Spatial and temporal variations in parameters at the upper boundary of gas hydrate stability zone of the Sea of Okhotsk

We model numerically regions of the Canadian continental shelves during successive glacio-eustatic cycles to illustrate past, current and future marine gas hydrate (GH) stability and instability. These models indicated that the marine GH resource has dynamic features and the formation age and resource volumes depend on the dynamics of the ocean-atmosphere system as it responds to both natural (glacial-interglacial) and anthropogenic (climate change) forcing. Our models focus on the interval beginning three million years ago (i.e., Late Pliocene-Holocene). They continue through the current interglacial and they are projected to its anticipated natural end. During the current interglacial the gas hydrate stability zone (GHSZ) thickness in each region responded uniquely as a function of changes in water depth and sea bottom temperature influenced by ocean currents. In general, the GHSZ in the deeper parts of the Pacific and Atlantic margins (≥1316 m) thinned primarily due to increased water bottom temperatures. The GHSZ is highly variable in the shallower settings on the same margins (~400–500 m). On the Pacific Margin shallow GH dissociated completely prior to nine thousand years ago but the effects of subsequent sea level rise reestablished a persistent, thin GHSZ. On the Atlantic Margin Scotian Shelf the warm Gulf Stream caused GHSZ to disappear completely, whereas in shallow water depths offshore Labrador the combination of the cool Labrador Current and sea level rise increased the GHSZ. If future ocean bottom temperatures remain constant, these general characteristics will persist until the current interglacial ends. If the sea bottom warms, possibly in response to global climate change, there could be a significant reduction to complete loss of GH stability, especially on the shallow parts of the continental shelf. The interglacial GH thinning rates constrain rates at which carbon can be transferred between the GH reservoir and the atmosphere-ocean system. Marine GH can destabilize much more quickly than sub-permafrost terrestrial GHs and this combined with the immense marine GH reservoir suggests that GH have the potential to affect the climate-ocean system. Our models show that GH stability reacts quickly to water column pressure effects but slowly to sea bottom temperature changes. Therefore it is likely that marine GH destabilization was rapid and progressive in response to the pressure effects of glacial eustatic sea level fall. This suggests against a catastrophic GH auto-cyclic control on glacial-interglacial climate intervals. It is computationally possible but, unfortunately in no way verifiably, to analyze the interactions and impacts that marine GHs had prior to the current interglacial because of uncertainties in temperature and pressure history constraints. Thus we have the capability, but no confidence that we can contribute currently to questions regarding the relationships among climate, glacio-eustatic sea level fluctuations and marine GH stability without improved local temperature and water column histories. We infer that the possibility for a GH control on climate or oceanic cycles is speculative, but qualitatively contrary to our model results.

Subsurface Temperatures beneath Southern Hydrate Ridge

The offshore Bangladesh includes the northern Bengal fan, where sediment supply from the Ganges and Brahmaputra rivers has resulted in the accumulation of up to 20 km of shallow-marine, fluvio-deltaic and slope sediments that have accumulated during rapid tectonic subsidence since the late Miocene. The high sedimentation rates, along with high organic matter content, make this area favorable for the formation of natural gas from both microbial and thermogenic sources. Here we use multichannel seismic reflection profiles and modelling of the gas hydrate stability zone (GHSZ) to present the first evidence for the occurrence of natural gas hydrate in the offshore Bangladesh. First, we analyze the sediments of the shelf and slope areas, which are characterized by downslope sediment transport features and by the presence, in places, of faults/fractures as well as widely distributed amplitude anomalies and seismic facies that we relate to the presence of gas. A high-amplitude reversed polarity reflection of variable continuity that mimics the seafloor and cross-cut stratigraphy is interpreted as a Bottom Simulating Reflector (BSR). The BSR is observed in several areas that are predominantly located in the E-SE of the study area, in water depths of 1300–1900 m and at depths below seafloor of 250–440 m. Sediments above BSR locations generally show higher seismic interval velocities reaching values of ∼1920–1940 m/s, which are consistent with the presence of gas hydrate in shallow marine sediments. Furthermore, the BSR lies at approximately the same depth as the theoretical base of the gas (methane) hydrate stability zone (BGHSZ), calculated assuming a 3.5 % wt pore water salinity and using existing geothermal gradient and seafloor temperature data from the study region. However, in places, the BSR lies deeper or shallower than the base of the modelled BGHSZ. These discrepancies include areas where faults/fractures and seismic evidence linked to fluid flow from deeper reservoirs reach the GHSZ disrupting its stratigraphic continuity. At these locations, we suggest that faults/fractures act as fluid migration pathways causing localized heat-flow perturbations and/or changes in the hydrate-forming gas composition both likely affecting the depth of the GHSZ. Our results provide the first evidence of the gas hydrate potential in the offshore Bangladesh and should drive future research and data acquisition aiming to understand the composition, saturation and thickness of the gas hydrate-bearing sediments in this region.

Gas hydrates in sub-seabed sediments is an unexploited source of energy with estimated reserves larger than those of conventional oil. One of the methods for recovering methane from gas hydrates involves injection of Carbon Dioxide (CO2), causing the dissociation of methane and storing CO2. The occurrence of gas hydrates offshore Portugal is well known associated to mud volcanoes in the Gulf of Cadiz. This article presents a determination of the areas with conditions for the formation of biogenic gas hydrates in Portugal’s mainland geological continental margin and assesses their overlap with CO2 hydrates stability zones defined in previous studies. The gas hydrates stability areas are defined using a transfer function recently published by other authors and takes into account the sedimentation rate, the particulate organic carbon content and the thickness of the gas hydrate stability zone. An equilibrium equation for gas hydrates, function of temperature and pressure, was adjusted using non-linear regression and the maximum stability zone thickness was found to be 798 m. The gas hydrates inventory was conducted in a Geographic Information System (GIS) environment and a full compaction scenario was adopted, with localized vertical flow assumed in the accrecionary wedge where mud volcanoes occur. Four areas where temperature and pressure conditions may exist for formation of gas hydrates were defined at an average of 60 km from Portugal’s mainland coastline. Two of those areas coincide with CO2 hydrates stability areas previously defined and should be the subject of further research to evaluate the occurrence of gas hydrate and the possibility of its recovery coupled with CO2 storage in sub-seabed sediments.

Chapter 2 - Natural Gas Hydrate Systems

Gas hydrate (GH) stability modeling results explain why some major Holocene submarine landslides along the Norwegian‐Barents margin could have been triggered by GH dissociation during the early to middle Holocene, not during the lowest sea levels of the Last Glacial Maximum (LGM). Our model results show that subbottom depths of 170–260 m below the pre‐slide continental slope (ca. 350–475 m present water depth) must have passed out of gas hydrate stability zone (GHSZ) by 8.15 ka as the effect of warm bottom water inflow at 11 ka penetrated into the subbottom, overcoming the effects of pressure increase due to sea level rise (SLR). The component of local SLR due to the isostatic response to Fennoscandian deglaciation is shown to be relatively insignificant, particularly for the part of the upper continental slope where the slide probably began. The stability relations show that GH could have formed under the ice sheet before deglaciation, and below deeper shelf areas after sea levels began to rise, but before significant warming near the GHSZ base. To the extent water deeper than 800 m has remained cold (−1° to 0°C) since LGM times, the GHSZ continued to thicken in deep water and GH dissociation could not have triggered Holocene failure in that regime. The present distribution of GH stability is limited to water depths greater than about 400 m in the Storegga slide area, and the thickness of the GHSZ increases with water depth.

The base of the gas hydrate stability zone (GHSZ) is a critical interface, providing a first-order estimate of gas hydrate distribution. Sensitivity to thermobaric conditions makes its prediction challenging, particularly in the regions with dynamic pressure–temperature regime. In Green Canyon Block 955 (GC 955) in the northern Gulf of Mexico, the seismically inferred base of the GHSZ is 450 m (1476 ft) below the seafloor, which is 400 m (1312 ft) shallower than predicted by gas hydrate stability modeling using standard temperature and pressure gradient assumptions and an assumption of structure I (99.9% methane gas) gas hydrate. We use three-dimensional seismic log data and heat-flow modeling to explain the factor of the salt diapir on the observed thinning of the GHSZ. We also test the alternative hypothesis that the GHSZ base is actually consistent with the theoretical depth. The heat-flow model indicates a salt-induced temperature anomaly, reaching 8°C at the reservoir level, which is sufficient to explain the position of the base of the GHSZ. Our analyses show that overpressure does develop at GC 955, but only within an approximately 500-m (∼1640-ft)-thick sediment section above the salt top, which does not currently affect the pressure field in the GHSZ (∼1000 m [∼328 ft] above salt). Our study confirms that a salt diapir can produce a strong localized perturbation of the temperature and pressure regime and thus on the stability of gas hydrates. Based on our results, we propose a generalized evolution mechanism for similar reservoirs, driven by salt-controlled gas hydrate formation and dissociation elsewhere in the world.

We propose several physical/chemical causes to support the seismic results which find presence of Bottom Simulating Reflector (BSR) at site 1144 and site 1148 in Dongsha Region, North of South China Sea. At site 1144, according to geothermal gradient, the bottom of stability zone of conduction mode is in agreement with BSR. At site 1148, however, the stability zone of conduction mode is smaller than the natural gas presence zone predicted by the BSR. We propose three causes, that is, mixed convection and conduction thermal flow mode, multiple composition of natural gas and overpressure in deep sediment to explain the BSR presence or gas hydrate presence. Further, our numerical simulation results suggest yet another reason for the presence of BSR at site 1144 and site 1148. Because the temperatures in deep sediment calculated from the mixed convection and conduction thermal flow mode are lower than that from the single conduction mode, the bottom of gas hydrate stability zone (GHSZ) is deeper than the bottom of gas hydrate deposition zone (GHDZ) or BSR. The result indicates that occurrence zone of natural is decided by the condition that natural gas concentrate in the zone is greater than its solubility.

The Qilian Mountain permafrost is the only place where gas hydrate occurs onshore China at present and its gas hydrate distribution is very complex and irregular. What patterns affecting the accumulation of gas hydrate or what process controlling the formation of gas hydrate are not clear in the study area. Aiming at a gas hydrate geological system, the geological process of gas hydrate formation was studied, based on geological data and analytical results obtained from drilling wells in the Qilian Mountain permafrost. As a result, three stages for the geological process of gas hydrate formation are put forward in the study area. During the late Mid-Jurassic, the upper Triassic generated and provided a major gas source for gas hydrate, secondarily in combination with gas associated with oil generated from the middle Jurassic. The main gas source migrated upward via faults of F1 and F2, partly and occasionally mixed with the coal-bed methane and the microbial methane produced in the shallow strata. It was blocked jointly by thrust faults and thick mudstone or oil shale to be initially accumulated in gas reservoir. From Cretaceous to Pleistocene, the sedimentary strata experienced erosion and the initial gas accumulation turned into residual gas after series of the Qinghai-Tibet plateau uplift. Since the early middle Pleistocene, glaciations formed a gas hydrate stability zone (GHSZ) and the residual gas was coupled with GHSZ to form gas hydrate subsequently. Hence three patterns for the coupling of the residual gas with GHSZ are summarized in the study area. When the residual gas happened to lie within GHSZ, the residual gas directly formed gas hydrate, which was indicated by the drilling results that gas anomalies were encountered within GHSZ as well as occurrences of gas hydrate in the field. When the residual gas was below GHSZ, the residual gas would continually migrate into GHSZ to form gas hydrate, which was indicated by the drilling results that gas anomalies had ever been encountered even if below GHSZ as well as occurrences of gas hydrate within GHSZ in the field. When the residual gas was above GHSZ, the residual gas remained or escaped, which was indicated by the drilling results that gas anomalies even with a high pressure abnormity were encountered in the shallower strata above GHSZ without occurrences of gas hydrate within GHSZ in the field.

A comprehensive characterization of gas hydrate system offshore the western Black Sea was performed through an integrated analysis of geophysical data. We detected the bottom‐simulating reflector (BSR), which marks, in this area, the base of gas hydrate stability. The observed BSR depth does not fit the theoretical steady state base of gas hydrate stability zone (BGHSZ). We show that the disparity between the BSR and predicted BGHSZ is the result of a transient state of the hydrate system due to the ongoing reequilibrium since the Last Glacial Maximum. When gas hydrates are brought outside the stability zone due to changes in temperature and sea level, their dissociation generates an increase in interstitial pore pressure. This process is favorable to the recrystallization of gas hydrates and delays the upward migration of the hydrate stability zone explaining the anomalously deep BSR. The BSR depth, which is commonly used to derive geothermal gradient values by assuming steady state conditions, is used here to derive the maximum excess pore pressure at the BGHSZ. Derived excess pore pressure values of 1–2 MPa are probably the result of the low permeability of hydrate‐bearing sediments. Higher pore pressure values derived at the location of a fault system could cause hydrofracturing enabling the free gas to cross the gas hydrate stability zone and emerge at the seafloor, forming the flares observed in close vicinity to where the shallow gas hydrates were sampled.

Using an approximately analytical formation, we extend the steady state model of the pure methane hydrate system to include the salinity based on the dynamic model of the methane hydrate system. The top and bottom boundaries of the methane hydrate stability zone (MHSZ) and the actual methane hydrate zone (MHZ), and the top of free gas occurrence are determined by using numerical methods and the new steady state model developed in this paper. Numerical results show that the MHZ thickness becomes thinner with increasing the salinity, and the stability is lowered and the base of the MHSZ is shifted toward the seafloor in the presence of salts. As a result, the thickness of actual hydrate occurrence becomes thinner compared with that of the pure water case. On the other hand, since lower solubility reduces the amount of gas needed to form methane hydrate, the existence of salts in seawater can actually promote methane gas hydrate formation in the hydrate stability zone. Numerical modeling also demonstrates that for the salt-water case the presence of methane within the field of methane hydrate stability is not sufficient to ensure the occurrence of gas hydrate, which can only form when the methane concentration dissolved in solution with salts exceeds the local methane solubility in salt water and if the methane flux exceeds a critical value corresponding to the rate of diffusive methane transport. In order to maintain gas hydrate or to form methane gas hydrate in marine sediments, a persistent supplied methane probably from biogenic or thermogenic processes, is required to overcome losses due to diffusion and advection.

Trapping and migration of methane associated with the gas hydrate stability zone at the Blake Ridge Diapir: new insights from seismic data

The gas hydrate stability zone, the zone below the seafloor where gas and water should form gas hydrate is predicted to be present in the tophole sections in deepwater areasaround the world including the deepwater Gulf of Mexico. Few, if any, bottom simulating reflectors (BSR) or other manifestations of the gas hydrate stability zone have been interpreted in the Gulf of Mexico. This paper presents convincing images of the base of the gas hydrate stability zone extracted from exploration 3-D data in Northwest Walker Ridge in the Gulf of Mexico from the upper 1,050 m of sediment. This area is on the margin of an uplifted and compressed mini-basin and in the vicinity of numerous giant gas mounds at the seafloor. Interpreted tophole stratigraphy includes laterally extensive, steeply dipping basin floor silts and sands. This paper shows images of hydrate-trapped gas at multiple reflectors at depths that are coincident with the predicted base of the hydrate stability zone if modeled with a geothermal gradient of 19.6 ± 0.5 °C/km. A BSR is seen in the seismic data in vertical section, but the more convincing images of the base of gas hydrate stability are seen in map view. We expect that the subsurface conditions here are not unique and the hydrate stability zone is similarly imaged, but perhaps has not been recognized, in similar settings in other parts of the Gulf of Mexico and elsewhere in the world. Recognition of hydrate-trapped gas can aid in better decision making for shallow-gas avoidance strategy for exploration drilling and optimal design and placement of conductor strings for deepwater development wells. The possible concentrations of gas hydrate reservoirs in the updip margins of hydrate traps should also be of interest to those that are evaluating the feasibility of commercial exploitation of gas hydrates. Local Geologic Setting Figure 1 shows the study area in the northwestern part of Walker Ridge in the Gulf of Mexico. The focus of the study is in an uplifted and compressed minibasin with giant gas mounds to the northeast and a large salt wall with shallow buried salt or salt cropping out at the seafloor to the south. Thrust faults within the mini-basin have seafloor expression (Figures1 and 2). The source of compressive stress is apparently from shallow salt movement north-to-south on the northern margin of the minibasin. A larger, deeper salt withdrawal basin (not shown) is oriented northwest-to-southeast, northeast of the giant gas mounds. Both of the salt withdrawal basins filled with Pliocene and Pleistocene sediments transported from the shelf-edge towards the central Gulf of Mexico Basin. The minibasin is filled with typical deepwater sedimentation cycles composed of debris flows, basin floor splays, channels, thin turbidites, and hemipelagic clays.

Origin of gas in gas hydrates as interpreted from geochemistry data obtained during the National Gas Hydrate Program Expedition 02, Krishna Godavari Basin, offshore India

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.