Determination of the lower boundaries of the natural gas hydrates stability zone in the subpermafrost horizons of the Yakut arch of the Vilyui syneclise, saturated with bicarbonate-sodium type waters

This paper considers the possibility of the underground gas storage facilities creating in a hydrate state on the north-western slope of the Yakut arch of the Vilyui syneclise. For this, the boundaries of the hydrate stability zone were determined for 6 promising areas of the considered geological structure. Equilibrium conditions of the natural gas hydrates formation in the model porous media containing bicarbonate-sodium type water (mineralization 20 g/l), characteristic for the subpermafrost horizons of the Yakut arch, have been studied by the method of differential thermal analysis. On the basis of the obtained results, the boundaries of the natural gas hydrates stability zone were determined. It was shown that the upper boundaries of the hydrate stability zone are located in the thickness of permafrost rocks. It was found that the lower boundaries of the natural gas hydrates stability zone in moist unsalted porous medium lie in the range from 930 to 1120 m. When the samples are saturated with mineralized water, the boundaries are located 80-360 m higher. The obtained experimental results allow us to conclude that in subpermafrost aquifers of the Yakut arch has favorable conditions for the formation of natural gas hydrates. Keywords: natural gas hydrates; aquifers; underground gas storage; hydrate stability zone; geothermal gradient; equilibrium conditions of the hydrate formation; bicarbonate-sodium type water.

Dating submarine landslides using the transient response of gas hydrate stability

Submarine landslides are prevalent on the modern-day seafloor, yet an elusive problem is constraining the timing of slope failure. Herein, we present a novel technique for constraining the age of submarine landslides without sediment core dating. Underneath a submarine landslide in the Orca Basin, Gulf of Mexico, in 3D seismic data we map an irregular bottom simulating reflection (BSR), which mimics the geometry of the pre-slide seafloor rather than the modern bathymetry. Based on the observed BSR, we suggest that the gas hydrate stability zone (GHSZ) is currently adjusting to the post-slide sediment temperature perturbations. We apply transient conductive heat flow modeling to constrain the response of the GHSZ to the slope failure, which yields a most likely age of ~8 ka demonstrating that gas hydrate systems can respond to slope failures even on the millennia timescales. We also provide an analytical approach to rapidly determine the age of submarine slides at any location.

Gas hydrate stability zone in Muri coalfield, Qinghai Province, China

ABSTRACT The gas hydrate stability zone (GHSZ) is the essential condition for gas hydrate accumulation, which is controlled by three main factors: gas component, geothermal gradient and permafrost thickness. Based on the gas component of hydrate samples from drilling in Muri coalfield, the gas hydrate phase equilibrium curve was calculated using Sloan's natural gas hydrate phase equilibrium procedure (CSMHYD) program. Through temperature data processing of coalfield boreholes, some important data such as thickness of permafrost and geothermal gradient were obtained. The GHSZ parameters of a single borehole were calculated by programming based on the above basic data. The average thickness of GHSZ of 85 boreholes in Muri coalfield amounted to approximately 1000 m, indicating very broad space for gas hydrate occurrence. The isogram of GHSZ bottom depth drawn from single borehole data in Muri coalfield demonstrated the regional distribution characteristics of GHSZ, and identified three favourable areas of gas hydrate occurrence where the bottom of GHSZ had a burial depth >1500 m – namely, the southern part of Juhugeng Mining Area, the middle part of Duosuogongma Mining Area and the eastern part of Xuehuoli Mining Area.

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

<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>

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

<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>

A Geophysical Review of the Seabed Methane Seepage Features and Their Relationship with Gas Hydrate Systems

Seabed methane seepage has gained attention from all over the world in recent years as an important source of greenhouse gas emission, and gas hydrates are also regarded as a key factor affecting climate change or even global warming due to their shallow burial and poor stability. However, the relationship between seabed methane seepage and gas hydrate systems is not clear although they often coexist in continental margins. It is of significance to clarify their relationship and better understand the contribution of gas hydrate systems or the deeper hydrocarbon reservoirs for methane flux leaking to the seawater or even the atmosphere by natural seepages at the seabed. In this paper, a geophysical examination of the global seabed methane seepage events has been conducted, and nearby gas hydrate stability zone and relevant fluid migration pathways have been interpreted or modelled using seismic data, multibeam data, or underwater photos. Results show that seabed methane seepage sites are often manifested as methane flares, pockmarks, deep-water corals, authigenic carbonates, and gas hydrate pingoes at the seabed, most of which are closely related to vertical fluid migration structures like faults, gas chimneys, mud volcanoes, and unconformity surfaces or are located in the landward limit of gas hydrate stability zone (LLGHSZ) where hydrate dissociation may have released a great volume of methane. Based on a comprehensive analysis of these features, three major types of seabed methane seepage are classified according to their spatial relationship with the location of LLGHSZ, deeper than the LLGHSZ (A), around the LLGHSZ (B), and shallower than LLGHSZ (C). These three seabed methane seepage types can be further divided into five subtypes considering whether the gas source of seabed methane seepage is from the gas hydrate systems or not. We propose subtype B2 represents the most important seabed methane seepage type due to the high density of seepage sites and large volume of released methane from massive focused vigorous methane seepage sites around the LLGHSZ. Based on the classification result of this research, more measures should be taken for subtype B2 seabed methane seepage to predict or even prevent ocean warming or climate change.

Forecast of Distribution and Thickness of Gas Hydrate Stability Zone at the Bottom of the Caspian Sea

The Caspian Sea is a region of active hydrocarbon production, where apart from conventional accumulations, gas hydrates (GH) are known to exist. GH are a potential future source of energy, however, currently they pose danger for development of conventional fields. The goal of this research was to determine the area of GH distribution and thickness of their stability zone in the Caspian Sea using numerical modeling and to define how certain parameters affect the calculated thickness. As a result of the research, cartographic schemes were created for the South and Middle Caspian, where GH were predicted. For the South Caspian, conditions for methane hydrates formation exist at depths of more than 419–454 m, and for the Middle Caspian, more than 416–453 m. The maximal thicknesses of methane hydrates stability zones for the South Caspian can reach 900–956 m, and for the Middle Caspian, 226–676 m. Variations of parameters of seafloor depth, geothermal gradient and gas composition can significantly change the resulting thickness of GH stability zone.