High Gas Hydrate and Free Gas Concentrations: An Explanation for Seeps Offshore South Mocha Island
Abstract
:1. Introduction
2. Materials and Methods
2.1. Seismic Data
2.2. Inversion Modelling
2.3. BSR-Derived Geothermal Gradient
2.4. Gas-Phase Concentrations
3. Results
4. Discussion
5. Conclusions
- (a)
- The BSR was recognised in both seismic sections analysed. Near Mocha Island (SO161–35 seismic profile), the BSR is continuous, strong and shows a variable depth. On the contrary, the RC2901–731 seismic profile reveals a discontinuous and locally strong BSR, while a constant BSR depth (~250 mbsf) is evidenced.
- (b)
- Variation on the geothermal gradient is the main cause of BSR depth variability close to the Mocha Island, where values ranging from 65 to 110 °C/km were obtained. These values are related to fluids advection canalised by faults and fractures present in the area.
- (c)
- High gas hydrate concentrations up to 20% of total volume (average 7.5%) were found in both sections. On the contrary, the free gas average concentrations are different and equal to 0.4% and 0.12% of total volume in SO161–35 and RC2901–731 seismic profiles, respectively.
- (d)
- The highest free gas concentration was observed close to Mocha Island (up to 1.1% of total volume) and would be related to gas hydrate dissociation associated to past sliding processes or free gas stored below an impermeable gas hydrate layer.
- (e)
- The highest gas hydrate concentration close to Mocha Island can be explained as due to high fluid supply from deeper zones, which can favour gas hydrate formation.
- (f)
- Gas hydrate dissociation processes, caused by active faults, seismic activity and/or by warm fluids expulsion from deeper zones, can constitute the main seepage supply source along this part of the Chilean margin. In both cases, these processes could generate slope instability and landslides.
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Seismic Lines | Research Vessel | Long Streamer | Channels | Intertrace | Shot Spacing | Airguns/Total Volume |
---|---|---|---|---|---|---|
RC2901–731 | RV/Conrad | 3000 m | 240 | 12.5 m | 50 m | 10/61.3 L |
SO161–35 | RV/Sonne | 3000 m | 25 108 | 12.5 m 25 m | 50 m | 20/54.1 L |
RC2901–731 Seismic Section | SO161–35 Seismic Section | ||
---|---|---|---|
Layers | Iterations | Layers | Iterations |
Seawater | 3 | Seawater | 4 |
From SF to H1 | 5 | From SF to BSR | 15 |
From H1 to BSR | 18 | Free gas | 10 |
Free gas | 4 | - | - |
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Vargas-Cordero, I.; Tinivella, U.; Villar-Muñoz, L.; Bento, J.P. High Gas Hydrate and Free Gas Concentrations: An Explanation for Seeps Offshore South Mocha Island. Energies 2018, 11, 3062. https://doi.org/10.3390/en11113062
Vargas-Cordero I, Tinivella U, Villar-Muñoz L, Bento JP. High Gas Hydrate and Free Gas Concentrations: An Explanation for Seeps Offshore South Mocha Island. Energies. 2018; 11(11):3062. https://doi.org/10.3390/en11113062
Chicago/Turabian StyleVargas-Cordero, Iván, Umberta Tinivella, Lucía Villar-Muñoz, and Joaquim P. Bento. 2018. "High Gas Hydrate and Free Gas Concentrations: An Explanation for Seeps Offshore South Mocha Island" Energies 11, no. 11: 3062. https://doi.org/10.3390/en11113062