Tertiary Uplift in the Northern National Petroleum Reserve in Alaska (NPRA) - Geology, Timing, and Influence on Petroleum Systems

David W. Houseknecht¹, Kenneth J. Bird², Robert C. Burruss¹, and Christopher Connors^3
1USGS, Reston, VA.
²USGS, Menlo Park, CA.
³Washington and Lee University, Lexington, VA.

A broad, post-mid-Cretaceous uplift is defined in northern NPRA by truncation of strata beneath a Plio-Pleistocene unconformity, stratal dip, thermal maturity patterns, and amounts of exhumation. Progressively older strata, from Paleogene to Albian, are truncated westward beneath the unconformity. The uplift is more than 100 miles wide (south-north) and extends westward beneath the Chukchi shelf. Stratal dip and the truncation pattern of seismic reflections in Albian strata indicate that the uplift is asymmetrical and north-vergent. Albian beds dip 0.5-1° on the south flank, 1-2° on the east flank, and 4-7° on the north flank, where a clearly defined angular unconformity between Plio-Pleistocene and Cretaceous-Paleogene strata is evident on seismic data just below the sea floor of the inner Beaufort shelf.

Exhumation estimates range from <1,000 ft in eastern NPRA to >7,000 ft in western NPRA, based on analysis of regional stratigraphy, compaction curves derived from sonic logs, vertical and lateral thermal maturity trends, and apatite fission-track (AFT) data. AFT analysis of samples from three wells across the eastern flank of the uplift indicates Tertiary cooling in two pulses. Cooling was initiated at 75-65 Ma and continued through the Tertiary, with accelerated cooling starting at 35-15 Ma.

The origin of the uplift is enigmatic. Although it overlaps older positive structural elements, evidence of a genetic link is lacking. Nor is the geometry and magnitude of the uplift consistent with a flexural origin related to Brooks Range tectonic loading. The uplift appears to form the eastern end of a huge area of elevated basement and thin to absent Cretaceous-Tertiary strata extending west-northwestward across the Chukchi shelf, and the timing of the uplift is coincident with wrench faulting beneath the Chukchi shelf. New marine seismic data north of NPRA reveal growth strata above basement-rooted faults north of the Beaufort shelf edge. Ongoing analysis of these data may provide insights on the origin of the uplift.

The northern NPRA uplift, which post-dates oil generation by >10-20 my, significantly influenced petroleum systems. The regional distribution of oil and gas in sub-LCU (Lower Cretaceous unconformity) and older reservoirs, gas isotopic composition, and oil seeps are spatially correlated with uplift magnitude. Exhumation likely caused a decrease in confining pressure on subsurface fluids, resulting in expansion of free gas and degassing of oil in reservoirs, and degassing of formation water. Moreover, the southeast part of the uplift may have provided northward migration pathways for voluminous gas originating in the Brooks Range fold-thrust belt, resulting in a “gas flush” through the region of maximum uplift, likely displacing oil from sub-LCU reservoirs. Strata above the LCU do not appear to be influenced in a consistently similar manner, suggesting that shale draped on the LCU acted as a vertical barrier to the gas flush.

 

AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.

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