Frontiers and Opportunities in Antarctic Geosciences * Certosa di Pontignano * 29-31 July 2004
Pore Pressure Regime and Gas-Phase Distribution –
The South Shetland Margin (Antarctica) Case Study
F. ACCAINO*, U. TINIVELLA & R. GELETTI
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42C – 34010 Sgonico (TS) - Italy
*Corresponding author (accaino@ogs.trieste.it)
Gas hydrates are a solid phase composed of water and low-molecular-weight gases (predominantly methane) which
form under conditions of low temperature, high pressure, and adequate gas concentrations, conditions that are common
in the upper few hundred meters of rapidly accumulated marine sediments. Gas hydrates in marine environments have
been mostly detected from analysis of seismic reflection profiles, where they produce remarkable Bottom Simulating
Reflectors (BSRs). The importance of gas hydrates is mainly related to (i) their enormous resource of natural gas, (ii)
their important role in sediment tectonics, (iii) their role on stability of continental margin, (iv) their greenhouse gas.
The regional distribution of BSRs generated by gas hydrates and free gas trapped below the hydrate stability zone
provides an important clue to their origin, which is always in some way related to the geological setting of the area of
occurrence. In order to improve the interpretation of the genesis of these hydrocarbon gas accumulations, a question
must be asked on which degree of accuracy the spatial distribution of gas in either phase and its volumetric abundance
have to be known. Studies in the South Shetland margin (Antarctica) indicate that a free gas zone is present below the
BSR with variable thickness and a variable velocity. Moreover, there is a strong correlation between the free gas
content in the pore space and the strength of the BSR (Tinivella et al., 2002).
A strong BSR was identified on several multichannel seismic reflection profiles acquired during the Austral summers
1989/1990 and 1996/1997 on the South Shetland Margin (Tinivella and Accaino, 2002). One Ocean Bottom
Seismometer (OBS), deployed during the 1996-97 cruise, provided energy arrivals from below the BSR and, in
particular, the refraction from the base of the free gas zone and the converted waves. The inversion of such events
allowed us to reconstruct the compressional and shear wave velocity field in the gas hydrate and free-gas zones
(Tinivella and Accaino, 2002).
The good results obtained by the inversion of the OBS data suggested us to propose a new detailed acquisition survey
with the purpose to reconstruct the elastic properties of the sediments in the BSR region. In February-March 2004 we
acquired new reflection and OBS seismic data in the area where the BSR is particularly strong, deploying nine fourchannels OBSs along pre-existing multichannel seismic lines. Seismic reflection data were recorded simultaneously.
To compare the BSR seismic results with a BSR blank area, we deployed a single OBS in the trench and we fired along
a seismic line acquired in 1989. In fact, as pointed out in previous studies (i.e., Tinivella et al. 2002), the trench can be
considered as a reference zone to calibrate the theoretical models in absence of gas hydrate and free gas.
The reflection seismic data was processed to enhance the BSR and the Base of the free Gas Reflector (BGR). The
seismic lines acquired during the first two legs were re-processed to improve the seismic section. The pre-stack depth
migration was then applied iteratively to reconstruct the velocity field with higher accuracy.
During the February-March 2004 cruise, we acquired other geophysical data such as: multibeam, CTD profiles in the
water column, sub-bottom profiles and two gravity cores. The multibeam data were recorded over an area of about 5500
km2. They allowed us to recognise several important features, such as mud volcanoes and collapse throughs and recent
slides from a mud volcano ridge.
We acquired four CTD profiles; one of them close to a mud volcano indicated a significant positive thermal and salinity
anomaly some 60-80 m above the sea floor. Because the anomaly is slight and it was recorded downward only, it is
probably correlated with the far field transient of an episodic fluid expulsion event. This evidence is also consistent with
the sub-bottom profile acquired across the area, that reveals the presence of several fluid expulsion plumes in the water
column at the border of the gas hydrate reservoir.
The cores are not yet analysed, but one of them exhibited a strong smell of methane, when recovered onboard.
The analysis of the new geophysical dataset and the accurate processing of the previous multichannel seismic lines
allowed us to better reconstruct the 3D geometry and physical properties of the reservoir and to understand the
migration and outflow of the mud and fluids from below the gas hydrate zone. The gas hydrates zone corresponds to the
relatively undisturbed portion of the sedimentary cover, whereas the mud and fluid expulsion phenomena are
concentrated along the fault systems bordering the reservoir. Outside the field the BSR is very weak or absent. The
outflow phenomena, mud volcanoes, collapse troughs and slides are active and likely episodic, indicating that there is a
pressure build-up over time, below the BSR. Moreover, the gas hydrate reservoir is located in a structural high,
supporting the hypothesis that the free gas is trapped in the sediments because of the hydrate presence.
The quite extensive data set available in the BSR area allows us to determine the distribution and concentration of gas
phases and to estimate the methane release caused by gas hydrate. The contribution to gas hydrate dissociation, induced
by the global change, could also be estimated, evaluating the effects of the past and future changes in temperature and
sea level.
REFERENCES
U. Tinivella & F. Accaino. Marine Geology, 164 (2000), 13-27
U. Tinivella, F. Accaino, and A. Camerlenghi. Marine Geophysical Research (2002), 23, 109-123