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USGS publishes Susitna basin assessment
Estimates up to 4.6 tcf of gas and up to 10 million barrels of oil in the basin in the Susitna Valley to the north of Anchorage Alan Bailey Petroleum News
The U.S. Geological Survey has published an assessment of undiscovered, technically recoverable oil and gas in the Susitna basin, a broad sedimentary basin under the Susitna Valley to the north of Anchorage. The agency has estimated the possibility of up to 4.6 trillion cubic feet of natural gas and up to 10 million barrels of oil recoverable from the basin. Mean estimates are 1.6 trillion cubic feet of gas and 2 million barrels of oil.
Given that no oil or gas has yet been produced from the basin and few exploratory wells have been drilled in it, the wide range of the estimates indicates relatively high levels of uncertainty in the figures. The lack of commercial discoveries results in volumes of zero at the lower ends of both the oil and the gas estimate ranges.
Probable resources However, the USGS says that a detailed assessment of the basin by its scientists does indicate the probable existence of oil and gas that could be discovered, developed and produced using current technology. And, being situated close to Anchorage and the Alaska Railbelt transportation corridor, a commercial development in the basin seems an enticing possibility.
The Susitna basin sits adjacent the northeastern edge of the Cook Inlet basin, with that latter basin’s producing oil and gas fields. However, the two basins are separated by a major geologic fault and appear to have formed under different circumstances. So, although the two basins appear similar in that they hold significant thicknesses of Tertiary sediments, including coal seams, laid down in ancient river systems, the two basins also differ: Unlike the Cook Inlet basin, where a thick, oil sourcing Mesozoic sequence underlies the Tertiary, the Tertiary of the Susitna basin appears to sit directly on impervious metamorphic rocks.
Gas prone Consequently, the Susitna basin has tended to be viewed as gas prone rather than oil prone, a view that seems supported by the new USGS assessment. The gas would have originated from microbial action in coals and shales that are rich in organic matter. The USGS says that this gas may have become trapped in sandstone reservoirs, where sand in ancient river channels became confined by impermeable, fine-grained material on adjacent floodplains. Other gas accumulations could have become trapped in sandstones and conglomerates, by folds in the strata or by adjacent geologic faults.
In the resulting assessment unit, referred to as the Susitna Tertiary Sandstone Gas Assessment Unit, the USGS scientists think that there may anywhere from one to 264 gas fields, with field sizes possibly falling within a range from 3 billion to 2,500 billion cubic feet. There is an 86 percent probability of there being at least one 3 bcf field, the scientists determined.
Possibility of oil Although the coals, shales and mudstones in the basin contain material capable of generating oil, the basin is in general thought to be too shallow and cold to produce conditions under which oil might form. The possible exception is an area in the southwestern part of the basin, where faulting appears to have buried the basin rocks to depths where temperatures could have reached levels for oil generation. That has resulted in what the USGS scientists refer to as the Beluga Mountain Subthrust Oil Assessment Unit, the assessment unit that accounts for all the potential oil in the basin. The scientists have estimated the possibility of at least one and possibly as many as 15 oil fields in this assessment unit, with field sizes ranging from 500,000 barrels to 20 million barrels. The scientists determined a 27 percent probability of there being a field of at least 500,000 barrels.
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