Potential of microbial methane formation in a high-temperature hydrocarbon seep
Journal
Applied Geochemistry
Journal Volume
27
Journal Issue
8
Pages
1666-1678
Date Issued
2012
Author(s)
Abstract
Hydrocarbon seepage is a surface expression where fluids mixed with sediments and hydrocarbons are expelled through fracture systems that potentially tap into gas-petroleum reservoirs. Hydrocarbons released from most seeps appear to be thermogenic on the basis of their relative abundance and isotopic composition. The potential for subsurface microbial processes modifying these geochemical fingerprints remains poorly constrained. In this study, microcosm incubations were conducted on mud slurries supplied with/without various methanogenic precursors at temperatures ranging from ambient conditions to 90°C, in order to assess microbial CH 4 formation in the subsurface beneath hydrocarbon seeps. The analyses indicated that CH 4 production was positive at ≤80°C, regardless of whether or not or which precursors were added. However, the pattern of CH 4 production rates varied with the precursor and temperature. In general, the optimum CH 4 production from H 2/CO 2 and formate occurred over a wide range of temperatures (≥40°C), whereas that from acetate, methanol and methylamine was restricted to relatively lower temperatures (40-50°C). The CH 4 recoveries, together with the C isotopic compositions of CH 4, further indicated that the quantities of CH 4 produced could not completely account for the quantities of precursor consumed, suggesting that a complex metabolic network was involved in the transformation of the added precursor and organic C inherited from inoculated sediments. Microbial CH 4 was estimated to constitute 7-61% of the CH 4 observed using experimentally-derived apparent isotope fractionations as the end member compositions. This illustrates the possibility that microbial CH 4 produced at shallower depths could quantitatively and isotopically alter deeply-sourced thermogenic CH 4 in hydrocarbon seep environments. © 2012 Elsevier Ltd.
SDGs
Type
journal article