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TALE OF TWO SYSTEMS: OIL POTENTIAL ALONG THE NORTHERN GULF OF ALASKA

VAN KOOTEN, Gerald K., Department of Geology, Calvin College, 1740 Knollcrest Circle SE, Grand Rapids, MI 49546, [email protected] and SHORT, Jeff W., Auke Bay Laboratory, National Marine Fisheries Service, NOAA, 11305 Glacier Highway, Juneau, AK 99801-8626

About 70 surface oil seeps along ~220 km of the northern Gulf of Alaska between Katalla and Icy Bay reflect two active oil systems. The western oil system is centered near Katalla and is sourced by marine organic shale of the Oligocene Poul Creek Fm. The Organic Shale member averages ~4% TOC (range ~1.5-8%) and is strongly oil-prone.

Oil-prone coals of the Eocene Kulthieth Fm source numerous oil seeps between Cape Yakataga and the Samovar Hills and are source rocks for the eastern system. Oil-prone coals occur along ~130 km of the coastal margin. Recent glacial retreat in the Wrangell-St. Elias National Park in Tann Fiord near Icy Bay allows improved access to coals formerly covered by ice. Exposed coals range from 6 cm to 1.5 m thick and are laterally discontinuous due to both tectonic disruption and depositional thinning and pinchout. Five sampled coal seams contain TOC from 74-82%. Rock-Eval analysis yield S2 peaks from 252-300 mg/g and Ro values from 0.90-0.99%. These data show the coals are highly oil-prone with 25-30% of the coal by weight converting to liquid hydrocarbon during thermal maturation and are mature for oil generation. Samples of fragmental coaly debris pulverized by the Malaspina Glacier and transported into Icy Bay by the Caetani River also contain an oil-prone component. Oil-prone coals outcrop at the headwaters of the Kosakuts River, the Sullivan anticline as inclusions in the Yakataga Fm, Tann Fiord, and the Samovar Hills.

The genetic link between Kulthieth coal and oil seeps along the Sullivan Anticline is supported by: 1. coincident distribution of coals and seeps, 2. absence of other identified source rocks, 3. petrographic identification of amorphous organic material in the Kulthieth coal, and 4. similarity of resistant sterane biomarkers between Kulthieth coal and weathered oil. Petrographic examination of coals from the Kosakuts and Tann Fiord outcrops identified abundant lipid-rich vitrinite (mainly desmocollinite). The lipid component within the desmocollinite grains are dispersed as tiny grains that are integrated within the detrital vitrinite macerals. The lipid material is possibly derived from algae growing in clear water swamps, mixed with detrital, degraded vitrinite, and is evidently responsible for the oil generative potential of the Kulthieth coals.

AAPG Search and Discovery Article #90058©2006 AAPG Pacific Section Meeting, Anchorage, Alaska