BRISTOL BAY BASIN, ALASKA PENINSULA, FRONTIER HYDROCARBON BASIN: NEW INSIGHTS

REIFENSTUHL, Rocky, Alaska Division of Geological & Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709, [email protected]

Regional hydrocarbon-focused fieldwork and analyses by the Alaska Division of Geological and Geophysical Surveys and the Division of Oil and Gas, along with a consortium of consulting geologists provides abundant new baseline exploration data. New field data codifies the fundamental reservoir characteristics of potential basin targets, places the reservoir data in a stratigraphic and sequence stratigraphic framework, and summarizes the structure, organic geochemistry, and hydrocarbon potential of this frontier basin (see wwwdggs.dnr.state.ak.us). Project data catalyzed interest in the October 2005 area wide lease sale ($1.3 million; 37 tracts) near Port Moller town site and Herendeen Bay. The area hosts a prolific seep of dry natural gas which, based on carbon and deuterium isotopes, suggest thermally mature Mesozoic strata lie at depth. Petroleum system plays in the Herendeen Bay region are within both the Mesozoic and Tertiary sections. Oil seeps (~1/2 barrel/day) from the Jurassic age Shelikof Formation south of Puale Bay. The Kamishak Formation, a Triassic age shallow-water limestone yields total organic carbon (TOC) to 2.4%, hydrogen index (HI) of 598 and oxygen index (OI) of 22. Middle Jurassic age Kialagvik clastic marine rocks yield up to 3.5% TOC, up to 680 HI, and average 22 OI. Such values are encouraging for potential hydrocarbon sources. The Miocene age Bear Lake Formation, an important reservoir in play models, is at least 3,700 feet (~1,200 meters) thick, and comprises a deepening-upward succession. Porosity and permeability values range from 1 to 35 percent and 0.001 to 1,000 millidarcies. The sand/organic siltstone ratio is estimated at 80/1. Siliceous coal and carbonaceous siltstone occur throughout the formation. Thin, siliceous coal deposits also occur in the underlying Upper Cretaceous rocks. These thin, but common, coal deposits create the possibility for biogenic methane production. The Oligocene age Stepovak Formation and the Eocene and Paleocene age Tolstoi Formation include organic-rich marine strata. Tolstoi Formation rocks yield TOC to 8.9%, and 253 HI and 8 OI. Publicly-available seismic and field data suggest structural traps and unconformities, particularly at the base of the Pliocene Milky River Formation. Seals are presumed to be thin and intraformational.