Combined seismic inversion and gravity modeling of a shallow anomaly in the southern Barents Sea

Geophysics ◽  
2003 ◽  
Vol 68 (4) ◽  
pp. 1140-1149 ◽  
Author(s):  
Bjørn Ursin ◽  
Christian Bauer ◽  
Huasheng Zhao ◽  
Christine Fichler
Keyword(s):  
Gas Hydrate ◽  
Barents Sea ◽  
Rock Physics ◽  
Seismic Inversion ◽  
Gravity Modeling ◽  
Generation Process ◽  
Unknown Parameters ◽  
Water Contact ◽  
Free Gas ◽  
Seismic Anomaly

A combination of prestack elastic inversion with gravity modeling has been applied to a shallow seismic anomaly in the southern Barents Sea. The anomaly is crosscutting dipping layers of late Paleocene age. Earlier seismic interpretations indicate a possible origin in gas‐hydrated sediments trapping underlying free gas. Our interpretation includes seismic inversion of a seismic model which consists of a stack of isotropic, homogeneous, and anelastic layers. The unknown parameters are the P‐ and S‐wave velocities, density, and thickness for each layer. As densities are poorly determined by this method, we included gravity modeling, which improved the density estimates. The resulting parameters have been integrated by rock physics calculations and the knowledge of typical attributes of gas hydrate occurrences. The top of the seismic anomaly is interpreted as a reflection from the base of a gas‐hydrated sediment trapping underlying free gas. The base of the anomaly is the gas‐water contact, which deviates from a flat spot due to capillary pressure differences related to lithology and the generation process of gas hydrate. The estimated density has been interpreted together with well results and indicates a significant amount of gas filling the pore volume.

Quantitative interpretation for gas hydrate accumulation in the eastern Green Canyon Area, Gulf of Mexico using seismic inversion and rock physics transform

Geophysics ◽  
2011 ◽  
Vol 76 (4) ◽  
pp. B139-B150 ◽  
Author(s):  
Zijian Zhang ◽  
De-hua Han ◽  
Qiuliang Yao
Keyword(s):  
Gulf Of Mexico ◽  
Gas Hydrate ◽  
Pore Space ◽  
Rock Physics ◽  
Seismic Inversion ◽  
Bright Spot ◽  
Quantitative Interpretation ◽  
Free Gas ◽  
Qualitative And Quantitative ◽  
Amplitude Data

Gas hydrate can be interpreted from seismic data through observation of bottom simulating reflector (BSR). It is a challenge to interpret gas hydrate without BSR. Three-dimensional qualitative and quantitative seismic interpretations were used to characterize gas hydrate distribution and concentration in the eastern Green Canyon area of the Gulf of Mexico, where BSR is absent. The combination of qualitative and quantitative interpretation reduces ambiguities in the estimation and identification of gas hydrate. Sandy deposition and faults are qualitatively interpreted from amplitude data. The 3D acoustic impedance volume was interpreted in terms of high P-impedance hydrate zones and low P-impedance free gas zones. Gas hydrate saturation derived from P-impedance is estimated by a rock physics transform. We interpreted gas hydrate in the sand-prone sediments with a maximum saturation of approximately 50% of the pore space. Sheet-like and some bright spot gas hydrate accumulations are interpreted. The interpretation of sheet-like gas hydrate within sand-prone sediments around faults suggests that fluid moves into the sand zones laterally by conduits. Variations in depths of interpreted gas hydrate zones imply nonequilibrium conditions. Low P-impedance free gas zones within high P-impedance gas hydrate zones imply possible coexistence of hydrate and free gas within the hydrate stability zone. We propose that gas hydrate distribution and concentration are associated with structures, buried sedimentary bodies, sources of gas, and fluid flux.

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.

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 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.

De-risking Appraisal Phase in the Recent Gas Discovery in Banggai-Sula Basin, Central Sulawesi: Application of AVO Analysis and Pre-stack Inversion in the Sub-ophiolite Reef Carbonate Play

2021 ◽  
Author(s):  
M. Ahmad
Keyword(s):  
Reservoir Characterization ◽  
Linear Trend ◽  
Rock Physics ◽  
Seismic Inversion ◽  
Added Value ◽  
Quality Data ◽  
Water Contact ◽  
Avo Analysis ◽  
Amplitude Versus Offset ◽  
Avo Attributes

Following the success in the exploration drilling campaign in the last few years, Pertamina EP puts the recently discovered Wol Structure into the appraisal stage. The exploration wells Wol-001 and Wol-002 were spudded in 2017 and 2019 respectively, and both flowed a significant gas rate from an excellent reservoir of Miocene Reef of Minahaki Formation. A good understanding of the reservoir distribution was essential in such a stage. Therefore, a proper reservoir characterization was then carried out for further appraisal purposes. Using the improved quality data from the latest 5D interpolation-PSDM as input, integration of amplitude versus offset (AVO) techniques and rock physics analysis was conducted to investigate the hydrocarbon extent. The AVO class IIp was observed at the boundary between overlying Kintom Shale and gas saturated Minahaki limestone. It is indicated by a positive intercept (Ro), decreased amplitudes with offsets, and negative amplitudes in the far offsets. This polarity reversal characteristic is clearly seen from both AVO modeling and actual CDP in the well locations. Several CDPs inside and outside the closure were also examined to check the consistency. The slice of partial stack volumes has also exhibited a similar trend within the closure where class IIp is suggestive. Since the AVO attributes such as intercept and gradient solely were not able to visualize the reservoir extent properly, the pre-stack seismic inversion was performed to obtain a more accurate reservoir distribution through quantitative interpretation. A cross plot of P-impedance (Ip) over S-impedance (Is) differentiates the gas zone clearly from the wet linear trend. A depth slice at GWC (gas water contact) level describes that most of the Wol Structure is gas-saturated including the newly identified closure in the northwest. It is a three-way dip closure formed by limestone that was dragged upward by a thrust fault. Interestingly, it has a similar AVO response to the main Wol Structure which suggests a gas-bearing reservoir. This work brings an added value to the use of AVO analysis and pre-stack inversion for hydrocarbon mapping for appraisal purposes. Not only it has largely reduced the subsurface uncertainty, but also revealed an upside potential that is worth considering in future exploration.

AVO inversion of BSRs in marine gas hydrate studies

Geophysics ◽  
2007 ◽  
Vol 72 (2) ◽  
pp. C31-C43 ◽  
Author(s):  
Marc-André P. Chen ◽  
Michael Riedel ◽  
Roy D. Hyndman ◽  
Stan E. Dosso
Keyword(s):  
Gas Hydrate ◽  
Probability Distributions ◽  
Rock Physics ◽  
Inversion Method ◽  
Physical Parameters ◽  
Sufficient Information ◽  
Variable Degree ◽  
S Wave ◽  
Free Gas ◽  
Marine Gas Hydrate

We examine the usefulness of amplitude versus offset (AVO) analysis of bottom-simulating reflections (BSRs) for estimating associated marine gas hydrate and free-gas concentrations. A nonlinear Bayesian inversion is applied to estimate marginal probability distributions (MPDs) of physical parameters at a BSR interface, which are related to overlying gas hydrate and underlying free-gas concentrations via rock physics modeling. The problem is constrained further by prior information and re-parameterization of inversion results. Inversion of BSR AVO data from offshore Vancouver Island, Canada, shows that gas hydrate and free-gas concentrations are, respectively, 0%–23% and0%–2% of the pore volume, at a 90% credibility level. This result indicates that the data do not provide sufficient information to independently resolve gas hydrate and free-gas concentrations to useful accuracy. The study is directed primarily at AVO for gas-hydrate-related BSRs, but may have important applicability in testing the degree of constraint of formation characteristics in other AVO studies. The inversion method is applied also to syn-thetic AVO data generated from Ostrander’s gas-sand model. In this case, MPDs sufficiently constrain the relationship between P- and S-wave velocities in the sandstone unit to determine if it is gas-charged. The variable degree of model constraint obtained in this AVO study highlights the need to include rigorous quantita-tive uncertainty analysis in all AVO studies.

Sign in / Sign up

Export Citation Format

Share Document