The Matanuska Valley: An Exposed Analog to the Tertiary Cook Inlet Forearc Basin, Alaska

By

D.L. Doherty, L. Higgins (Prodigy Oil and Gas LLC), and R.V. Nelson (Forest Oil Corporation)

Recent analysis of well-exposed Paleogene sedimentary rocks and related structures in the Matanuska Valley allows exploration geologists to develop depositional frameworks of reservoir facies in the Cook Inlet forearc basin. The Matanuska Valley is the up-plunge and aerially exposed extension of the mostly submerged Cook Inlet forearc basin. This basin contains thick accumulations of Mesozoic and Cenozoic clastic sedimentary rocks. Major oil-bearing reservoirs in the Cook Inlet basin are of Paleogene and early Neogene age and include the alluvial and fluvial Hemlock and Tyonek formations. There are few exposures of these rocks along the flanks of the Cook Inlet. However, similar deposits of equivalent age are well exposed with associated basin-bounding faults in the uplifted MatanuskaValley extension of the basin, providing an excellent laboratory to study relationships between tectonism, sedimentation and volcanism. Here the Tertiary section consists of arkosic to lithic sandstone, conglomerate, siltstone, mudstone, and locally abundant coal. Two facies tracts are evident in the Matanuska Valley analogous to subsurface facies within the Cook Inlet: large braided-stream fed, wet alluvialfan deposits along the basin margins (Arkose Ridge, Wishbone, and Tsadaka formations), and a coal-bearing, mud-dominated basincenter sequence (Chickaloon formation). Depositionally and sedimentologically the Arkose Ridge,Wishbone and Tsadaka formations are analogous to the extensive reservoirs in the Hemlock and Tyonek formations in the giant fields along the west side of the Cook Inlet basin. The Chickaloon formation is analogous to the mud-dominated coaly portion of the Tyonek formation in the central part of the basin.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.