Reservoir Potential of Tertiary Sandstones, Bristol Bay Basin, Alaska Peninsula

Kenneth P. Helmold¹, Paul L. Decker¹, Rocky R. Reifenstuhl², and Michael D. Wilson^3
1 Alaska Division of Oil & Gas, Anchorage, AK
² Alaska Division of Geological & Geophysical Surveys, Fairbanks, AK
³ Geological Consultant, Lakewood, CO

Conventional core and outcrop samples of Tertiary sandstones were examined as part of a regional study to characterize the petroleum potential of the Bristol Bay frontier basin. Prospective sandstone reservoirs include the Pliocene Milky River, Miocene Bear Lake and Oligocene Stepovak formations which are exposed along the northern side of the Alaska Peninsula.

Tertiary sandstones vary in grain size from lower very fine-grained to upper coarse-grained and are well to very poorly sorted. Poor sorting reflects abundant detrital matrix and is particularly common in the Bear Lake formation. The sandstones are highly lithic with an average framework composition of Q₂₂F₁₆L₆₁. Monocrystalline quartz is more common than polycrystalline varieties, and plagioclase is more abundant than K-feldspar. The average lithic composition of the sandstones is Ls₄₁Lv₄₃Lm₁₆ with grains consisting primarily of felsic and mafic volcanic fragments, chert, phyllite, schist, quartzite, felsic plutonic fragments, mudstone and siltstone. Milky River sandstones are the most volcanogenic, with an average composition of Ls₂₄Lv₆₅Lm₁₁. Authigenic phases are minor, consisting of patchy siderite, calcite and kaolinite. Primary intergranular pores form the bulk of the pore system. Compaction and ductile grain deformation are the key agents of porosity destruction, particularly in the Stepovak formation.

Reservoir quality varies from excellent (φ > 30%, k > 100 md) to poor (φ < 10%, k < 1 md). Porosities in excess of 20% and permeabilities over 10 md are present at depths approaching 10,000 feet in the offshore portion of the basin. The majority of samples analyzed could be effective oil reservoirs and a significant number of additional samples could be effective gas reservoirs, particularly in the presence of a well developed fracture network.