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

Energies ◽  
2018 ◽  
Vol 11 (11) ◽  
pp. 3062 ◽  
Author(s):  
Iván Vargas-Cordero ◽  
Umberta Tinivella ◽  
Lucía Villar-Muñoz ◽  
Joaquim Bento
Keyword(s):  
Gas Hydrate ◽  
Active Faults ◽  
Geothermal Gradient ◽  
Slope Instability ◽  
Cold Seeps ◽  
Supply Source ◽  
Hydrate Dissociation ◽  
Free Gas ◽  
The Past ◽  
Seismic Sections

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.

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

2018 ◽  
Author(s):  
Iván Vargas-Cordero ◽  
Umberta Tinivella ◽  
Lucía Villar-Muñoz ◽  
Joaquim P. Bento
Keyword(s):  
Gas Hydrate ◽  
Geothermal Gradient ◽  
Supply Source ◽  
Hydrate Dissociation ◽  
Geothermal Gradients ◽  
Free Gas ◽  
Gas Phases ◽  
Seismic Sections ◽  
Chilean Margin ◽  
High Geothermal Gradient

Recent studies have reported shallow and deep seep areas offshore Mocha island. Gas hydrate occurrences along the Chilean margin could explain seeps presence. Gas phases (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. These values are in agreement with a variable and high geothermal gradient (65 to 110 °C/km) related to high supply deep fluids canalised by faults and fractures. Faraway from Mocha island (more than 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 triggered by ancient sliding reported in literature.

Identification of gas hydrate dissociation from wireline-log data in the Shenhu area, South China Sea

Geophysics ◽  
2012 ◽  
Vol 77 (3) ◽  
pp. B125-B134 ◽  
Author(s):  
Xiujuan Wang ◽  
Myung Lee ◽  
Shiguo Wu ◽  
Shengxiong Yang
Keyword(s):  
Wave Velocity ◽  
Gas Hydrate ◽  
Seismic Data ◽  
Pore Space ◽  
P Wave ◽  
Well Log ◽  
Hydrate Dissociation ◽  
Free Gas ◽  
P Wave Velocity

Wireline logs were acquired in eight wells during China’s first gas hydrate drilling expedition (GMGS-1) in April–June of 2007. Well logs obtained from site SH3 indicated gas hydrate was present in the depth range of 195–206 m below seafloor with a maximum pore-space gas hydrate saturation, calculated from pore water freshening, of about 26%. Assuming gas hydrate is uniformly distributed in the sediments, resistivity calculations using Archie’s equation yielded hydrate-saturation trends similar to those from chloride concentrations. However, the measured compressional (P-wave) velocities decreased sharply at the depth between 194 and 199 mbsf, dropping as low as [Formula: see text], indicating the presence of free gas in the pore space, possibly caused by the dissociation of gas hydrate during drilling. Because surface seismic data acquired prior to drilling were not influenced by the in situ gas hydrate dissociation, surface seismic data could be used to identify the cause of the low P-wave velocity observed in the well log. To determine whether the low well-log P-wave velocity was caused by in situ free gas or by gas hydrate dissociation, synthetic seismograms were generated using the measured well-log P-wave velocity along with velocities calculated assuming both gas hydrate and free gas in the pore space. Comparing the surface seismic data with various synthetic seismograms suggested that low P-wave velocities were likely caused by the dissociation of in situ gas hydrate during drilling.

scholarly journals Gas Hydrate System Offshore Chile

Energies ◽  
2021 ◽  
Vol 14 (3) ◽  
pp. 709
Author(s):  
Ivan Vargas-Cordero ◽  
Michela Giustiniani ◽  
Umberta Tinivella ◽  
Lucia Villar-Muñoz ◽  
Giulia Alessandrini
Keyword(s):  
Gas Hydrate ◽  
Slope Failure ◽  
Natural Phenomena ◽  
Hydrate Dissociation ◽  
Free Gas ◽  
Hydrate Reservoir ◽  
Porosity Reduction ◽  
Gas Hydrate Reservoir ◽  
High Concentrations ◽  
Seismic Lines

In recent decades, the Chilean margin has been extensively investigated to better characterize the complex geological setting through the geophysical data. The analysis of seismic lines allowed us to identify the occurrence of gas hydrates and free gas in many places along the margin and the change of the pore fluid due to the potential hydrate dissociation. The porosity reduction due to the hydrate presence is linked to the slope to identify the area more sensitive in case of natural phenomena or induced by human activities that could determine gas hydrate dissociations and/or leakage of the free gas trapped below the gas-hydrate stability zone. Clearly, the gas hydrate reservoir could be a strategic energy reserve for Chile. The steady-state modelling pointed out that the climate change could determine gas hydrate dissociation, triggering slope failure. This hypothesis is supported by the presence of high concentrations of gas hydrate in correspondence of important seafloor slope. The dissociation of gas hydrate could change the petrophysical characteristics of the subsoil triggering slopes, which already occurred in the past. Consequently, it is required to improve knowledge about the behavior of the gas hydrate system in a function of complex natural phenomena before the exploitation of this important resource.

scholarly journals Effectiveness of Fluorescent Viability Assays in Studies of Arctic Cold Seep Foraminifera

2021 ◽  
Vol 8 ◽  
Author(s):  
Katarzyna Melaniuk
Keyword(s):  
Water Depth ◽  
Rose Bengal ◽  
Gas Hydrate ◽  
Barents Sea ◽  
Fluorescence Probe ◽  
Isotopic Signature ◽  
Cold Seep ◽  
Cold Seeps ◽  
Gas Reservoirs ◽  
The Past

Highly negative δ13C values in fossil foraminifera from methane cold seeps have been proposed to reflect episodes of methane release from gas hydrate dissociation or free gas reservoirs triggered by climatic changes in the past. Because most studies on live foraminifera are based on the presence of Rose Bengal staining, that colors the cytoplasm of both live and recently dead individuals it remains unclear if, and to what extent live foraminifera incorporate methane-derived carbon during biomineralization, or whether the isotopic signature is mostly affected by authigenic overgrowth. In this paper, modern foraminiferal assemblages from a gas hydrate province Vestnesa Ridge (∼1,200 m water depth, northeastern Fram Strait) and from Storfjordrenna (∼400 m water depth in the western Barents Sea) is presented. By using the fluorescent viability assays CellTrackerTM Green (CTG) CMFDA and CellHunt Green (CHG) together with conventional Rose Bengal, it was possible to examine live and recently dead foraminifera separately. Metabolically active foraminifera were shown to inhabit methane-enriched sediments at both investigated locations. The benthic foraminiferal faunas were dominated by common Arctic species such as Melonis barleeanus, Cassidulina neoteretis, and Nonionellina labradorica. The combined usage of the fluorescence probe and Rose Bengal revealed only minor shifts in species compositions and differences in ratios between live and recently dead foraminifera from Storfjordrenna. There was no clear evidence that methane significantly affected the δ13C signature of the calcite of living specimens.

scholarly journals Gas origin linked to paleo BSR

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Iván de la Cruz Vargas-Cordero ◽  
Lucia Villar-Muñoz ◽  
Umberta Tinivella ◽  
Michela Giustiniani ◽  
Nathan Bangs ◽  
...  
Keyword(s):  
Gas Hydrate ◽  
Geothermal Gradient ◽  
Last Deglaciation ◽  
Seismic Line ◽  
Central Chile ◽  
Free Gas ◽  
Bottom Simulating Reflector ◽  
South Central ◽  
Gas System ◽  
Central South

AbstractThe Central-South Chile margin is an excellent site to address the changes in the gas hydrate system since the last deglaciation associated with tectonic uplift and great earthquakes. However, the dynamic of the gas hydrate/free gas system along south central Chile is currently not well understood. From geophysical data and modeling analyses, we evaluate gas hydrate/free gas concentrations along a seismic line, derive geothermal gradients, and model past positions of the Bottom Simulating Reflector (BSR; until 13,000 years BP). The results reveal high hydrate/free gas concentrations and local geothermal gradient anomalies related to fluid migration through faults linked to seafloor mud volcanoes. The BSR-derived geothermal gradient, the base of free gas layers, BSR distribution and models of the paleo-BSR form a basis to evaluate the origin of the gas. If paleo-BSR coincides with the base of the free gas, the gas presence can be related to the gas hydrate dissociation due to climate change and geological evolution. Only if the base of free gas reflector is deeper than the paleo-BSR, a deeper gas supply can be invoked.

scholarly journals From scenario-based seismic hazard to scenario-based landslide hazard: rewinding to the past via statistical simulations

2021 ◽  
Author(s):  
Luguang Luo ◽  
Luigi Lombardo ◽  
Cees van Westen ◽  
Xiangjun Pei ◽  
Runqiu Huang
Keyword(s):  
Ground Motion ◽  
Land Use Planning ◽  
Landslide Susceptibility ◽  
Additive Model ◽  
Slope Instability ◽  
Susceptibility Map ◽  
Jiuzhaigou Earthquake ◽  
Full Spectrum ◽  
The Past ◽  
Time Invariant

AbstractThe vast majority of statistically-based landslide susceptibility studies assumes the slope instability process to be time-invariant under the definition that “the past and present are keys to the future”. This assumption may generally be valid. However, the trigger, be it a rainfall or an earthquake event, clearly varies over time. And yet, the temporal component of the trigger is rarely included in landslide susceptibility studies and only confined to hazard assessment. In this work, we investigate a population of landslides triggered in response to the 2017 Jiuzhaigou earthquake ($$M_w = 6.5$$ M w = 6.5 ) including the associated ground motion in the analyses, these being carried out at the Slope Unit (SU) level. We do this by implementing a Bayesian version of a Generalized Additive Model and assuming that the slope instability across the SUs in the study area behaves according to a Bernoulli probability distribution. This procedure would generally produce a susceptibility map reflecting the spatial pattern of the specific trigger and therefore of limited use for land use planning. However, we implement this first analytical step to reliably estimate the ground motion effect, and its distribution, on unstable SUs. We then assume the effect of the ground motion to be time-invariant, enabling statistical simulations for any ground motion scenario that occurred in the area from 1933 to 2017. As a result, we obtain the full spectrum of potential coseismic susceptibility patterns over the last century and compress this information into a hazard model/map representative of all the possible ground motion patterns since 1933. This backward statistical simulations can also be further exploited in the opposite direction where, by accounting for scenario-based ground motion, one can also use it in a forward direction to estimate future unstable slopes.

Review of natural gas hydrate dissociation effects on seabed stability

2021 ◽  
Author(s):  
Min Zhang ◽  
Ming Niu ◽  
Shiwei Shen ◽  
Shulin Dai ◽  
Yan Xu
Keyword(s):  
Natural Gas ◽  
Gas Hydrate ◽  
Natural Gas Hydrate ◽  
Hydrate Dissociation

Characterization of elastic properties of near-surface and subsurface deepwater hydrate-bearing sediments

Geophysics ◽  
2013 ◽  
Vol 78 (3) ◽  
pp. D169-D179 ◽  
Author(s):  
Zijian Zhang ◽  
De-hua Han ◽  
Daniel R. McConnell
Keyword(s):  
Wave Velocity ◽  
Elastic Constants ◽  
Gas Hydrate ◽  
Effective Medium Theory ◽  
Pore Space ◽  
Seismic Interpretation ◽  
P Wave ◽  
Near Surface ◽  
Free Gas ◽  
Hydrate Bearing Sediments

Hydrate-bearing sands and shallow nodular hydrate are potential energy resources and geohazards, and they both need to be better understood and identified. Therefore, it is useful to develop methodologies for modeling and simulating elastic constants of these hydrate-bearing sediments. A gas-hydrate rock-physics model based on the effective medium theory was successfully applied to dry rock, water-saturated rock, and hydrate-bearing rock. The model was used to investigate the seismic interpretation capability of hydrate-bearing sediments in the Gulf of Mexico by computing elastic constants, also known as seismic attributes, in terms of seismic interpretation, including the normal incident reflectivity (NI), Poisson’s ratio (PR), P-wave velocity ([Formula: see text]), S-wave velocity ([Formula: see text]), and density. The study of the model was concerned with the formation of gas hydrate, and, therefore, hydrate-bearing sediments were divided into hydrate-bearing sands, hydrate-bearing sands with free gas in the pore space, and shallow nodular hydrate. Although relations of hydrate saturation versus [Formula: see text] and [Formula: see text] are different between structures I and II gas hydrates, highly concentrated hydrate-bearing sands may be interpreted on poststack seismic amplitude sections because of the high NI present. The computations of elastic constant implied that hydrate-bearing sands with free gas could be detected with the crossplot of NI and PR from prestack amplitude analysis, and density may be a good hydrate indicator for shallow nodular hydrate, if it can be accurately estimated by seismic methods.

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