the deeper rock strata into temperatures where oil might be generated, Swenson said. However, the basin is thought to be primarily gas prone.
A group of companies, including Native regional corporation Doyon Corp., is engaged in exploration of the Nenana basin and drilled one well in the basin in 2009, later reporting that this well did not encounter a commercial gas accumulation. Two wells drilled several decades ago in earlier exploration programs only penetrated relatively shallow depths toward the margins of the basin. Doyon is currently planning a seismic survey in the northern part of the basin.
Yukon Flats The relatively large, 15,000-square-mile Yukon Flats basin, to the north of Fairbanks near the Canadian border, is another classic Alaska Tertiary basin. With a typical Tertiary rock sequence of sandstones and coals, and with depths up to 23,000 feet, this basin has a fair amount of oil and gas potential, Swenson said. There is industry interest in the basin, he said (editor’s note: Doyon has been spearheading efforts to attract exploration of the basin).
The Susitna basin, to the north of Anchorage and separated from the Cook Inlet basin by a major geologic fault, actually consists of two smaller basins — a major basin within the main sweep of lowlands around the Susitna River, and a minor basin to the northwest, in the area of the village of Skwentna. With Tertiary strata having a maximum thickness of 15,000 feet in the major basin, the basin lacks sufficient depth for likely oil generation and probably holds gas rather than oil. In addition, the older Tertiary strata that host oil in the neighboring Cook Inlet basin appear to be missing in the Susitna basin, and no one has yet found any evidence for the existence of a viable oil source rock in the Susitna basin area, Swenson said.
With two wells drilled in the basin some years ago and just some 1960s era seismic data available, exploration of the basin has been very limited. Cook Inlet Energy LLC currently operates a state exploration license in the basin. And, especially given the proximity of the basin to the Alaska Railbelt, the Division of Geological and Geophysical Surveys is this year starting a detailed study of the petroleum geology of the basin.
The Copper River basin, lying under the Copper River Valley around the town of Glennallen, encompasses a large area of territory but is relatively shallow. There are only about 3,000 feet of Tertiary strata in the basin, but in the western part of the basin these strata overlay similar marine Mesozoic rocks to those that generated oil in the Cook Inlet basin, Swenson said.
“There is some (hydrocarbon) potential in the Mesozoic marine section and that has been drilled to try to capture that potential,” Swenson said.
However, other than in that western part of the basin, metamorphic and volcanic rocks with no hydrocarbon potential lie under the Tertiary.
Bering Sea In addition to various basins onshore Alaska, there are several large, similar Tertiary basins under the waters of the Bering Sea, on the relatively shallow Bering Sea shelf. The fact that these basins contain Tertiary non-marine rocks, despite the fact that the basins are now subsea, reflects the previous existence of the so-called “Bering land bridge,” the land that at one time connected Siberia to Alaska, Swenson said.
The Bering Sea basins include the Norton basin, under the Norton Sound, and the Hope basin, to the northwest of Kotzebue.
The existence of complex geologic structures, especially large geologic faults, in the Norton basin has given rise to a number of potential exploration plays, resulting in some exploration interest, Swenson said. The basin contains up to 23,000 feet of Tertiary stratigraphy, he said.
ARCO drilled two stratigraphic test wells in the basin in 1980 and 1982, and the federal government leased 59 tracts in the basin following a March 1983 lease sale. Companies drilled six exploration wells in the basin in the mid-1980s, with all of the wells showing indications of the presence of natural gas. One or two wells had weak oil shows, Swenson said.
The U.S. Minerals Management Service, the precursor to the Bureau of Ocean Energy, Management, Regulation and Enforcement, assessed the possibility of finding up to 1.6 trillion cubic feet of gas in the basin, he said.
Navarin basin The largest and most remote of the Bering Sea basins is the 32,000-square-mile, 36,000-foot-deep Navarin basin, 350 miles west of the Yukon-Kuskokwim Delta. One stratigraphic test well and eight exploration wells have been drilled in this basin, with these wells finding gas shows and, in the lower part of the Tertiary section, the limited existence of oil-prone source rocks, Swenson said. The wells found seven potential oil and gas reservoir rock units in the Tertiary, he said.
MMS assessed the possibility of 500 million barrels of oil and six trillion cubic feet of natural gas in the basin, but the basin would be extremely expensive to explore and develop, Swenson said.
Another Bering Sea basin, the North Aleutian basin, has high oil and gas potential and has been the subject of oil and gas exploration in the past (editor’s note: the North Aleutian basin is now subject to an oil and gas lease sale moratorium, ordered by President Obama in March 2010).
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