Geochemical detection of minor hydrocarbon seepage in marine sediment

All hydrocarbon (HC) reservoirs experience some degree of leakage, so HCs will enter the overlying sediment. While strong leakage causes surface manifestations, minor leakage can remain undetected as the hydrocarbons are completely metabolized during their ascent to the sediment surface. However, even minor seepage affects the sediment’s geochemistry and microbiology as it adds electron donors. The PROSPECTOMICS project aims to use these sometimes minute microbiological and geochemical changes as a tool for HC prospecting.

We recovered fifty 2-3 m long sediment cores in the Barents Sea from three potential HC seepage zones (HC zones) and two zones without seepage (REF zones) and sampled sediment and pore water with high spatial resolution.

We measured sulfate reduction rates and quantified microbial cell abundance, and characterized the organic matter via FT-ICR-MS. We also quantified anions and cations in the pore water via ion chromatography and ICP-MS and determined alkalinity via titration.

FT-ICR-MS and cell counts did not show any differences between HC zones and REF zones. Sulfate concentration profiles decrease linearly with depth and show a much steeper decline and greater variability in the HC zones than in the REF zones. The linear profiles imply the absence of active sulfate reduction within the cored depth intervals and a sink for sulfate at greater depth, most probably sulfate-driven anaerobic oxidation of methane (AOM). This would also explain the correlating linear increase in alkalinity. At some sites in the HC zones pore water sulfide profiles also increase linearly with depth whereas at other HC sites and at all REF sites, sulfide concentrations remain below our detection limit throughout the entire core.

Using highly sensitive ³⁵SO₄^2- radiotracer incubations, we were able to detect low rates of microbial sulfate reduction in the pmol*cm^-3*d^-1 range in some single samples from deeper layers in HC cores. Thus, despite apparently linear pore water sulfate gradients indicating no net sulfate reduction, we observed minor but detectable microbial turnover of sulfate. Modeling of the sulfate reduction rate based on pore water concentration data also yielded rates in the same order of magnitude as the radiotracer measurements, confirming microbial activity.

Looking at the pore water cation concentration profiles, manganese and calcium show different linear trends in the HC zones compared to REF zones. In HC zones, manganese decreases with depth, while in the REF zones, manganese concentrations increase. Calcium concentrations decrease at HC sites while they remain constant at REF sites. These findings can partly be explained by microbial activity and associated alteration of clays, potentially due to microbial reduction of structural metals, ion exchange processes and mineral dissolution and formation. Barium was only detected in the pore water of some cores originating from HC zones where it might have been released during sulfate reduction accompanied with destabilization of baryte.

In summary, relative differences in pore water ion concentration trends and the occurrence of sulfate reduction may be indicators of HC seepage.