Alaska's Interior rift basins: a new frontier for discovery

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Regional potential

The presence of excellent oil-prone source rocks in the Nenana basin allows a more optimistic evaluation of other interior Alaska rift basins. Deep nonmarine rift basins, such as the oil-rich Bohai basin in northeastern China, often contain oil-prone lacustrine source rocks deposited in deep, stratified and anoxic lakes. Similar lacustrine source rocks may exist in the Alaska rift basins, possibly in addition to the coaly source rocks encountered in Nunivak-1. Deep, stratified and anoxic lakes are most likely to form during the initial rifting phase when fault movement is greatest. The presence of lacustrine source rocks in the deepest parts of these basins would add to the oil charge.

The Yukon Flats contains subbasins similar in depth (~25,000 ft) but considerably larger than the Nenana basin. Outcrop cores from the 1980s around the margin of the Yukon Flats found coal with similar HI contents as Nunivak-1. For example, one coaly shale cored at 70 ft along the southern rim of the Yukon Flats is 1 ft thick with TOC = 34%, HI = 379, and T_max = 442° C., indicating oil window maturity. This coal is an excellent oil source rock and is assigned "Late Paleocene or (?)Eocene," a similar age to oil-prone coals in the Nenana basin. A slightly deeper coal has TOC = 52.1% and HI = 342, while several additional coals were not analyzed.

The surface microseeps in Nenana basin and Yukon Flats may be sourced by these coaly source rocks. Oil volume in the microseeps is so far too small to geochemically type to the oil source, but correlation may be possible if more concentrated surface seepage is found.

Acknowledgments

Many people and companies contributed to this study. Jim Mery, John Woodman, and Jeff Filut (Doyon Ltd.) provided support, data, encouragement, and discussion. Alicia Orange, Greg Jurkowski, Peggy Gallagher, and Tom Walsh (Petrotechnical Resources Alaska) provided critical data, discussion, and review. David West collected field samples for surface hydrocarbons, and David Seneshen (Vista Geoscience) analyzed the results. Gary Thompson (Thompson Solutions) produced the gravity map and gravity inversion and made the Birch Creek depth interpretation (Getech). Charlie James (Platte River Associates) performed the basin geohistory analysis and expulsion calculations.

References

1. Rowan, E.L., and Stanley, R.G., "The Yukon Flats Cretaceous (?)-Tertiary Extensional Basin, East-Central Alaska: Burial and Thermal History Modeling," US Geological Survey, SIR 2007-5281, 2008, 19 pp.

2. Frost, G.M., and Stanley, R.G., "Compiled Geologic and Bouguer Gravity Map of the Nenana Basin Area, Central Alaska," USGS Open File Report 91-0562, 1991, 30 pp.

3. Stanley, R.G., and Lillis, P.G., "Preliminary Interpretation of Rock-Eval Pyrolysis and Vitrinite Reflectance Results From the Nunivak 1 Well in the Nenana Basin, Central Alaska," AAPG Search and Discovery Article #90125, AAPG Pacific Section Meeting, Anchorage, Alas., May 8-11, 2011.

4. Petersen, H.I., "The petroleum generation potential and effective oil window of humic coals related to coal composition and age," Coal Geology, Vol. 67, 2006, pp. 221-48.

The authors

Gerry Van Kooten ([email protected]) is professor of geology at Calvin College, Grand Rapids, Mich., and a consultant on energy projects with Petrotechnical Resources Alaska. He has worked in geothermal, coal, and oil and gas exploration and development for over 30 years from ARCO offices in Dallas, Denver, and Anchorage. He also has investigated the ecological effects of natural oil seeps along the southern coast of Alaska. He has a PhD from the University of California Santa Barbara, an MS from Arizona State University, and a BS degree from the University of Washington.

Michael A. Richter is owner of Michael Richter Exploration Consulting LLC, an oil and gas exploration consulting company located near Denver. He has more than 35 years of experience in oil and gas exploration, working projects for ARCO in Asia, the Middle East, Russia, Central Asia, and Alaska. He was ARCO Alaska's vice-president of exploration and land when Phillips Petroleum purchased ARCO Alaska's assets. He has an MS in geophysics from the University of Wisconsin-Milwaukee and a BS in geology from the University of Wisconsin-Oshkosh.

Pierre A. Zippi is principal scientist and owner of Biostratigraphy.com LLC, a biostratigraphic service company located near Dallas. He has over 30 years' experience in applied biostratigraphy. Previously he was the lead biostratigrapher for ARCO International Oil & Gas Co. and palynologist at ARCO Alaska. He is a research professor at Southern Methodist University. He has a PhD from the University of Toronto, an MS from the University of Georgia, and BS from Pennsylvania State University.

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