Methane hydrate saturations at the Southern Hikurangi margin (New Zealand) estimated from seismic and rock physics inversion

2020 ◽  
Author(s):  
Francesco Turco ◽  
Andrew Gorman ◽  
Gareth Crutchley ◽  
Leonardo Azevedo ◽  
Dario Grana ◽  
...  
Keyword(s):  
New Zealand ◽  
Gas Hydrate ◽  
Methane Hydrate ◽  
Waveform Inversion ◽  
Hydrate Formation ◽  
Rock Physics ◽  
Stability Zone ◽  
Hydrate Saturation ◽  
Physics Model ◽  
Hydrate Stability

<p>Geophysical data indicate that the Hikurangi subduction margin on New Zealand’s East Coast contains a large gas hydrate province. Gas hydrates are widespread in shallow sediments across the margin, and locally intense fluid seepage associated with methane hydrate is observed in several areas. Glendhu and Honeycomb ridges lie at the toe of the Hikurangi deformation wedge at depths ranging from 2100 to 2800 m. These two parallel four-way closure systems host concentrated methane hydrate deposits. The control on hydrate formation at these ridges is governed by steeply dipping permeable strata and fractures, which allow methane to flow upwards into the gas hydrate stability zone. Hydrate recycling at the base of the hydrate stability zone may contribute to the accumulation of highly concentrated hydrate in porous layers.<br>To improve the characterisation of the hydrate systems at Glendhu and Honeycomb ridges, we estimate hydrate saturation and porosity of the concentrated hydrate deposits. We first estimate elastic properties (density, compressional and shear-wave velocities) of the gas hydrate stability zone through full-waveform inversion and <span>iterative geostatistical seismic amplitude versus angle (AVA) inversion</span>. We then perform a petrophysical inversion based on a rock physics model to predict gas hydrate saturation and porosity of the hydrate bearing sediments along the two ridges.<br>Our results indicate that the high seismic amplitudes correspond to the top interface of highly concentrated hydrate deposit, with peak saturations around 35%. Because of the resolution of the seismic data we assume that the estimated properties are averaged over layers of 10 to 20 meters thickness. These saturation values are in agreement with studies conducted in other areas of concentrated hydrate accumulations in similar geologic settings.</p>

Seismic attribute enhancement of weak and discontinuous gas hydrate bottom-simulating reflectors in the Pegasus Basin, New Zealand

2019 ◽  
Vol 7 (3) ◽  
pp. SG11-SG22 ◽  
Author(s):  
Heather Bedle
Keyword(s):  
New Zealand ◽  
Gas Hydrates ◽  
Gas Hydrate ◽  
Seismic Data ◽  
Rock Physics ◽  
Stability Zone ◽  
Seismic Attribute ◽  
Rock Physics Modeling ◽  
Physics Modeling ◽  
Hydrate Stability

Gas hydrates in the oceanic subsurface are often difficult to image with reflection seismic data, particularly when the strata run parallel to the seafloor and in regions that lack the presence of a bottom-simulating reflector (BSR). To address and understand these imaging complications, rock-physics modeling and seismic attribute analysis are performed on modern 2D lines in the Pegasus Basin in New Zealand, where the BSR is not continuously imaged. Based on rock-physics and seismic analyses, several seismic attribute methods identify weak BSR reflections, with the far-angle stack data being particularly effective. Rock modeling results demonstrate that far-offset seismic data are critical in improving the imaging and interpretation of the base of the gas hydrate stability zone. The rock-physics modeling results are applied to the Pegasus 2009 2D data set that reveals a very weak seismic reflection at the base of the hydrates in the far-angle stack. This often-discontinuous reflection is significantly weaker in amplitude than typical BSRs associated with hydrates. These weak far-angle stack BSRs often do not appear clearly in full stack data, the most commonly interpreted seismic data type. Additional amplitude variation with angle (AVA) attribute analyses provide insight into identifying the presence of gas hydrates in regions lacking a strong BSR. Although dozens of seismic attributes were investigated for their ability to reveal weak reflections at the base of the gas hydrate stability zone, those that enhance class 2 AVA anomalies were most effective, particularly the seismic fluid factor attribute.

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 Erosion of the seafloor at the top of the gas hydrate stability zone on the Hikurangi Margin, New Zealand

2005 ◽  
Vol 32 (24) ◽  
Author(s):  
I. A. Pecher ◽  
S. A. Henrys ◽  
S. Ellis ◽  
S. M. Chiswell ◽  
N. Kukowski
Keyword(s):  
New Zealand ◽  
Gas Hydrate ◽  
Stability Zone ◽  
Hydrate Stability

scholarly journals Rock Physics Model and Seismic Dispersion and Attenuation in Gas Hydrate-Bearing Sediments

2021 ◽  
Vol 9 ◽  
Author(s):  
Zhiqi Guo ◽  
Xueying Wang ◽  
Jian Jiao ◽  
Haifeng Chen
Keyword(s):  
Gas Hydrate ◽  
Rock Physics ◽  
P Wave ◽  
Pore Filling ◽  
P Wave Velocity ◽  
Hydrate Saturation ◽  
Physics Model ◽  
Rock Physics Model ◽  
Hydrate Bearing Sediments ◽  
Dispersion And Attenuation

A rock physics model was established to calculate the P-wave velocity dispersion and attenuation caused by the squirt flow of fluids in gas hydrate-bearing sediments. The critical hydrate saturation parameter was introduced to describe different ways of hydrate concentration, including the mode of pore filling and the co-existence mode of pore filling and particle cementation. Rock physical modeling results indicate that the P-wave velocity is insensitive to the increase in gas hydrate saturation for the mode of pore filling, while it increases rapidly with increasing gas hydrate saturation for the co-existence mode of pore filling and particle cementation. Meanwhile, seismic modeling results show that both the PP and mode-converted PS reflections are insensitive to the gas hydrate saturation that is lower than the critical value, while they tend to change obviously for the hydrate saturation that is higher than the critical value. These can be interpreted that only when gas hydrate begins to be part of solid matrix at high gas hydrate saturation, it represents observable impact on elastic properties of the gas hydrate-bearing sediments. Synthetic seismograms are calculated for a 2D heterogeneous model where the gas hydrate saturation varies vertically and layer thickness of the gas hydrate-bearing sediment varies laterally. Modeling results show that larger thickness of the gas hydrate-bearing layer generally corresponds to stronger reflection amplitudes from the bottom simulating reflector.

scholarly journals AVO Analysis of a Weak BSR on the Hikurangi Margin, New Zealand

2021 ◽  
Author(s):  
◽  
Srinivasan Navalpakam Roopa
Keyword(s):  
New Zealand ◽  
Reflection Coefficient ◽  
Gas Hydrate ◽  
Rock Physics ◽  
High Amplitude ◽  
Low Permeability ◽  
High Permeability ◽  
Reservoir Rocks ◽  
Avo Analysis ◽  
Hydrate Stability

<p><b>Gas hydrates occur in deep, cold areas on the Hikurangi margin, New Zealand, generally at water depths of ≥ 600m and ≤ 8oC temperature. In these areas elevated hydrostatic pressures and low temperatures create stable conditions for hydrate formation. The occurrence of Bottom-Simulating Reflections (BSRs) is known to indicate the Base of the Gas Hydrate Stability (BGHS) zone, below which solid hydrates cannot exist due to increasing temperatures of sediments. BSRs in most settings worldwide are thought to be largely caused by free gas at the base of the gas hydrate stability zone. They are characterized by a large negative reflection coefficient due to significant decrease in P-wave velocity attributed to the presence of gas below the BSR. On the Hikurangi margin however, many BSRs appear relatively weak. This study presents the results of Amplitude Variation with Offset (AVO) analysis of a weak BSR beneath Puke Ridge, a thrust ridge on the accretionary wedge east of Gisborne, North Island. Rock-physics modelling is used to interpret the findings.</b></p> <p>The 05CM04 seismic line has been processed by preserving the amplitude and care has been taken to not bias the variation of reflectivity coefficient with offset. The zero-offset reflection coefficient or AVO intercept (A) is in the range of -0.008 to - 0.015 and the AVO gradient (B) is between -0.015 and -0.03.</p> <p>Rock-physics modelling was employed to determine the possible concentrations of gas and hydrate that can yield the observed reflection coefficients. Negligible hydrate saturation above with a patchy gas distribution of 3% saturation beneath the BSR might explain this pattern. An alternative end-member estimation of 13% saturation of hydrate in a frame-supporting model with no gas beneath it could generate the observed reflection coefficient but it is geologically unlikely. Synthetic modelling reveals that the low reflectivity of the BSR could also be due to the presence of thin layers of more concentrated or evenly distributed gas but this scenario is considered to be geologically unlikely.</p> <p>BSRs beneath some thrust ridges in the southern Hikurangi margin, appear as a series of clearly separated bright spots, which indicate free gas accumulations which when connected mimic the geometry of the seafloor. The most likely lithologic explanation for these high amplitude patches within weak BSRs, is the concept of segmented BSRs which is also seen in the Gulf of Mexico. The bright ―gas‖ anomalies are inferred to correlate with sand-rich high permeability layers while the weak BSR could be due to low saturations of gas in clay-rich low permeability layers. The weak BSR beneath the Puke Ridge is indicative of low and patchy gas saturations in low-permeability reservoir rocks while high amplitude patches found in this area may indicate high-permeability sands that may be attractive reservoir rocks for future gas hydrate production.</p>

Hydrate quantification: Integrating full-waveform inversion, seismic attributes, and rock physics

2016 ◽  
Vol 4 (1) ◽  
pp. SA55-SA71 ◽  
Author(s):  
P. Jaiswal
Keyword(s):  
Velocity Field ◽  
Waveform Inversion ◽  
Rock Physics ◽  
Well Logs ◽  
Full Waveform Inversion ◽  
Secondary Porosity ◽  
Full Waveform ◽  
Hydrate Saturation ◽  
Physics Model ◽  
Rock Physics Model

Hydrate quantification from seismic data is a two-pronged challenge. The first is creating a velocity field with high enough resolution and accuracy such that it is a meaningful representation of hydrate variability in the host sediments. The second is constructing a rock-physics model that accounts for the appropriate growth of the hydrate and allows for the interpretation of the velocity field in terms of hydrate saturation. In this paper, both challenges are addressed in a quantification workflow that uses 2D seismic and colocated well logs. The study area is situated in the Krishna-Godavari Basin, offshore eastern Indian coast, where hydrate was discovered in the National Gas Hydrate Program Expedition 01 (NGHP-01). The workflow hinges on a rock-physics model that expresses total hydrate saturation in terms of primary (matrix) and secondary (fractures, faults, voids, etc.) porosities and their respective primary and secondary saturations and incorporates hydrate-filled secondary porosity into the rock as an additional grain type using the Hashin-Shtrikman bounds. The model is first applied to a set of well logs at a colocated site, NGHP-01-10, following which the application is extended into the seismic domain by (1) the incoherency attribute as a proxy for secondary porosity and (2) a full-waveform inversion-based P-wave velocity ([Formula: see text]) model as a proxy for primary saturation. The remaining — the primary porosity and secondary saturation — are assumed to remain the same across the seismic profile as at the site NGHP-01-10. The resulting, seismically estimated, hydrate saturation compares well with saturations from core depressurization at colocated sites NGHP-01-10 and NGHP-01-13. The quantification workflow presented here is potentially adaptable to other geographical areas with the caveat that empirical relations between porosity, saturation, and seismic attributes may have to be locally established.

scholarly journals Adaptations ofAtribacteriato life in methane hydrates: hot traits for cold life

2019 ◽  
Author(s):  
Jennifer B. Glass ◽  
Piyush Ranjan ◽  
Cecilia B. Kretz ◽  
Brook L. Nunn ◽  
Abigail M. Johnson ◽  
...  
Keyword(s):  
Carboxylic Acids ◽  
Gas Hydrate ◽  
Methane Hydrate ◽  
Inositol Phosphate ◽  
Hyperthermophilic Archaea ◽  
Rrna Gene ◽  
Stability Zone ◽  
Cold Temperatures ◽  
Hydrate Bearing Sediments ◽  
Hydrate Stability

SummaryGas hydrates harbor gigatons of natural gas, yet their microbiomes remain mysterious. We bioprospected methane hydrate-bearing sediments from under Hydrate Ridge (offshore Oregon, USA, ODP Site 1244) using 16S rRNA gene amplicon, metagenomic, and metaproteomic analysis.Atribacteria(JS-1 Genus 1) sequences rose in abundance with increasing sediment depth. We characterized the most complete JS-1 Genus 1 metagenome-assembled genomic bin (B2) from the deepest sample, 69 meters below the seafloor (E10-H5), within the gas hydrate stability zone. B2 harbors functions not previously reported forAtribacteria, including a primitive respiratory complex and myriad capabilities to survive extreme conditions (e.g. high salt brines, high pressure, and cold temperatures). SeveralAtribacteriatraits, such as a hydrogenase-Na+/H+antiporter supercomplex (Hun) and di-myo-inositol-phosphate (DIP) synthesis, were similar to those from hyperthermophilic archaea. ExpressedAtribacteriaproteins were involved in transport of branched chain amino acids and carboxylic acids. Transporter genes were downstream from a novel helix-turn-helix transcriptional regulator, AtiR, which was not present inAtribacteriafrom other sites. Overall,Atribacteriaappear to be endowed with unique strategies that may contribute to its dominance in methane-hydrate bearing sediments. Active microbial transport of amino and carboxylic acids in the gas hydrate stability zone may influence gas hydrate stability.Originality-Significance StatementThis work provides insights into the metabolism and adaptations of elusiveAtribacteria(JS-1 clade) that are ubiquitous and abundant in methane-rich ecosystems. We show that JS-1 (Genus 1) from methane hydrate stability zones contain metabolisms and stress survival strategies similar to hyperthermophilic archaea.

Sign in / Sign up

Export Citation Format

Share Document