Petroleum Geochemistry of Oils and Rocks in Arctic National Wildlife Refuge, Alaska

Leslie B. Magoon, Donald E. Anders

Thirteen oil seeps or oil-stained outcrops in or adjacent to the coastal plain of the Arctic National Wildlife Refuge (ANWR) in northeastern Alaska indicate that commercial quantities of hydrocarbons may be present in the subsurface. The area is flanked by two important petroleum provinces: the Prudhoe Bay area on the west and the Mackenzie delta on the east.

Organic carbon content (wt. %), organic matter type, and pyrolysis hydrocarbon yield show that rock units such as the Kingak Shale (average 1.3 wt. %), pebble shale unit (2.1 wt. %), and Canning Formation (1.9 wt. %) contain predominantly type III organic matter. The exception is the Hue Shale (5.9 wt. %), which contains type II organic matter. Pre-Cretaceous rocks that crop out in the Brooks Range could not be adequately evaluated because of high thermal maturity. Thermal maturity thresholds for oil, condensate, and gas calculated from vitrinite reflectance gradients in the Point Thomson area are 4,000, 7,300, and 9,330 m, respectively (12,000, 22,500, and 28,000 ft).

Time-temperature index (TTI) calculations for the Beli-1 and Point Thomson-1 wells immediately west of ANWR indicate that maturity first occurred in the south and progressed north. The Cretaceous Hue Shale matured in the Beli-1 well during the Eocene and in the Point Thomson-1 well in the late Miocene to early Pliocene.

In the Point Thomson area, the condensate and gas recovered from the Thomson sandstone and basement complex based on API gravity and gas/oil ratio (GOR) probably originated from the pebble shale unit, and on the same basis, the oil recovered from the Canning Formation probably originated from the Hue Shale. The gas recovered from the three wells in the Kavik area is probably thermal gas from overmature source rocks in the immediate area.

None of the oils in or adjacent to ANWR correlate with oils from the Prudhoe Bay area or farther west. However, the Jago oil type, which includes oil from Marsh Creek anticline, Jago River, and Angun Point, does correlate with the Hue Shale based on stable carbon isotope data and the C₁₉/C₂₃ tricyclic terpane ratio.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.