Petroleum Geology of Norton Basin, Alaska
Basement rocks beneath the main part of the Norton basin were deformed and heated during the Late Jurassic and Early Cretaceous to the extent that these rocks were not capable of generating hydrocarbons when the basin formed during the latest Cretaceous or early Paleogene. Consequently, source rocks for oil, if they exist, are most likely to be within the basin fill. If the Norton basin began to form 65 m.y. ago, subsided at a nearly constant rate, and had an average geothermal gradient of between 35 and 45°C/km, then rocks as young as late Oligocene are in the oil window (vitrinite reflectance between 0.65 and 1.30%). The appearance on seismic sections of reflections from rocks in and below the calculated oil window suggests that these rocks were deposited in a nonm rine environment. Thus, gas and condensate are the most likely hydrocarbons to be present in the basin. Because of their shallow depth of burial, Neogene (possibly marine) rocks are not likely to be thermally mature anywhere in the basin. Deep parts of the basin formed as isolated fault-bounded lows; consequently, the volume of mature rocks makes up at most 11% of the total basin fill. Numerous potential traps for hydrocarbons exist in the Norton basin; the traps include fractured or weathered basement rocks in horsts, strata in alluvial fans on the flanks of horsts, and arched strata over horsts.