**2.3 Gas hydrate and related features**

The bathymetric map of the SSM provides evidence of four mud volcanoes (**Figure 3**; [18, 31]), which are associated to the presence of gas hydrate. This active mud volcanism might be favored by the reactivation of pre-existing faults and weakness zones because of the regional extensional tectonics of the South Shetland trench and margin [34], the adjacent Bransfield Strait back-arc basin [35], and the complex tectonic interaction at the Elephant Island triple junction [36].

*Map of the investigated area with the location of the acquired data during the surveys. Red rectangle indicates the seafloor reflectivity reported in Figure 5. Green rectangles indicate the position of the zoom reported in Figure 6.*

The chirp data (location in **Figure 4**) confirm the presence of a few slides, which are probably related to gas hydrate dissociation and several fluid expulsion sites, probably related to active mud volcanism sustained by a hydrocarbon reservoir. Note that, as expected, low values of seafloor reflectivity are identified in correspondence of these features (**Figure 5**). Moreover, the bathymetry shows the presence of ancient slides that could be linked to gas hydrates (**Figure 6**).

Fluid analyses made to the gravity cores revealed the presence of several hydrocarbon gases, such as methane, ethane, propane, butane, pentane and hexane, and traces of aromatic hydrocarbons of >C12 carbon chain length, suggesting a thermogenic origin of the gas.

A strong and continuous BSR was recognized through the analysis of the seismic lines and OBS data, allowing the detection of a large gas hydrate reservoir on the SSM. [33] reported an example of seismic and OBS data. The elastic properties of the different layers across the BSR were modeled by using Tinivella theoretical models in order to quantify the concentrations of gas hydrates and free gas in the pore space [37, 38].

Poisson's ratio near the OBS location was evaluated by the joint inversion of compressional and shear wave arrivals in the vertical and horizontal components of OBS data. Useful information about physical properties of marine sediments in areas where no well data are available were obtained through Amplitude Versus Offset (AVO) analysis and OBS data [29, 38]. In detail, the sediments seems not cemented by the presence of hydrate (due to AVO behavior) and the free gas below the BSR seems uniformly distributed in the pore space (due to the low Poisson ratio) and not in overpressure condition (due to low P-converted wave amplitude; [39]).

Analysis of geophysical data evidences that the accumulation of fluids within sediments is strictly related to tectonics features, such as faults and folds. The concept of hydrate porosity (HP), which is directly related to the fluid content, was introduced in order to better understand the relationship between gas hydrate presence and geological features. HP represents the difference between the

**7**

**Figure 5.**

*in Figure 3.*

reference porosity (i.e., the porosity without gas hydrate) and the effective porosity (i.e., the porosity reduced by the gas hydrate presence; [40]). The detailed analysis of the reservoir revealed a close relationship between HP, and consequently gas hydrate accumulation, and geological features, such as syncline-anticline structures and fractures distribution within sediments [40]. In particular, a relationship is

*Map of the seafloor reflectivity extracted from the CHIRP DATA. The location of the area is reports* 

*Gas Hydrates in Antarctica*

*DOI: http://dx.doi.org/10.5772/intechopen.94306*

**Figure 4.** *Map of the investigated area with the location of the CHIRP data.*

*Glaciers and the Polar Environment*

genic origin of the gas.

pore space [37, 38].

The chirp data (location in **Figure 4**) confirm the presence of a few slides, which are probably related to gas hydrate dissociation and several fluid expulsion sites, probably related to active mud volcanism sustained by a hydrocarbon reservoir. Note that, as expected, low values of seafloor reflectivity are identified in correspondence of these features (**Figure 5**). Moreover, the bathymetry shows the presence of ancient slides that could be linked to gas hydrates (**Figure 6**).

Fluid analyses made to the gravity cores revealed the presence of several hydrocarbon gases, such as methane, ethane, propane, butane, pentane and hexane, and traces of aromatic hydrocarbons of >C12 carbon chain length, suggesting a thermo-

A strong and continuous BSR was recognized through the analysis of the seismic lines and OBS data, allowing the detection of a large gas hydrate reservoir on the SSM. [33] reported an example of seismic and OBS data. The elastic properties of the different layers across the BSR were modeled by using Tinivella theoretical models in order to quantify the concentrations of gas hydrates and free gas in the

Poisson's ratio near the OBS location was evaluated by the joint inversion of compressional and shear wave arrivals in the vertical and horizontal components of OBS data. Useful information about physical properties of marine sediments in areas where no well data are available were obtained through Amplitude Versus Offset (AVO) analysis and OBS data [29, 38]. In detail, the sediments seems not cemented by the presence of hydrate (due to AVO behavior) and the free gas below the BSR seems uniformly distributed in the pore space (due to the low Poisson ratio) and not in overpressure condition (due to low P-converted wave amplitude; [39]). Analysis of geophysical data evidences that the accumulation of fluids within sediments is strictly related to tectonics features, such as faults and folds. The concept of hydrate porosity (HP), which is directly related to the fluid content, was introduced in order to better understand the relationship between gas hydrate presence and geological features. HP represents the difference between the

**6**

**Figure 4.**

*Map of the investigated area with the location of the CHIRP data.*

#### **Figure 5.**

*Map of the seafloor reflectivity extracted from the CHIRP DATA. The location of the area is reports in Figure 3.*

reference porosity (i.e., the porosity without gas hydrate) and the effective porosity (i.e., the porosity reduced by the gas hydrate presence; [40]). The detailed analysis of the reservoir revealed a close relationship between HP, and consequently gas hydrate accumulation, and geological features, such as syncline-anticline structures and fractures distribution within sediments [40]. In particular, a relationship is

**Figure 6.**

*Example of ancient slides highlighted with a black dash lines on the bathymetric data. The position of the two zoom is reported in Figure 3. The bathymetric data scale is reported in Figure 3.*

underlined between the HP values and the distance from the hinge of the anticline: the HP increases toward the limbs of anticline. The microfracturing model supports the idea that the synclines favors the hydrate accumulation above the BSR, while the anticlines favors the free gas accumulation below the BSR, when important faults acting as preferential path-way for fluids escapes [40].

All available seismic profiles and OBS data were analyzed in order to obtain 2D seismic velocity models, then translated in terms of concentrations of gas hydrate and free gas in the pore space by using Tinivella theoretical models [37, 38]. The jointly interpolation of the 2D models allowed obtaining a 3D model of gas hydrate concentration from the seafloor to the BSR, as shown in **Figure 7**. The total volume of hydrate, estimated in the area (600 km2 ) where the interpolation is reliable, is 16 × 109 m3 . The gas hydrate concentration is affected by error estimated equal to about ±25%, as deduced from sensitivity tests and from error analysis related to the interpolation procedure. The estimated amount of gas hydrate can vary in a range of 12 × 109 –20 × 109 m3 . Moreover, considering that 1 m3 of gas hydrate corresponds to about 140 m3 of free gas in standard conditions, the total free gas trapped in this reservoir ranges between 1.68 × 1012 and 2.8 × 1012 m3 . This estimation does not take into account the free gas contained within pore space below the hydrate layer, so this values could be underestimated [41].
