Geologic Setting and Source Depth of the Norton Basin Gas Seep

Abstract

The Norton basin gas seep was discovered in 1976. Seismic reflection records across the seep zone show two types of acoustic anomalies indicative of gas-charged sediment: reflector pull-downs and abrupt reflector terminations. The seeping gas apparently originates from a secondary hydrocarbon accumulation trapped at the basin margin by a combination of structural and stratigraphic features. Introduction A field program for measuring concentrations of low-molecular-weight hydrocarbons in the waters of the southern Chukchi Sea and the northern Bering Sea (Norton Sound) was conducted Sept 1976 by the Pacific Meteorological and Environmental Pacific Meteorological and Environmental Laboratory (NOAA). During this survey, a plume of hydrocarbon-rich water was discovered in Norton Sound. Fig. 1 shows the isopleths of ethane concentration in the near-bottom waters. The elevated hydrocarbon levels in the plume could be detected for 150 km downcurrent from what apears to be a source or sources on the seafloor about 45 km south of Nome, AK. Preliminary dynamic modeling estimates of the initial gas-phase composition predict methane/ethane and ethane/propane ratios of 24 and 1.7, respectively, assuming the hydrocarbons were introduced by bubbles from a submarine seepage of natural gas. The low ethane/propane ratio indicates gas from a petroleum source rather than from nonassociated or biogenic natural gas. A detailed discussion of the geochemistry of the seep gases is given by Cline and Holmes and Kvenvolden et al.; Nelson et al. discuss bottom sediment geochemistry and possible seafloor locations of the seep. Unsubstantiated reports indicate the existence of natural hydrocarbon seeps near the mouth of the Inglutalik River (Fig. 1) in the northeastern part of Norton Bay, and in the Sinuk River Valley (Fig. 1) about 35 km northwest of Nome. The source rocks for the Inglutalik and Sinuk seeps have not yet been identified definitely. A mantle of Quaternary deposits rests on Cretaceous rocks of the Yukon-Koyukuk geosyncline at the Inglutalik River site. Hopkins (in Miller et al.) reports that much of the trench formed by the Sinuk and Stewart river valleys may be underlain by infolded, unmetamorphosed Cretaceous or Tertiary sediments. Miller et al. also mention a reported occurrence of oil shale on Besboro Island northwest of Unalakleet (Fig. 1). Limited prospecting for oil was conducted at Hastings Creek near Cape Nome (Fig. 1) in 1906 and 1918. Two wells were drilled during the 1906 operation; one showed a trace of oil and the other encountered flammable gas at a depth of 37 m. The gas was at sufficient pressure to blow a 550-kg stem 23 m up the hole. The two holes drilled in 1918 reached depths of 45 and 65 m without encountering any hydrocarbon shows; another attempt in 1919 reached 107 m with similar negative results. The impetus for these drilling episodes was the repeated sighting of oil-like films on the lagoons near Nome and Cape Nome, and a beach, foam thought to be paraffin, brought in by the onshore winds. paraffin, brought in by the onshore winds.