Geochemical detection of minor hydrocarbon seepage in marine subsurface sediments

All hydrocarbon reservoirs leak slightly. At sites with only minor leakage the hydrocarbons (HC) infiltrating the sediment from below are completely metabolized before reaching the sediments surface due to microbial activity, thus not creating any surface manifestations, e.g. seeps. Nevertheless, the HC influx will change geochemistry as well as microbial community composition and activity in the affected area because electron donors are added into the system. The PROSPECTOMIS project wants to detect these changes to develop a minimally invasive and low-cost tool do detect HC reservoirs using omics and geochemistry techniques.

In November 2021 we recovered fifty 2-3 m long sediment cores from three minor HC seepage zones and two non-seepage reference zones in the southern Barents Sea and sampled sediment and pore water with high spatial resolution.

While Fourier-transform ion cyclotron resonance mass spectrometry and cell abundances did not show any differences between HC seepage zones and reference zones, multiple pore water concentration profiles e.g. for Manganese, Calcium, Silicon, Strontium, Sulfide revealed differences. Also, higher fluxes of sulfate and alkalinity in HC zones indicate that the sink for sulfate and the source of alkalinity must be shallower in HC zones, most probably caused by anaerobic oxidation of methane (AOM) below the sampled depth interval. Linear sulfate pore water profiles indicate no net turnover of sulfate. Nevertheless, we observed low rates of sulfate reduction using radiotracer incubations, mostly in HC zones.

Combining these parameters with multi-omics datasets will reveal potential indicators for minimal HC seepage.