Hydrocarbon plays of the Antarctic Peninsula margin: Determining fluid flow pathways

The continental margin off the Antarctic Peninsula hosts significant gas hydrates accumulations off the South Shetland Islands. The area has experienced remarkable isostatic rebound due to ice sheet retreat since the Last Glacial Maximum (LGM). Considering heat flow data reported in the area, hydrates could undergone active dissociation. Such dissociation may modify the mechanical properties of hydrate bearing sediments, eventually leading to slope failures and related fluid seepage may also translate in methane emissions to the ocean. Here we use legacy seismic data to map the occurrence of gas hydrates and free gas and their relation to tectonic structures with the aim of determining the nature of fluid emissions (diffuse or focused).

The subduction of the former Phoenix plate beneath the Shetland plate is the main tectonic control of the area. Normal faults are particularly apparent in the upper part of the slope. These faults disrupt the seafloor or the upper subsurface (within 0.25 to 1 s TWTT below seafloor).

Seismic indicators related to the presence of marine gas hydrates, referred to as bottom simulator reflectors (BSRs), have been observed in the continental slope between Snow and Greenwich Islands and among the Shackleton Fracture Zone and Nelson Island. They are located at water depths between 450 and 4800 m and 0.12 to 0.9 s TWTT below seafloor, becoming shallower towards the shelf edge.  The BSRs are commonly affected by normal faults.

In seismic data, free gas is inferred by acoustic blanking and chimneys. However, the same acoustic response could result from intense tectonic activity, particularly at the foot of the slope, where the Shetland block overthrusts the Phoenix plate. Acoustic blanking and seismic chimneys are often found on the slope in water depths between 590 to 5175m and 0 to 2 s TWTT below seafloor. In general the acoustic blanking is identified in the upper part of the sedimentary record, particularly in the accretionary prism. In most seismic profiles, BSRs occur together with faults, chimneys and acoustic blanking.

According to our results, faulting plays a significant role in the migration of fluids trapped in the sedimentary record. In addition, the presence of chimneys and acoustic blanking in replacement of BSRs suggests unstable gas hydrates under the present-day Pressure/Temperature conditions.