NEW INSIGHTS INTO THE SUBSURFACE GEOLOGY AND PETROLEUM POTENTIAL OF THE SUSITNA BASIN, SOUTH-CENTRAL ALASKA

Exploratory wells, seismic reflection, gravity, aeromagnetic, and organic geochemical studies provide insights into the subsurface geology and petroleum potential of the Susitna basin, located about 80 km NW of Anchorage. The Susitna basin consists of Cenozoic strata about 4-5 km thick that are broadly similar in lithology and age to the nearby petroleum-producing Cook Inlet basin but are mostly unexplored.

Basement rocks beneath the Susitna basin have not been penetrated by wells. In the eastern and central parts of the basin, basement is inferred to consist of pre-Cenozoic metamorphic and plutonic rocks of the Wrangellia terrane, similar to those exposed in the nearby Talkeetna Mountains. In the western part of the Susitna basin, west of the inferred Broad Pass fault, basement likely includes strongly deformed and metamorphosed marine strata of the Jurassic and Cretaceous Kahiltna assemblage.

Overlying the basement rocks are volcanic and sedimentary rocks, more than 700 m thick, that contain Paleocene palynomorphs and yield ⁴⁰Ar/³⁹Ar ages of about 58-54 Ma on basalt and andesite. The volcanic package is overlain by more than 1,300 m of nonmarine coal-bearing strata of early to middle Eocene age. The Eocene sequence is unconformably overlain by Miocene and younger nonmarine coal-bearing strata that are capped by Quaternary glacial and fluvial deposits. Seismic profiles and new aeromagnetic data show that the Cenozoic strata are deformed into open folds and cut by reverse and thrust faults. Gravity modeling indicates that the southwestern margin of the basin is the Beluga Mountain fault, a NW-striking, SW-dipping thrust fault.

Eocene and Miocene coal and shale are potential source rocks of microbial methane and are shown by Rock-Eval pyrolysis analyses to be possible sources of thermogenic gas and some liquids. Mesozoic marine shales that are the source rocks of oil in the Cook Inlet basin have not been found in the Susitna basin. Potential reservoirs include Eocene and younger fluvial sandstones; potential seals include fine-grained fluvial overbank deposits. Many prospective anticlinal and fault traps appear on seismic profiles. Seven exploratory wells drilled in the basin during 1964-2005 found no commercial deposits of oil or gas, but the oil industry has said that new wells will be drilled in the near future.