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Gathering data on the Nenana basin DGGS-led team reports on the status of its field work, sampling and analysis, assessing the basin’s oil and gas potential Alan Bailey Petroleum News
Alaska’s Division of Geological and Geophysical Surveys, or DGGS, has published a status report on research it is conducting into the oil and gas potential of the Nenana basin, a large sediment-containing trough in Alaska’s Interior, about 50 miles southwest of Fairbanks. Geologists have long viewed the basin as likely to contain natural gas. And, with the basin being close to the transportation corridor between the cities of Anchorage and Fairbanks, the possibility of a significant gas find in the basin offers the tantalizing possibility of a new source of gas for space heating or power generation in the Alaska Railbelt.
Doyon exploration Doyon Ltd., the Native regional corporation for the Alaska Interior, has been spearheading efforts to explore the basin — in 2009 the corporation drilled the Nunivak No. 1 well in the basin near the village of Nenana and the corporation plans another well nearby this summer. Doyon has said that, while the Nunivak well did not encounter commercial quantities of gas, the well did provide intriguing evidence for the hydrocarbon potential of the basin. And the corporation says that its exploration efforts have uncovered indications that the basin may hold oil as well as gas.
With very little rock exposed at the surface within the area of the basin, geologic assessments of the basin depend on examinations of rock exposures in the hills around the basin margin, coupled with insights from gravity and seismic data gathered in the basin. Some limited subsurface data are also available from the Nunivak well and two earlier wells, drilled several years ago on the basin’s flanks.
Some results from the Doyon exploration have indicated that the basin may be much deeper than previously thought, with the possibility that potential source rocks have been buried to depths where temperatures would drive oil or gas formation. Conventional thinking about potential natural gas formation in the basin has tended to assume that, rather than forming from the application of heat, gas would be generated from the action of bacteria on organic material in coal seams.
Reconnaissance fieldwork The DGGS research involves a team of scientists from DGGS, Alaska’s Division of Oil and Gas, the U.S. Geological Survey and the University of Alaska Fairbanks. In 2012, the year when the Nenana basin research started, the team carried out reconnaissance fieldwork, verifying, measuring and sampling the rock sequences exposed on the southeast side of the basin and on the south side of the adjoining Tanana basin on the north side of the Alaska Range.
“The goal of this work was to improve our understanding of the geologic development of the Nenana basin and to collect a suite of samples to better evaluate hydrocarbon potential,” the report says.
The DGGS-led team visited well-exposed sections of the basin fill, at the edge of the basin near and to the east of Healy, the site of Alaska’s only commercial coal mining operation. The team made detailed measurements of the sequence of exposed rock strata and took rock samples, assessing some rock units for their potential as hydrocarbon sources and other units for their possible effectiveness as hydrocarbon reservoirs.
Rivers and lakes Geologists think that the Nenana basin, formed as a consequence of the movement of the Earth’s crust around some major geologic faults, is probably filled with relatively young sediments, primarily Tertiary in age and laid down from systems of rivers and lakes, and with periods of luxuriant vegetation growth giving rise to substantial coal seams.
Coal seams in the basin could source gas, and perhaps oil. However, the DGGS-led team also sees possible comparisons between lake-deposited sediments in the Nenana basin and organic-rich lake sediments elsewhere in the world that can source oil. The team has taken samples from a rock formation of this type exposed near Healy for laboratory analysis, the DGGS report says.
And field observations appeared to confirm that many of the rocks in the basin were laid down as sediments deposited from rivers. However, geologic features that the team encountered during its fieldwork included a fossil forest, buried and preserved in volcanic ash, the DGGS report says.
Samples for timing The field team also collected rock samples from the older rocks which form the basement to the basin and which are exposed in the hills on the north and south sides of the basin, and in the Yukon-Tanana uplands to the east. These rocks, igneous and metamorphic in nature, contain minerals that the team hopes will provide insights into the thermal history of the basin floor, thus providing evidence for the timing of Nenana basin formation.
DGGS says that it anticipates publishing detailed results from its interpretations of hydrocarbon reservoir and source rock quality in the basin by the end of 2013. In due course the team will also publish the results from the laboratory analyses of rock samples collected in 2012. In 2013 the research team may conduct an additional evaluation of rock cuttings from the Nunivak well, including a supplementary analysis of data that can be gleaned from those cuttings. The team is also considering some further field studies in the northern Alaska Range, the DGGS report says.
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