Influence of minor hydrocarbon seepage on sulfur cycling in marine subsurface sediments and its significance for hydrocarbon reservoir detection

Abstract. All hydrocarbon (HC) reservoirs leak to some extent. When small quantities of HCs escape offshore reservoirs and migrate through overlying organic-poor marine sediments towards the surface, these HCs are often completely metabolized by microbial activity before reaching the sediment-water interface. However, inconspicuous HC fluxes still affect the geochemistry of the surrounding sediment, thereby exerting a subtle influence on the composition and activity of microbial populations in shallow subseafloor environments.

In this study, we investigated how localized HC seepage affects microbial sulfate reduction in organic-poor sediment from the SW Barents Sea. We focused on three areas overlying known HC deposits and two reference areas of pristine seabed for comparison. The analysis of 50 gravity cores revealed significant variability in the predicted depth of sulfate depletion across sampling sites, ranging from 3 to 12 m below the seafloor. Although we observed nearly linear pore water sulfate and alkalinity profiles, we measured and modeled low rates (pmol × cm³ × d⁻¹) of sulfate reduction. Metagenomic and metatranscriptomic data on functional marker genes supported microbial turnover associated with active processes of sulfate reduction and anaerobic oxidation of methane (AOM). Marker genes for taxonomy (i.e. SSU rRNA, rpoD), sulfate reduction (i.e. dsrAB, aprAB), methanogenesis and methanotrophy (i.e. mcrA) revealed metabolic activities by a consortium of sulfate-reducing bacteria and ANME archaea, capable of harnessing energy for cell division (i.e. ftsAZ) from HC traces diffusing through the sediment.

Overall, our study demonstrates that the gradient in pore water geochemistry, the rates of sulfate reduction processes, and the genetic features of microbial populations actively involved in sulfate-driven AOM processes are all affected by inconspicuous HC seepage. This slight HC seepage resulted in sulfate depletion at shallower depth and produced concomitant biogeochemical signatures in the shallow subsurface that enable the inference of deeply buried reservoirs.