Miocene Tyonek Cook Inlet Stratigraphy as Seen in Cores from Cosmopolitan Oil Field

Doherty, D. J. and Fitzgerald, M.
[email protected]

The Cook Inlet forearc basin contains 30,000’ of non-marine sedimentary rocks from Paleocene to Late Pliocene age. The Tertiary geology of the southern Kenai Peninsula within the Cook Inlet basin consists of two depositional systems: an alluvial fan system along the faulted eastern margin of the basin and a thru-flowing axial fluvial system that varies from mud dominated to sand dominated through time. This study focuses on the Miocene Tyonek Formation, in the vicinity of the shut-in Cosmopolitan oil field. The Cosmopolitan field is named for the oil accumulation discovered by Penzoil, and Mobil in the 1960’s with the drilling of Starichkof St. Unit #1 and Starichkof St. #1 wells. Penzoil cut 7 cores in the northern down dip well, Starichkof State Unit #1.The cores in this well are scattered through the Tyonek formation (4039-7459’). Cores 1-4 were cut in sequence from 4039’ to 4260’. These cores penetrate the axial fluvial system with fluvial channels, crevasse splays, flood plain, swamp deposits. Some sandstone in this sequence is of excellent reservoir quality. A second set of cores cut in the lower part of the Tyonek; Cores 5-6 contain flood basin–floodplain, and lacustrine deposits, typical of much of the Tyonek Formation. Beneath the “normal” Tyonek Formation is a thick sequence of sandstone, with a marked absence of coals. This unit is informally known as the Starichkof Sand, and is about 450’ thick in the Starichkof St. Unit #1 well. Core 7 (7402-7459’) penetrates 57’ of the upper part of the Starichkof Sand. This core contains a thick sequence of chert rich pebble and granule conglomerates and an apparent total lack of coal. Based on a regional study of conglomerate clast lithologies throughout the basin, the high chert content indicates the source terrain is the uplifted Chugach Mountain to the east. This conglomeritic sand is remarkable in that it represents a “cleaned-up” version of the Chugach derived alluvial fan system. Typically these fan deposits are of poor reservoir quality due to abundant clays forming conglomeritic debris flows in more proximal settings. The sand in this core represents a reworking of these fan deposits and re-deposition basin-ward by braided stream processes. Oil has been tested from the Starichkof sands and deeper Hemlock sands on the Cosmopolitan structure. The Starichkof sand with similar characteristics to those seen in core #7 should be an excellent exploration target elsewhere in the southern Kenai Peninsula.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013