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DGGS publishes Yukon Flats results Geologic fieldwork conducted on the perimeter of the Interior basin finds evidence hinting at the basin’s oil and gas potential ALAN BAILEY Petroleum News
Alaska’s Division of Geological and Geophysical Services has published the results of some fieldwork that its geologists carried out around the perimeter of the Yukon Flats basin. The basin lies to the north of Fairbanks, between the Dalton Highway and the Canadian border, and is prospective for oil and gas.
The DGGS fieldwork was done in 2002 in conjunction with a U.S. Geological Survey assessment of hydrocarbon resources in the basin. The USGS assessment was published in 2004 and suggested that there might be 173 million barrels of undiscovered, technically recoverable oil and 5.5 trillion cubic feet of natural gas in the basin. There may be nearly 127 million barrels of natural gas liquids in the basin, the USGS estimated.
Doyon Ltd., the Native regional corporation for the Interior, owns some subsurface land within the basin and in recent years has become interested in exploring for oil and gas in the area.
Tertiary basin The basin is associated with movement along a major fault system, the Tintina fault system, which runs along the southern side of the basin. The basin is thought to contain sediments of Tertiary age. These sediments are understood to be terrestrial in nature, deposited from an ancient system of rivers and lakes. There are similarities between the geology of this basin that of other Tertiary basins around Alaska, including the Tertiary component of the Cook Inlet basin with its prolific oil and gas fields.
The DGGS geologists conducted some reconnaissance geology around the perimeter of the Yukon Flats basin, at locations where some of the sedimentary strata of the basin are exposed at the surface.
In places, significant thicknesses of strata can be seen. For example, at one location on the east bank of the Yukon River, north of the village of Rampart, the rock succession is more than 5,000 feet thick, with the strata dipping steeply to the north.
Rock types and sedimentary structures observed confirmed that the sedimentary material was deposited from a system of rivers.
Reservoir potential The DGGS report says that, with most of the rocks examined having been deposited at the basin margin, or in troughs extending away from the basin, the nature of the rocks indicates that coarse grained detritus was available for deposition in the basin, and that rivers had existed that could transport this detritus into the basin. The porosity and permeability of the sediments, key factors in their suitability as hydrocarbon reservoirs, would have been excellent at the time of depositions but would have degraded when buried at depth. Although the quality of potential reservoir rocks buried deep in the basin is unknown, rock samples collected during the DGGS fieldwork showed porosities and permeabilities ranging from negligible to good, the report says.
Fine-grained relatively impermeable rocks observed at the surface are also presumed to exist at depth. These rocks could act as oil and gas seals, leading to the existence of stratigraphic hydrocarbon traps.
“Fair to good reservoir quality combined with good potential for stratigraphic traps suggests that viable reservoir rock is present in the subsurface of the Yukon Flats basin,” the report says. “Test holes at Fort Yukon demonstrate the presence of coal seams in the shallow subsurface. If coals, carbonaceous mudstones or other petroleum source rocks are present in the deeper stratigraphy of the basin, a functioning petroleum system may be present.”
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