METHANE AND OTHER HYDROCARBON GASES IN MARINE SEDIMENT

Abstract

Hydrocarbon gases are common in marine sediment accumulating in present-day oceans. Such gases originate from the decomposition of organic matter by biochemical and chemical processes. We consider seven hydrocarbon gases that occur in marine sediment (Table 1). In addition, inorganic gases such as nitrogen (N2), argon (Ar), carbon dioxide (C02), and helium (He) are present, but usually as minor or trace components in natural gas. These will not be discussed here. Methane (C1) is almost always the dominant component of the natural gas mixtures. Usually accompanying C1 are other hydrocarbon gases, including ethane (C2), propane (C3), isobutane (i-C4), and normal butane (n­ C4), that are present in variable amounts from traces to 30-40 percent collectively. Marine sediments also contain volatile hydrocarbons of higher molecular weight, e.g. Cs through at least C7 (Hunt 1975), but our discussion is confined to permanent gas hydrocarbons C1 through C4. In addition to gaseous alkanes, the alkenes, ethene (C2;) and propene (C3;), are found also, but mainly in sediment near the seafloor and in the overlying water column. There are three main stages of natural gas formation during the burial history of sediment. The earliest stage is biological C1 formation, which