Play Concepts for Reopening the Bristol Bay Basin: Tertiary and Mesozoic Petroleum Systems of the Alaska Peninsula

Paul L. Decker¹, Rocky R. Reifenstuhl², Emily S. Finzel², and Kenneth P. Helmold^1
1 Alaska Division of Oil & Gas, Anchorage, AK
² Alaska Division of Geological & Geophysical Surveys, Fairbanks, AK

Alaska's frontier Bristol Bay basin reopened for state leasing in 2005 after a two decade hiatus. The new annual areawide sale, encompassing 5.8 million acres, is dramatically under-explored with only 11 wells drilled, all between 1959 and 1985. Interest focuses on the northwestern Alaska Peninsula, which parallels the thick southeastern side of the asymmetric back-arc basin. Cenozoic basin fill ranges from > 15,000 feet thick below state waters to zero in nearby uplifts with exposed oil-window Mesozoic strata. A complex subsidence and uplift history makes this margin prospective for structural and stratigraphic plays in both Mesozoic and Tertiary hydrocarbon systems.

Mesozoic sources generate oil and gas seeps on the eastern Alaska Peninsula, and probably also source a vigorous seep of thermogenic methane on a major surface anticline in the sale area. Mesozoic sandstones are degraded by zeolites, but may constitute gas reservoirs, particularly where highly fractured. Lower Cretaceous marine mudstones may seal Jurassic gas reservoirs.

Depending on maturity, Upper Cretaceous to Tertiary coals and carbonaceous mudstones may be excellent sources for either thermogenic or biogenic gas. RockEval and kerogen analyses suggest that Paleogene coals and shales may generate some light oil or condensate, and isotopic and gas wetness data support a liquids-associated thermogenic origin for some Tertiary gas shows. Several Tertiary sandstones would be fair to excellent oil or gas reservoirs, particularly the Miocene Bear Lake Formation and equivalents. Mudstones interbedded with reservoir sands are likely to provide intraformational seals, as in the partially analogous Tertiary system of Cook Inlet.