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AAPG Bulletin


AAPG Bulletin, V. 98, No. 4 (April 2014), P. 631–661.

Copyright copy2014. The American Association of Petroleum Geologists. All rights reserved.


Sedimentology, stratigraphy, and reservoir properties of an unconventional, shallow, frozen petroleum reservoir in the Cretaceous Nanushuk Formation at Umiat field, North Slope, Alaska

Grant T. Shimer,1 Paul J. McCarthy,2 Catherine L. Hanks3

1Department of Geology and Geophysics, University of Alaska Fairbanks, Fairbanks, Alaska 99775; present address: Department of Geology, Whitman College, Walla Walla, Washington 99362; [email protected]
2Department of Geology and Geophysics, and Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska 99775; [email protected]
3Department of Geology and Geophysics, Petroleum Engineering, and Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska 99775; [email protected]


Numerous oil and gas accumulations exist in the Brooks Range foothills of the National Petroleum Reserve in Alaska (NPRA). We use cores and well logs from 12 abandoned legacy wells at Umiat field, near the southeastern boundary of the NPRA, to characterize the sedimentology and stratigraphy of unconventional shallow frozen reservoirs in sandstones of the Cretaceous (Albian–Cenomanian) Nanushuk Formation. The Nanushuk Formation at Umiat has five facies associations: offshore and prodelta, lower shoreface, upper shoreface, delta front, and delta plain.

Three stratigraphically distinct, regionally extensive Nanushuk Formation depositional systems at Umiat contain several potential petroleum reservoirs. The lower Nanushuk Formation, including a reservoir interval known informally as the lower Grandstand, primarily consists of marine mudstone and shoreface sandstones. The middle Nanushuk Formation is dominantly deltaic and contains a second major reservoir interval in the informal upper Grandstand sandstone. Both the upper Grandstand and lower Grandstand are regressive. The transgressive upper Nanushuk Formation contains an additional potential reservoir interval in shoreface sandstones of the informal Ninuluk interval. The primary reservoir intervals at Umiat field are upper shoreface and delta-front sandstones in the upper Grandstand and lower Grandstand, where increased sorting and decreased bioturbation in high-energy depositional environments affect overall permeability and permeability anisotropy.

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