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.

Estimating the amount of gas hydrate and free gas from marine seismic data

Geophysics ◽  
2000 ◽  
Vol 65 (2) ◽  
pp. 565-573 ◽  
Author(s):  
Christine Ecker ◽  
Jack Dvorkin ◽  
Amos M. Nur
Keyword(s):  
Gas Hydrate ◽  
Seismic Data ◽  
Rock Physics ◽  
Free Gas ◽  
Interval Velocity ◽  
Blake Ridge ◽  
Hydrate Saturation ◽  
Physics Model ◽  
Rock Physics Model ◽  
Marine Seismic

Marine seismic data and well‐log measurements at the Blake Ridge offshore South Carolina show that prominent seismic bottom‐simulating reflectors (BSRs) are caused by sediment layers with gas hydrate overlying sediments with free gas. We apply a theoretical rock‐physics model to 2-D Blake Ridge marine seismic data to determine gas‐hydrate and free‐gas saturation. High‐porosity marine sediment is modeled as a granular system where the elastic wave velocities are linked to porosity; effective pressure; mineralogy; elastic properties of the pore‐filling material; and water, gas, and gas‐hydrate saturation of the pore space. To apply this model to seismic data, we first obtain interval velocity using stacking velocity analysis. Next, all input parameters to the rock‐physics model, except porosity and water, gas, and gas hydrate saturation, are estimated from geologic information. To estimate porosity and saturation from interval velocity, we first assume that the entire sediment does not contain gas hydrate or free gas. Then we use the rock‐physics model to calculate porosity directly from the interval velocity. Such porosity profiles appear to have anomalies where gas hydrate and free gas are present (as compared to typical profiles expected and obtained in sediment without gas hydrate or gas). Porosity is underestimated in the hydrate region and is overestimated in the free‐gas region. We calculate the porosity residuals by subtracting a typical porosity profile (without gas hydrate and gas) from that with anomalies. Next we use the rock‐physics model to eliminate these anomalies by introducing gas‐hydrate or gas saturation. As a result, we obtain the desired 2-D saturation map. The maximum gas‐hydrate saturation thus obtained is between 13% and 18% of the pore space (depending on the version of the model used). These saturation values are consistent with those measured in the Blake Ridge wells (away from the seismic line), which are about 12%. Free‐gas saturation varies between 1% and 2%. The saturation estimates are extremely sensitive to the input velocity values. Therefore, accurate velocity determination is crucial for correct reservoir characterization.

scholarly journals Seismic characterization of geothermal sedimentary reservoirs: A field example from the Copenhagen area, Denmark

2020 ◽  
Vol 8 (2) ◽  
pp. T275-T291 ◽  
Author(s):  
Kenneth Bredesen ◽  
Esben Dalgaard ◽  
Anders Mathiesen ◽  
Rasmus Rasmussen ◽  
Niels Balling
Keyword(s):  
Reservoir Characterization ◽  
Energy Resource ◽  
Rock Physics ◽  
Seismic Inversion ◽  
Amplitude Variation With Offset ◽  
Seismic Characterization ◽  
Rock Physics Modeling ◽  
Physics Modeling ◽  
Physics Model ◽  
Rock Physics Model

We have seismically characterized a Triassic-Jurassic deep geothermal sandstone reservoir north of Copenhagen, onshore Denmark. A suite of regional geophysical measurements, including prestack seismic data and well logs, was integrated with geologic information to obtain facies and reservoir property predictions in a Bayesian framework. The applied workflow combined a facies-dependent calibrated rock-physics model with a simultaneous amplitude-variation-with-offset seismic inversion. The results suggest that certain sandstone distributions are potential aquifers within the target interval, which appear reasonable based on the geologic properties. However, prediction accuracy suffers from a restricted data foundation and should, therefore, only be considered as an indicator of potential aquifers. Despite these issues, the results demonstrate new possibilities for future seismic reservoir characterization and rock-physics modeling for exploration purposes, derisking, and the exploitation of geothermal energy as a green and sustainable energy resource.

Rock-physics diagnostics of an offshore gas field

Geophysics ◽  
2017 ◽  
Vol 82 (4) ◽  
pp. MR121-MR132 ◽  
Author(s):  
Uri Wollner ◽  
Yunfei Yang ◽  
Jack P. Dvorkin
Keyword(s):  
Rock Physics ◽  
Rock Properties ◽  
Gas Field ◽  
Amplitude Variation With Offset ◽  
Rock Physics Modeling ◽  
Physics Modeling ◽  
Using Data ◽  
Physics Model ◽  
Well Data ◽  
Rock Physics Model

Seismic reflections depend on the contrasts of the elastic properties of the subsurface and their 3D geometry. As a result, interpreting seismic data for petrophysical rock properties requires a theoretical rock-physics model that links the seismic response to a rock’s velocity and density. Such a model is based on controlled experiments in which the petrophysical and elastic rock properties are measured on the same samples, such as in the wellbore. Using data from three wells drilled through a clastic offshore gas reservoir, we establish a theoretical rock-physics model that quantitatively explains these data. The modeling is based on the assumption that only three minerals are present: quartz, clay, and feldspar. To have a single rock-physics transform to quantify the well data in the entire intervals under examination in all three wells, we introduced field-specific elastic moduli for the clay. We then used the model to correct the measured shear-wave velocity because it appeared to be unreasonably low. The resulting model-derived Poisson’s ratio is much smaller than the measured ratio, especially in the reservoir. The associated synthetic amplitude variation with offset response appears to be consistent with the recorded seismic angle stacks. We have shown how rock-physics modeling not only helps us to correct the well data, but also allows us to go beyond the settings represented in the wells and quantify the seismic signatures of rock properties and conditions varying in a wider range using forward seismic modeling.

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.

Identification of morphologies of gas hydrate distribution based on amplitude variation with angle analysis

Geophysics ◽  
2018 ◽  
Vol 83 (3) ◽  
pp. B143-B154 ◽  
Author(s):  
Tao Liu ◽  
Xuewei Liu
Keyword(s):  
Gas Hydrate ◽  
Northern South China Sea ◽  
Rock Physics ◽  
Class Iii ◽  
Pore Filling ◽  
Amplitude Variation ◽  
Bottom Simulating Reflector ◽  
Physics Modeling ◽  
Amplitude Variation With Angle ◽  
Theoretical Results

There are two basic distribution morphologies of gas hydrates in nature: pore filling and fracture filling. Identification of gas hydrate morphologies is essential for improved resource evaluation and exploitation. Improper exploitation may cause seafloor instability, lead to atmospheric venting, and exacerbate the greenhouse effect. To identify gas hydrate morphologies, we combine rock-physics modeling and amplitude variation with angle (AVA) analysis to study the theoretical AVA patterns of the bottom simulating reflector (BSR) beneath the pore-filling and fracture-filling gas hydrate bearing sediments (GHBS), respectively. The theoretical results indicate completely different AVA patterns between these two morphologies. The AVA of the BSR beneath the pore-filling GHBS shows a class III anomaly with negative intercept and gradient. However, the AVA of the BSR beneath the fracture-filling GHBS exhibits a class IV anomaly with a negative intercept and positive gradient. In addition, the theoretical AVA intercept and gradient are affected by the fracture orientation, manifesting an anisotropic signature varying with the fracture dip and azimuth. The processed prestack data at well sites 08 and 16 in the northern South China Sea are selected for testing our method. The AVA analysis of the actual BSRs beneath the pore-filling and fracture-filling GHBS at these sites shows class III and IV responses, respectively, in agreement with our theoretical results. The gas hydrate saturations are also estimated by comparing the theoretical AVA with the actual AVA patterns. The estimations at site 08 are close to those estimated from the pore-water freshening.

Assessment of gas hydrate using pre-stack seismic inversion in Mahanadi Basin, offshore eastern India

2021 ◽  
pp. 1-42
Author(s):  
Maheswar Ojha ◽  
Ranjana Ghosh
Keyword(s):  
Gas Hydrate ◽  
Rock Physics ◽  
Type Equation ◽  
Seismic Inversion ◽  
Log Analysis ◽  
Well Log ◽  
Seismic Section ◽  
Bottom Simulating Reflector ◽  
Hydrate Saturation ◽  
Well Log Analysis

The Indian National Gas Hydrate Program Expedition-01 in 2006 has discovered gas hydrate in Mahanadi offshore basin along the eastern Indian margin. However, well log analysis, pressure core measurements and Infra-Red (IR) anomalies reveal that gas hydrates are distributed as disseminated within the fine-grained sediment, unlike massive gas hydrate deposits in the Krishna-Godavari basin. 2D multi-channel seismic section, which crosses the Holes NGHP-01-9A and 19B located at about 24 km apart shows a continuous bottom-simulating reflector (BSR) along it. We aim to investigate the prospect of gas hydrate accumulation in this area by integrating well log analysis and seismic methods with rock physics modeling. First, we estimate gas hydrate saturation at these two Holes from the observed impedance using the three-phase Biot-type equation (TPBE). Then we establish a linear relationship between gas hydrate saturation and impedance contrast with respect to the water-saturated sediment. Using this established relation and impedance obtained from pre-stack inversion of seismic data, we produce a 2D gas hydrate-distribution image over the entire seismic section. Gas hydrate saturation estimated from resistivity and sonic data at well locations varies within 0-15%, which agrees well with the available pressure core measurements at Hole 19. However, the 2D map of gas hydrate distribution obtained from our method shows maximum gas hydrate saturation is about 40% just above the BSR between the CDP (common depth point) 1450 and 2850. The presence of gas-charged sediments below the BSR is one of the reasons for the strong BSR observed in the seismic section, which is depicted as low impedance in the inverted impedance section. Closed sedimentary structures above the BSR are probably obstructing the movements of free-gas upslope, for which we do not see the presence of gas hydrate throughout the seismic section above the BSR.

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