The Ellesmerian Petroleum System--North Slope of Alaska

BIRD, KENNETH, J., U.S. Geological Survey, Menlo Park, CA

The Ellesmerian petroleum system contains some of the largest oil and gas accumulations in North America, including the supergiant Prudhoe Bay oil field. Recoverable hydrocarbons in the Ellesmerian system amount to approximately 12 billion barrels of oil and 35 tcf of gas. Carbon isotope and molecular biomarker data indicate that the oil, with an API gravity of about 25 degrees and an intermediate sulfur content of about 1%, originated in the Shublik Formation, Kingak Shale, and pebble shale unit. Burial and thermal history reconstruction indicate that most of the petroleum was generated beneath the Brooks Range front and the Colville basin during Cretaceous time and migrated northward to the Barrow arch where it accumulated in combination traps. The traps contain carbonate and silici lastic reservoir rocks ranging in age from Mississippian to Early Cretaceous. During Tertiary time, the petroleum trapped in older, deeper reservoir rocks remigrated into shallower reservoirs of Late Cretaceous and Tertiary age. The combination traps formed during an Early Cretaceous episode of continental rifting, when the Ellesmerian sequence was faulted, uplifted, and partially eroded. Subsidence and deposition of the pebble shale unit, a regional seal, completed the traps.

The essential elements of this petroleum system (source, reservoir, seal, and overburden rocks) were deposited and critical processes (generation, migration, trap formation, and accumulation) occurred from Mississippian (approx.350 Ma) to about mid-Tertiary (approx.40 Ma)--a duration of about 310 m.y. The Ellesmerian petroleum system encompasses the entire length of the Barrow arch (approx.800 km) and the north flank of the Colville basin (approx.300 km). The stratigraphic extent of the system includes late Paleozoic to Tertiary rocks.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)