High Gas Hydrate and Free Gas Concentrations: An Explanation for Seeps Offshore South Mocha Island

Abstract

Recent studies have reported cold seeps offshore of Mocha Island. Gas hydrate occurrences along the Chilean margin could explain seeps presence. Gas-phase (gas hydrate and free gas) and geothermal gradients were estimated analysing two seismic sections. Close to Mocha Island (up to 20 km) were detected high (up to 1900 m/s) and low (1260 m/s) velocities associated with high gas hydrate (up to 20% of total volume) and free gas (up to 1.1% of total volume) concentrations, respectively. A variable and high geothermal gradient (65-110 • C/km) was obtained. These results are related to high supply of deep fluids canalised by faults and fractures. Faraway from Mocha Island (>60 km), free gas concentrations decrease to 0.3% of total volume and low geothermal gradient (from 35 to 60 • C/km) are associated with low fluids supply. Finally, we propose gas hydrate dissociation processes as the main supply source for seeps in the vicinity of Mocha Island. These processes can be caused by: (a) active faults and seismic activity; and (b) warm fluid expulsion from deeper zones altering hydrate stability conditions. In both cases, gas hydrate dissociation could generate slope instability and landslides, as occurred in the past in this region and reported in the literature.Energies 2018, 11, 3062 2 of 13 close to 3000 km (from 33 • S until 56 • S). In the second case, some authors have modelled possible scenarios for gas hydrate dissociation in shallow water (from 150 to 600 m water depth) considering the increasing temperature due to the global warming and the consequent potential methane release into the atmosphere. They concluded that the gas hydrate dissociation processes are induced just by a slight increment of temperature [25][26][27][28][29][30][31]. In this context, gas hydrate estimates contribute to evaluate submarine geohazards, global warming contribution related to gas hydrate dissociation and the potential methane reservoir. Moreover, estimates of methane stored in marine sediments as gas hydrate and free gas phases can be used to model more realistic scenarios associated with gas hydrates dissociation and its effects as a greenhouse gas.Relationships between gas hydrates and cold seeps have been documented in active and passive margins [5,[31][32][33][34][35][36], in which chemosynthetic communities, authigenic carbonate nodules, enrichment stable isotopes in pore water, and faults and fractures, connected with BSR and detected in seismic profiles, can be related to gas hydrate presence.The present study adds new information regarding gas-phase concentrations of the southern Chilean margin by using seismic and theoretical velocity models (Figure 1). Mocha Island is characterised by active seismicity and constitutes an emerged block uplifted during the Quaternary in the Arauco peninsula [37,38]. Further, Mocha Island is known for intertidal and subtidal gas seepage system [39,40] and deeper seeps at 1400 water depth [41], whose presence probably is related to gas hydrates dissociation. In this context, gas-phase est...

Gas Hydrate Estimate in an Area of Deformation and High Heat Flow at the Chile Triple Junction

“…The Chile Triple Junction (CTJ) area is a spectacular example of tectonic erosion (e.g., [39]). Even though many investigations are associated with seepage identification and gas expulsion quantification (gas bubbles) (e.g., [29,38,[40][41][42]), there are few cases where the objective was to estimate the size of the gas source, as concentrations of gas hydrate and free gas [43].…”

Potential Instability of Gas Hydrates along the Chilean Margin Due to Ocean Warming

“…Mean volume concentrations of 18% and 1% have been observed for gas hydrate and free gas, respectively [20]. Recently, seismic data analysis confirmed the presence of gas hydrate and free gas, estimating concentration values in agreement with direct measurements [13,27,28,30,31]. In addition, studies off Valdivia estimated about 3.5% gas hydrate saturation in the pore space of marine sediments [21,30].…”

“…The release of huge quantities of natural gas in the water column could affect the marine ecosystem resulting in significant impact to benthic organisms [6]. Furthermore, methane is an important greenhouse gas, so after its release into the ocean, it could reach the atmosphere, resulting in positive feedback for global warming, as underlined by previous studies [7][8][9], although this is still the subject of debate among the scientific community (i.e., [10][11][12][13]). In fact, many factors prevent the methane from gas hydrate from reaching the atmosphere, such as methane release velocities and rates from the subsurface, and methane oxidation to carbon dioxide by microbial and chemical processes [14].…”

“…Gas hydrate has been detected in water depths up to 4 km with a depth range of 100-600 m below the seafloor (m b.s.f.) [13,[23][24][25][26][27][28]. During ODP Leg 141, drill holes near the Chile Triple Junction sampled gas hydrates [29].…”

Gas Hydrate: Environmental and Climate Impacts

“…In the last few years, the integration of geophysical (mainly seismic and electromagnetic data), geochemical, and heat-flow data have allowed for detecting and characterizing gas hydrate and free gas volumes and their distribution in the sediments, i.e., [20][21][22][23]. Thus, reviews of extensive geophysical surveys and direct measurements combined with geological interpretation and theoretical modelling will increase our understanding of the occurrence, distribution, and concentration of gas hydrate and the underlying free gas beneath the ocean bottom and in the permafrost regions, i.e., [24][25][26][27][28][29].…”