Geological Setting Of The Norton Basin Gas Seep
Authors: Holmes Mark L. , Cline Joel  L. , Johnson Janice  L. 
Published: October 2008

Geochemical evidence of a submarine seepage of natural gas beneath Norton Sound was discovered during a 1976 environmental survey of petroleum hydrocarbon levels in waters covering Alaskan continental shelves. Relatively high concentrations of C2-C4 alkanes are apparently emanating from a small area on the sea floor 40-47 km south of Nome. Preliminary estimates of the initial gas phase composition suggest that the gas may originate from a liquid petroleum source.

Norton basin contains more than 3.9 km of sedimentary section, and is floored by an acoustic basement having seismic velocities ranging from 5.5 to 6.5 km/sec. The basin fill consists of three major units distinguishable on the basis of compressional velocities; unconformities appear to separate each of these units. The basal unit, with a velocity of 4.9 km/sec, occurs only in the deeper central portion of the basin. A thick (2 km) section with velocities ranging from 2.3 to 3.7 km/sec rests on this lower unit and acoustic basement. Compressional velocities in the 1.2 km-thick upper unit range from 1.6 to 2.1 km/sec. All direct evidence suggests that the lower two units are mainly non-marine Cretaceous and lower to middle Tertiary sedimentary rocks, respectively, but the size of the basin is such that transitions to marine facies could occur within these sections. The upper unit probably consists of upper Tertiary and Quaternary sedimentary rocks and sediment.

A large southerly dipping fault or steep flexure offsets acoustic basement approximately 700 m along the northern margin of the basin just north of the seep area. Several steeply dipping faults which might provide favorable migration avenues for mobile hydrocarbons from deep reservoirs to the sea floor cut through the sedimentary section in the vicinity of the seep. Also conspicuous on the seismic reflection records are two different types of acoustic anomalies: reflector pull-downs and abrupt reflector terminations. Both of these anomalous acoustic responses are indicative of gas-charged sediment. The seep is located directly over an anomaly of the latter type, having a horizontal extent in excess of 9 km and a general appearance strikingly similar to reflection records over gas caps associated with known major oil fields.

INTRODUCTION

A field program for the measurement of concentrations of low-molecular-weight hydrocarbons in the waters of the southern Chukchi Sea and the northern Bering Sea (Norton Sound) was carried out in September, 1976, by the Pacific Meteorological and Environmental Laboratory (NOAA). In the course of this survey, a plume of hydrocarbon-rich water was discovered in Norton Sound (1). Figure 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 down current from what appears to be a source or sources on the sea floor approximately 45 km south of Nome. 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.