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 Assessment of gas-hydrate saturations in the Makran accretionary prism using the offset dependence of seismic amplitudes

Geophysics ◽  
2010 ◽  
Vol 75 (2) ◽  
pp. C1-C6 ◽  
Author(s):  
Maheswar Ojha ◽  
Kalachand Sain ◽  
Timothy A. Minshull
Keyword(s):  
Gas Hydrate ◽  
Seismic Velocity ◽  
Rock Physics ◽  
Accretionary Prism ◽  
Seismic Line ◽  
Amplitude Variation With Offset ◽  
Free Gas ◽  
Bottom Simulating Reflector ◽  
Physics Modeling ◽  
Physics Model

We estimate the saturations of gas hydrate and free gas based on measurements of seismic-reflection amplitude variation with offset (AVO) for a bottom-simulating reflector coupled with rock-physics modeling. When we apply the approach to data from a seismic line in the Makran accretionary prism in the Arabian Sea, the results reveal lateral variations of gas-hydrate and free-gas saturations of 4–29% and 1–7.5%, respectively, depending on the rock-physics model used to relate seismic velocity to saturation. Our approach is simple and easy to implement.

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.

Estimation of gas hydrate and free gas saturation, concentration, and distribution from seismic data

Geophysics ◽  
2002 ◽  
Vol 67 (2) ◽  
pp. 582-593 ◽  
Author(s):  
Shaoming Lu ◽  
George A. McMechan
Keyword(s):  
Gas Hydrate ◽  
Seismic Data ◽  
Acoustic Impedance ◽  
Single Channel ◽  
Pore Space ◽  
Theoretical Models ◽  
Free Gas ◽  
Bottom Simulating Reflector ◽  
Saturation Concentration ◽  
Empirical Relations

Gas hydrates contain a major untapped source of energy and are of potential economic importance. The theoretical models to estimate gas hydrate saturation from seismic data predict significantly different acoustic/elastic properties of sediments containing gas hydrate; we do not know which to use. Thus, we develop a new approach based on empirical relations. The water‐filled porosity is calibrated (using well‐log data) to acoustic impedance twice: one calibration where gas hydrate is present and the other where free gas is present. The water‐filled porosity is used in a combination of Archie equations (with corresponding parameters for either gas hydrate or free gas) to estimate gas hydrate or free gas saturations. The method is applied to single‐channel seismic data and well logs from Ocean Drilling Program leg 164 from the Blake Ridge area off the east coast of North America. The gas hydrate above the bottom simulating reflector (BSR) is estimated to occupy ∼3–8% of the pore space (∼2–6% by volume). Free gas is interpreted to be present in three main layers beneath the BSR, with average gas saturations of 11–14%, 7–11%, and 1–5% of the pore space (6–8%, 4–6%, and 1–3% by volume), respectively. The estimated saturations of gas hydrate are very similar to those estimated from vertical seismic profile data and generally agree with those from independent, indirect estimates obtained from resistivity and chloride measurements. The estimated free gas saturations agree with measurements from a pressure core sampler. These results suggest that locally derived empirical relations between porosity and acoustic impedance can provide cost‐effective estimates of the saturation, concentration, and distribution of gas hydrate and free gas away from control wells.

Bottom‐simulating reflectors: Seismic velocities and AVO effects

Geophysics ◽  
2000 ◽  
Vol 65 (1) ◽  
pp. 54-67 ◽  
Author(s):  
José M. Carcione ◽  
Umberta Tinivella
Keyword(s):  
Gas Hydrate ◽  
Porous Matrix ◽  
Seismic Velocities ◽  
Two Phase ◽  
Average Pore ◽  
Average Pore Radius ◽  
Phase Theory ◽  
Free Gas ◽  
Bottom Simulating Reflector ◽  
Offset Curves

We obtain the wave velocities of ice‐ and gas hydrate‐bearing sediments as a function of concentration and temperature. Unlike previous theories based on simple slowness and/or moduli averaging or two‐phase models, we use a Biot‐type three‐phase theory that considers the existence of two solids (grain and ice or clathrate) and a liquid (water), and a porous matrix containing gas and water. For consolidated Berea sandstone, the theory underestimates the value of the compressional velocity below 0°C. Including grain‐ice interactions and grain cementation yields a good fit to the experimental data. Strictly speaking, water proportion and temperature are closely related. Fitting the wave velocity at a given temperature allows the prediction of the velocity throughout the range of temperatures, provided that the average pore radius and its standard deviation are known. The reflection coefficients are computed with a viscoelastic single‐phase constitutive model. The analysis is carried out for the top and bottom of a free‐gas zone beneath a gas hydrate‐bearing sediment and overlying a sediment fully saturated with water. Assuming that the bottom‐simulating reflector is caused solely by an interface separating cemented gas hydrate‐ and free gas‐bearing sediments, we conclude that (1) for a given gas saturation, it is difficult to evaluate the amount of gas hydrate at low concentrations. However, low and high concentrations of hydrate can be distinguished, since they give positive and negative anomalies, respectively. (2) Saturation of free gas can be determined from the reflection amplitude, but not from the type of anomaly. (3) The P to S reflection coefficient is a good indicator of high amounts of free gas and gas hydrate. On the other hand, the amplitude‐variation‐with‐offset curves are always positive for uncemented sediments.

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.

scholarly journals Evaluation Method of the Gas Hydrate and Free Gas System and Its Application in the Shenhu Area, South China Sea

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-16
Author(s):  
Pibo Su ◽  
Tingwei Li ◽  
Shurong Liang ◽  
Jinqiang Liang ◽  
Xiaoxue Wang ◽  
...  
Keyword(s):  
Gas Hydrate ◽  
Seismic Reflection ◽  
Effective Porosity ◽  
The Body ◽  
Effective Thickness ◽  
Space Distribution ◽  
Depth Range ◽  
Free Gas ◽  
Gas System ◽  
Reflection Characteristics

As a new alternative energy source, gas hydrate has attracted wide attention all over the world. Since gas hydrate is always associated with free gas, the evaluation of the gas hydrate and free gas system is an important aspect of hydrate reservoir exploration and development. In this study, based on identifying gas hydrate and free gas by well logging, the seismic reflection characteristics of gas hydrate and free gas are determined by an accurate well-to-seismic calibration method. On account of seismic reflection characteristics, AVO attributes are used to identify gas hydrate and free gas qualitatively. Using prestack and poststack inversion to get the ratio of P -wave impedance and P -wave-to- S -wave velocities, we determine the three-dimensional space distribution of gas hydrate and free gas, predict their effective porosity and saturation, and eventually achieve the meticulous depiction of gas hydrate and free gas in the body, which is necessary in subsequent estimation of gas hydrate and free gas resources. Results show that according to logging interpretation, gas hydrate of the B-well is located in the depth range of 1460–1510 mbsl and free gas is in 1510–1542 mbsl. Moreover, gas hydrate of the A-well is located in the depth range of 1425–1512 mbsl, and no obvious free gas is identified. Gas hydrate is located above free gas and distributed continuously. In plane form, gas hydrate and free gas both present subelliptical distribution in the NW-SE direction. Gas hydrate has an effective porosity of 0.30–0.40, an average saturation of 0.33–0.40, and an effective thickness of 3.0–10.5 m, whereas free gas possesses an effective porosity of 0.35–0.40, a saturation of 0.24–0.32, and an effective thickness of 2.0–5.0 m.

scholarly journals The differences between the measured heat flow and BSR heat flow in the Shenhu gas hydrate drilling area, northern South China Sea

2018 ◽  
Vol 37 (2) ◽  
pp. 756-769 ◽  
Author(s):  
Miao Dong ◽  
Jian Zhang ◽  
Xing Xu ◽  
Shi-Guo Wu
Keyword(s):  
Heat Flow ◽  
Gas Hydrate ◽  
Northern South China Sea ◽  
Geothermal Gradient ◽  
Topographic Effects ◽  
Fluid Flux ◽  
Flow Measurements ◽  
Upward Migration ◽  
Bottom Simulating Reflector ◽  
The Impact

Temperature is an important factor that affects the stability of a gas hydrate. To investigate the geothermal characteristics in the gas hydrate drilling area, heat flow measurements were performed in the surrounding area of the SH2 well. The measured heat flow was compared with the bottom simulating reflector heat flow, which was calculated by using the depth of the bottom simulating reflector in the seismic data. Combined with the geological background of the Shenhu drilling area, we analyzed the reasons for the differences between the measured heat flow and the bottom simulating reflector heat flow. In addition to analyzing the differences caused by the calculation parameters, we calculated the 3-D topographic effects on the measured heat flow by using the finite element numerical simulation method. The results show that the measured heat flow was seriously affected by the topography and produced a −50–30% error in the study area. After terrain correction of the measured heat flow, we found that the data were greater than the bottom simulating reflector heat flow at almost all sites. Therefore, we considered the impact of fluid activity and calculated the relationship among the thickness of the gas hydrate stability zone, the fluid flux and the heat flow. The results show that when the base of the bottom simulating reflector was at a certain depth, the geothermal gradient increased with the increasing upward migration of the fluid flux. Therefore, when upward fluid migration is present, the measured heat flow in the seafloor sediments is greater than the heat flow in the deep layers. In general, we showed that the influences of the topography and fluid activity are the main factors leading to the inconsistency between the bottom simulating reflector heat flow and the measured heat flow in the Shenhu gas hydrate drilling area.

Gas hydrate, fluid flow and free gas: Formation of the bottom-simulating reflector

2007 ◽  
Vol 261 (3-4) ◽  
pp. 407-420 ◽  
Author(s):  
R. Ross Haacke ◽  
Graham K. Westbrook ◽  
Roy D. Hyndman
Keyword(s):  
Fluid Flow ◽  
Gas Hydrate ◽  
Free Gas ◽  
Bottom Simulating Reflector ◽  
Gas Formation
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