ABSTRACT: Oil Source Packages, Generation, Migration, and Entrapment in a Tertiary Forearc Basin, Cook Inlet, Southern Alaska

STERNE, EDWARD J., JOHN C. DOLSON, RANDAL L. BILLINGLEY, and JEFF A. COOK, Amoco Production Company, Denver, CO

Viewed regionally, the productive portion of the Cook Inlet is a geological anomaly that occupies only a minor fraction of the forearc terrane developed along the convergent margin of southern Alaska.

Oil to source rock correlations show that the 1.3 billion barrels of oil produced to date in the inlet were sourced from widely distributed Triassic and Jurassic Talkeetna and Tuxedni marine shales. Only in the Cook Inlet area is it known that these source packages were stripped of overlying, impermeable Cretaceous flysch and then deeply buried and matured beneath thick Tertiary basin fill.

Low geothermal gradient, undersaturated oil fields, and recent structural trap development indicate that oil generation occurred late in the history of the basin and is probably ongoing.

Oil generated below the deepest part of the basin migrated updip beneath regional seals in the Jurassic Naknek, Cretaceous Matanuska, and Paleocene Chickaloon formations to their respective zero edges where it escaped into carrier beds in the Tertiary. Once in the Tertiary, the oil migrated updip, filling structures in its path, such as Granite Point, Middle Ground Shoal, and MacArthur River, before spilling to the next structure. The basin is charge limited, and partially filled structural traps at Trading Bay and Swanson River mark the culmination of regional migration pathways. Beyond these fields, closed structures fall within shadow zones to migration and are dry.

Understanding the unique set of conditions that have allowed oil generation, migration, and entrapment in the Cook Inlet will help guide future exploration within the basin and force development of new exploration concepts elsewhere along the southern Alaska convergent margin and in forearc basins worldwide.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)