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Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Resolution of Conflicting Data from the Beaufort Sea Aurora#1 Well Clarifies the Petroleum Source Rock Potential of the Lower Cretaceous Succession, Alaska

Margaret A. Keller and Kenneth J. Bird
DOI/US Geol Survey, 345 Middlefield Road, Mail Stop 969, Menlo Park, CA 94025, [email protected]

The 18,325 ft Aurora #1 well in the Beaufort Sea north of the eastern coastal area of the Arctic National Wildlife Refuge (ANWR) is important because it has the only subsurface data available to determine the petroleum potential of nearby Alaska. Although extensively studied since completion in 1988, the significance of conflicting interpretations of organic richness from cuttings versus sidewall cores for the succession considered Lower Cretaceous was not recognized. In this study, we independently determined organic richness of the Lower Cretaceous succession by a relatively new technique of analysis applied to borehole geophysical logs.

Using sonic, resistivity, gamma ray, and caliper logs and the Delta logR technique, a profile of total organic carbon (TOC) was determined for the fine-grained siliciclastic succession of Early Cretaceous age (≈ 15,500-16,446 ft). Results show that interval average TOC from 15,500-16,070 ft ranges mainly from 1.5-3.0%, whereas the range is only ≈ 1.0 to 1.5% in deeper units below that (16,070-16,446 ft). Prior geochemical studies of sidewall cores through the same succession showed notably high TOC (2.5-6.0 wt %) measured on 5 cores in the interval 15,580-15,840 ft, and consistently low TOC of approximately 1% in 6 cores from 15,940-16,420 ft. These core data are consistent with our log results, but differ from the well cuttings data over the same interval. Furthermore, the core and cuttings samples have different foraminifera species occurrences, and paper shale, which is common in the cores, is rare in the cuttings. We conclude that the cuttings are contaminated with material that caved into the borehole from above.

Results from geophysical log analysis and from sidewall core analysis using organic geochemistry, lithology, and paleontology indicate that approximately 300-400 ft of the 1,000-ft Lower Cretaceous succession in the Aurora well includes intervals of organic-rich mudstone and shale. The high thermal maturity (approximately 1.3-1.5% Ro) of this succession precludes a more definitive interpretation of the original petroleum source potential.

Posted with permission of The Geological Society of America; abstract also online ( © Copyright 2005 The Geological Society of America (GSA).