Abstract: Hydrocarbon Gases in Sediments of Eastern Gulf of Alaska

Keith A. Kvenvolden, George D. Redden, Paul R. Carlson

Hydrocarbon gases were measured in nearshore, near-surface sediments recovered by grab samples and/or gravity cores at 12 stations. Methane (C₁) was detected in all samples and ranged in concentration from 0.32 to 23.0 nl/g of wet sediment. Hydrocarbon gases of higher molecular weights--i.e., ethane (C₂), propane (C₃), isobutane (i-C₄), n-butane (n-C₄), ethylene and propylene--were detected in most of the samples at concentrations at least an order of magnitude less than the concentration of C₁. The highest amounts of C₁, as well as C₂, C₃, and n-C₄, were found in a sample of clayey silt from near the mouth of the Copper River. High concentrations of gases also we e found in clayey-silt samples southeast of Kayak Island. These areas of high gas concentrations show discontinuous seismic reflectors suggesting the presence of gas-charged sediments. Gravity-core samples from depths of about 50 cm below water-sediment interface at the stations near Kayak Island yielded C₁ at a concentration of about 14 and 20 nl/g. The ratios of C₁/(C₂ + C₃) were 287 and 235. In addition, ethylene and propylene were observed. These data are consistent with a source from biochemical and low-temperature chemical diagenesis of sedimentary organic materials. If the gases observed in these cores are related to apparent gas-charged sediments deeper in the Holocene section, then this deeper gas probably has resulted from early diage etic processes and is not the product of seepage of petrogenically derived gases.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC