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

• Published in: Energies (Basel) Vol. 11; no. 11; p. 3062
• Main Authors: Vargas-Cordero, Iván, Tinivella, Umberta, Villar-Muñoz, Lucía, Bento, Joaquim
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.11.2018
• Subjects: bottom simulating reflector (BSR); Climate change; Earth; Earthquakes; Estimates; gas hydrate; Gas hydrates; Global warming; Greenhouse effect; Greenhouse gases; Hydrates; Marine sediments; Methane; Mocha Island; Ocean floor; Sediments; Seepage; Seismicity; Studies; Temperature; Vapor phases; Velocity; Water depth; Chile; United States > US; Alaska
• ISSN: 1996-1073; 1996-1073
• DOI: 10.3390/en11113062

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.