Interstitial Light Hydrocarbon Gases in Jumbo Piston Cores Offshore Indonesia: Thermogenic or Biogenic?

Abstract Interstitial light hydrocarbon gases were measured to 12 m below seabed in two Jumbo piston cores acquired in deepwater sediments offshore Indonesia. Determinations were made for dissolved methane, ethene, ethane, propene, propane, iso-butane, n-butane, iso-pentane, and n-pentane, as well as for carbon dioxide. Stable carbon isotopic ratios of methane and ethane were also measured. Such gas measurements are typically performed in deepwaters around the world as elements of surface geochemical exploration programs for the purpose of distinguishing biogenic from thermogenic sources in seep gases. Various gas source models have been developed in the literature to aid in accurate interpretation for systems that have been biodegraded, fractionated, and/or mixed during migration. In these models, assumptions are made about the ranges of composition of the end-member gas types and maturities. The compositional ranges of the thermogenic and biogenic end members have been empirically derived from a large database of drilled-hole gas data and various theoretical considerations. In addition to this literature base, tens of thousands of near-surface marine sediment cores have been acquired worldwide and analyzed for interstitial gases on a proprietary SGE basis over the last 35-40 years. Interpretation of these results has helped to fine-tune the accepted end-member compositions of the source models. However, essentially all of the SGE sediment cores have reached a maximum 5 m below the seabed. Our acquisition and analysis of cores down to 12 m has revealed information about gas compositions not apparent from the former SGE coring efforts. In particular, ethane concentrations are in the accepted thermogenic range, but their stable carbon isotopic compositions are not. Near-surface gases formerly interpreted to have an unambiguous thermogenic component are here shown to be purely biogenic. Our conclusions may change the boundaries of the biogenic end member used for the indication of thermogenic gas traces in near-surface sediments.