Light hydrocarbons in marine sediments

Abstract

Concentrations of the C₁-C₃ hydrocarbons and stable carbon isotope compositions of methane in recent sediments and seep gases of the Texas-Louisiana shelf-slop region were determined. These gases have been produced by both microbial and thermocatalytic processes. Microbially-produced gases consist almost exclusively of methane, having C₁/(C₂+ C₃) hydrocarbon ratios greater than 1000 and σ¹³ (subscript PDB) values of methane more negative than -60 °/oo. Petroleum-related hydrocarbon gases generally have C₁/(C₂+ C₃) ratios smaller than 50 and isotopic ratios more positive than -50 °/oo. A geochemical model based on these two parameters is used to show that natural gas compositions can be altered due to mixing of gases from the two sources as well as by microbial action and migration through sediments. Light hydrocarbons in the upper few meters of Gulf of Mexico sediments are almost entirely of microbial origin. In the Mississippi Delta region, interstitial sulfate is quickly depleted and extensive production of methane is observed within one or two meters of the seawater-sediment interface. This "horizon" of high methane concentration occurs progressively deeper in sediments in an offshore direction. Bacterial production of methane is not restricted to the sulfate-free zone, however, but apparently also occurs within "microenvironments" in sediments having near-seawater interstitial sulfate concentrations..