Base Of Gas Hydrate Stability Research Articles

Slow response of the gas hydrate system to ridge erosion and sea-level rise: Insights from double BSRs on the southern Hikurangi margin (New Zealand)

We analyse reflection seismic profiles across the outer accretionary wedge at the convergent New Zealand Hikurangi margin. We identify several, in some case stacked, bottom simulating reflections (BSRs). We interpret these multiple BSRs to record changes in gas hydrate stability. With the aid of gas hydrate systems modelling, we identify two geological drivers that affect gas hydrate stability: (1.) rapid sedimentation in trough basins and (2.) uplift and erosion of thrust ridges. Rapid sedimentation in trough basins buries gas hydrates that formed above the former base of gas hydrate stability (BGHS). Locally, we observe a remnant BSR from this process, likely due to residual gas and possibly gas hydrate. The combined effects of uplift and erosion, in contrast, result in the preservation of a remnant BSR within the gas hydrate stability zone, whilst a new BSR forms locally at the present-day BGHS. However, the limited occurrence of double BSRs in seismic data and the model both suggest that the formation of a deeper BSR is limited by gas supply. Formation of significant gas hydrate at this deeper level only occurs in areas of focused gas migration. This slow formation of gas hydrate also has implications for the response to glacio-eustatic sea-level rise: gas hydrates are more likely to accumulate above the BGHS corresponding to the last glacial maximum, whereas only small amounts formed above the deeper present-day BGHS. Hence, future bottom water warming will, at least initially, not lead to significant methane release from dissociating gas hydrates in deep water.

Episodic Venting of a Submarine Gas Seep on Geological Time Scales: Formosa Ridge, Northern South China Sea

AbstractThe Formosa Ridge cold seep is among the first documented active seeps on the northern South China Sea passive margin slope. Although this system has been the focus of scientific studies for decades, the geological factors controlling gas release are not well understood due to a lack of constraints of the subsurface structure and seepage history. Here, we use high‐resolution 3D seismic data to image stratigraphic and structural relationships associated with fluid expulsion, which provide spatio‐temporal constraints on the gas hydrate system at depth and methane seepage at modern and paleo seafloors. Gas has accumulated beneath the base of gas hydrate stability to a critical thickness, causing hydraulic fracturing, propagation of a vertical gas conduit, and morphological features (mounds) at paleo‐seafloor horizons. These mounds record multiple distinct gas migration episodes between 300,000 and 127,000 years ago, separated by periods of dormancy. Episodic seepage still seems to occur at the present day, as evidenced by two separate fronts of ascending gas imaged within the conduit. We propose that episodic seepage is associated with enhanced seafloor sedimentation. The increasing overburden leads to an increase in effective horizontal stress that exceeds the gas pressure at the top of the gas reservoir. As a result, the conduit closes off until the gas reservoir is replenished to a new (greater) critical thickness to reopen hydraulic fractures. Our results provide intricate detail of long‐term methane flux through sub‐seabed seep systems, which is important for assessing its impact on seafloor and ocean biogeochemistry.

Sedimentary bottom simulating reflection muting—A new model of hydrate and fluid redistribution from the Pegasus Basin, New Zealand

AbstractBottom simulating reflections (BSRs) interpreted in seismic data are one of the most used indicators for the presence of gas hydrates. In numerous hydrate provinces, including the Hikurangi margin, east of New Zealand, distinct and anomalous gaps in reflectivity punctuate otherwise continuous BSRs. We undertake a seismic stratigraphic and structural interpretation of a dense grid of two‐dimensional reflection seismic data to investigate possible causes of widespread BSR gaps that occur near the hinge area of synclines. We explain these BSR gaps with a tectono‐sedimentary model where sedimentation into accretionary wedge synclines leads to an upward migration of the base of gas hydrate stability with respect to stratigraphy, causing dissociation of gas hydrates to water and free gas. A trough‐shaped syncline's radially dipping beds promotes along‐strata, upward migration of gas away from the hinge area, incrementally depleting synclinal hinges of gas and resulting in a BSR gap or muted BSR. This model may be applicable to observed gaps in BSRs in tectonically active slope basins worldwide.

Integrated geochemical approach to determine the source of methane in gas hydrate from Green Canyon Block 955 in the Gulf of Mexico

Massive volumes of gas are sequestered within gas hydrate in subsurface marine sediments in the Gulf of Mexico. Methane associated with gas hydrate is a potentially important economic resource and a significant reservoir of carbon within the global carbon cycle. Nevertheless, uncertainties remain about the genetic source (e.g., microbial, thermogenic) and possible migration history of natural gas incorporated into hydrate. Previous studies have primarily used the hydrocarbon molecular (CH4/C2H6+) and isotopic (δ13C-CH4, δ2H-CH4) compositions of natural gas to address these uncertainties. However, hydrocarbon tracers are altered by mixing, oxidation, secondary methanogenesis, or fluid migration, which presents challenges when deciphering the mechanisms responsible for methane formation and accumulation. To evaluate the genetic source of natural gases from Green Canyon Block 955 (GC 955), east of the Sigsbee escarpment, we collected and analyzed samples from the first pressurized hydrate-bearing sediment cores collected from a coarse-grained hydrate reservoir in the Gulf of Mexico. Gas samples were analyzed for hydrocarbon (C1- C5), major gas (e.g., N2, CO2), and noble gas (He-Xe) abundance and isotopic (e.g., δ13C-CH4, δ2H-CH4, δ13C-CO2, δ15N-N2, 3He/4He, 4He/20Ne) compositions. We determined that natural gas in hydrates from this location are predominantly of primary microbial origin (conservatively at least 76%) and are formed by the hydrogenotrophic (CO2 reduction) methanogenesis pathway. We also note increased thermogenic proportions (~6%) in a hydrate-bearing layer below the main hydrate-bearing interval (separated by a 5 m water-bearing layer). Our results suggest that microbial methane may be abundant below the base of gas hydrate stability at GC 955.

Scientific Results of the Hydrate-01 Stratigraphic Test Well Program, Western Prudhoe Bay Unit, Alaska North Slope

The United States Department of Energy, the MH21-S Research Consortium of Japan, and the United States Geological Survey are collaborating to enable gas hydrate scientific drilling and extended-duration reservoir response testing on the Alaska North Slope. To feasibly execute such a test, a location is required that is accessible from existing roads and gravel pads and that can be occupied without disrupting ongoing industry operations. A review of potential locations meeting these criteria determined the likely occurrence of gas hydrate in two fine-grained marginal-marine sands of Tertiary age in the vicinity of the inactive “Kuparuk State 7-11-12” exploration pad in the western Prudhoe Bay Unit (PBU). Existing well and seismic data for that site were insufficient to preclude the potential for free gas occurrence within the deeper (and most prospective) target sand. Therefore, with support from the PBU Working Interest Owners, Alaska Department of Natural Resources, and Petrotechnical Resources Alaska, the Hydrate-01 Stratigraphic Test Well (STW) was drilled in December 2018 to confirm the suitability of the site for future gas hydrate scientific testing. The Hydrate-01 well was successfully drilled to −3290 ft (1003 m) subsea vertical depth at a bottom hole location of approximately 900 ft (∼275 m) east of the surface location. The drilling program featured acquisition of a full suite of logging while drilling data, the collection of side-wall pressure cores, and the installation of distributed temperature and distributed acoustic sensor fiber-optic cables. The log data acquired confirmed the occurrence of gas hydrate at high saturation in two target sands. Integrated evaluation of log and sidewall core data provide petrophysical and geomechanical property information that allow for potential reservoir response to depressurization to be simulated. The deeper “B1 sand” is deemed to be most favorable for reservoir response testing as a result of confirmed gas hydrate occurrence in sediments of high intrinsic permeability, location within 100 ft (30 m) of the base of gas hydrate stability, and minimal risk for direct communication with permeable water-bearing (hydrate-free) zones. The shallower “D1 sand” provides a secondary target that is differentiated by colder in situ temperatures and the interpreted direct hydraulic communication to a lower section of non-hydrate-bearing, water-saturated sand. The Hydrate-01 log data also confirm the occurrence of at least one sub-seismic fault in close proximity to the B1 sand reservoir. To better image the distribution of the gas-hydrate-bearing reservoir sections and associated faults, a three-dimensional (3D) vertical seismic profile was conducted in early 2019 using the distributed acoustic sensors installed as part of the Hydrate-01 STW completion. Detailed two-dimensional (2D) and 3D geologic models have been constructed to enable numerical simulations to inform the planning for potential future scientific tests of reservoir response to depressurization at the site.

Upward‐Doming Zones of Gas Hydrate and Free Gas at the Bases of Gas Chimneys, New Zealand's Hikurangi Margin

AbstractFocused gas migration through the gas hydrate stability zone in vertical gas conduits is a global phenomenon. The process can lead to concentrated gas hydrate formation and seafloor gas seepage, which influences seafloor biodiversity and ocean biogeochemistry. However, much is unknown about how gas and gas hydrate co‐exist within and around gas conduits. We present seismic imaging of the gas hydrate system beneath a four‐way closure anticlinal ridge at New Zealand's southern Hikurangi subduction margin. Gas has accumulated beneath the base of gas hydrate stability to a thickness of up to ∼240 m, which has ultimately led to hydraulic fracturing and propagation of a vertical gas conduit to the seafloor. Despite the existence of an array of normal faults beneath the ridge, these structures are not exploited as long‐range gas flow conduits. Directly beneath the conduit, and extending upward from the regional base of gas hydrate stability, is a broad zone characterized by both negative‐ and positive‐polarity reflections. We interpret this zone as a volume of sediment hosting both gas hydrate and free gas, that developed due to partial gas trapping beneath a mass transport deposit. Similar highly reflective zones have been identified at the bases of other gas conduits, but they are not intrinsic to all gas conduits through gas hydrate systems. We suggest that pronounced intervening sealing units within the gas hydrate stability zone determine whether or not they form.

Seismic velocity and reflectivity analysis of concentrated gas hydrate deposits on the southern Hikurangi Margin (New Zealand)

In the southern Hikurangi subduction margin, widespread gas hydrate accumulations are inferred based on the presence of bottom simulating reflections and recovered gas hydrate samples, mainly associated with thrust ridges. We present a detailed analysis of high- and medium-resolution seismic reflection data across Glendhu and Honeycomb ridges, two elongated four-way closure systems at the toe of the deformation wedge. High-amplitude reflections within the gas hydrate stability zone, coincident with high seismic velocities, suggest the presence of highly concentrated gas hydrate accumulations in the core regions of the anticlinal ridges. A novel method involving combined seismic velocity and reflectivity analysis and rock physics modelling is used to estimate hydrate saturations in localised areas. The effective medium model consistently predicts gas hydrate saturations of ~30% of the pore space at Glendhu Ridge and >60% at Honeycomb Ridge, whereas the empirical three-phases weighted equation likely underestimates the amount of gas hydrate present. We note that our estimates are dependent on the vertical resolution of the seismic data (5–14 m), and that the existence of thin layers hosting gas hydrate at higher concentrations is likely based on observations made elsewhere in similar depositional environments. A comparison between the two ridges provides insights into the evolution of thrust related anticlines at the toe of the accretionary wedge. We propose that the main driving mechanism for concentrated hydrate accumulation in the study area is along-strata gas migration. The vertical extent of these accumulations is a function of the steepness of the strata crossing the base of gas hydrate stability, and of the volume of sediments from which fluid flows into each structure. According to our interpretation, older structures situated further landward ofthe deformation front are more likely to host more extensive concentrated hydrate deposits than younger ridges situated at the deformation front and characterised by more gentle folding. The method introduced in this work is useful to retrieve quantitative estimates of gas hydrate saturations based on multi-channel seismic data.

Reply to Comments by N. Sultan on “Sedimentation Controls on Methane‐Hydrate Dynamics Across Glacial/Interglacial Stages: An Example From International Ocean Discovery Program Site U1517, Hikurangi Margin”

AbstractScreaton et al. (2019, https://doi.org/10.1029/2019GC008603) examined the role of sedimentation, sea level, and bottom water temperature (BWT) changes due to glaciation as drivers for the downward migration of the base of gas hydrate stability and gas hydrate formation. International Ocean Discovery Program (IODP) Site U1517 in the Hikurangi margin was used as a case study because data at this site document a marked increase in chloride over a broad depth range, which was attributed to recent gas hydrate formation. In a comment on Screaton et al. (2019, https://doi.org/10.1029/2019GC008603), Sultan (2020, https://doi.org/10.1029/2019gc008846) used a linear thermal profile to argue that inferences and characterization of methane hydrate at IODP Site U1517 were incorrect because some occur below his estimated base of gas hydrate stability (BGHS). Based on this apparent discrepancy, Sultan (2020, https://doi.org/10.1029/2019gc008846) further stated that low‐chloride spikes may be unreliable indicators of methane hydrate occurrence. In this reply, we emphasize that unsteady‐state, and thus nonlinear, thermal profiles are likely in areas experiencing active sedimentation and bottom‐water temperature (BWT) changes. The resulting deviation from steady‐state temperature profile shifts the BGHS downward. In addition, sedimentation has the potential to bury methane hydrate more rapidly than it dissociates, helping to explain how methane hydrate could be observed below the BGHS. We also review the supporting evidence for gas‐hydrate occurrence at Site U1517 and the criteria used for Site U1517 site selection.

Sedimentation Controls on Methane‐Hydrate Dynamics Across Glacial/Interglacial Stages: An Example From International Ocean Discovery Program Site U1517, Hikurangi Margin

AbstractDissolved chloride concentrations higher than seawater were observed over a broad depth range in pore water profiles from International Ocean Discovery Program Site U1517 on the Hikurangi Margin. This Cl maximum is not associated with an 87Sr/86Sr anomaly, indicating that it is not caused by hydration reactions during ash alteration. We use a numerical modeling approach to examine possible causes for recent gas hydrate formation that can result in the observed Cl high. Our approach considers sedimentation, sea level, and bottom water temperature (BWT) changes due to glaciation as drivers for the downward migration of the base of gas hydrate stability and gas hydrate formation. The modeling results reveal that lowering of sea level during glaciation can allow methane hydrate dissociation followed by postglacial hydrate formation as sea level rises. However, BWT cooling of 2 °C during glaciation followed by warming during deglaciation would mostly counteract the impacts of sea level change. Bottom water cooling during glaciation is expected in this region and many locations worldwide. As a result, our simulations do not support the previous hypotheses of large‐scale gas hydrate dissociation due to sea level drop during glaciation, which have been proposed as triggers for widespread gas release and slope failure. Such a mechanism is only possible where BWT remains constant or increases during glaciation. Our simulations indicate that sedimentation constitutes the largest factor driving recent methane hydrate formation at Site U1517, and we suggest that sedimentation may play a larger role in gas hydrate dynamics along margins than previously recognized.

Shallow-rooted mud volcanism in Lake Baikal

Abstract Lake Baikal is the only freshwater basin containing sediments with gas hydrate accumulations, some of which are associated with mud volcano activity. Twenty-two mud volcanoes have already been identified in different areas of Lake Baikal, but the formation process and source depth remained unknown due to a lack of conclusive evidences. Here we discuss a set of geological and geophysical data to report the discovery of the hydrate-bearing Akadem mud volcanic complex (AMVC) on the Academician Ridge in central Lake Baikal. The obtained results allowed for the first time to concretely estimate the source depth of the mobilized fluids and sediments. Analysis of diatom skeletons present in the mud breccia revealed that the oldest diatom specimen is Cyclotella Iris et var. This species is characteristic for a short age interval ranging between 4.8 and 5.6 Ma. The same diatom was also detected between 230 and 310 m below the lake floor (mblf) in the borehole BDP-98 drilled near the AMVC. Combining biostratigraphic correlation and seismostratigraphy, it is estimated that the same interval is located at 200–300 mblf below the mud volcanic field. The elevated heat flow measured at AMVC indicates that the original base of gas hydrate stability (regionally located at ∼212 mblf) is currently shifted upwards of ∼100 m. The acquired data are consistent with a scenario envisaging the rise of warm fluids throughout the mud volcano complex zone. We suggest that migration of deep fluids could have initiated the gas hydrate dissociation and, in turn, rapidly generated overpressured shallow mud chambers. The ultimate piercing and triggering of the mud volcanoes activity resulted in the eruption of mud breccia and formation of densely packed crater sites in the study area. The depicted scenario can be applied to many mud volcanoes in Lake Baikal where similar anomalous heat flow conditions have been measured. These findings also emphasize that the genetic association between gas hydrate dissociation and the initiation of eruptive activity explains numerous peculiarities of the “Baikal” sedimentary volcanism (e.g. lack of lithified rocks among mud breccia clasts, gas hydrates, moderately elevated heat flow). This type of mud volcanism differentiates from the typically deeply rooted piercements observed worldwide in mature (marine) sedimentary basins. Ultimately our findings open a new prospective for mud volcano research worldwide, emphasizing that gas hydrates are not just one of the common features for sedimentary volcanism, but may have an active role as a triggering mechanism for the process itself.

Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System in the Western Black Sea

AbstractA 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.

Free gas distribution and basal shear zone development in a subaqueous landslide – Insight from 3D seismic imaging of the Tuaheni Landslide Complex, New Zealand

The Hikurangi margin is an active continental margin east of New Zealand's North Island. It is well recognized as a seismically active zone and is known for the occurrence of free gas and gas hydrates within the shallow sediments. A variety of subaqueous landslides can be observed at the margin, including the Tuaheni Landslide Complex off Poverty Bay. This slide complex has been interpreted previously as a slowly creeping landform, as its morphology and internal deformation is comparable to terrestrial earthflows and rock glaciers. In 2014, we acquired a high-resolution 3D seismic volume covering major parts of the Tuaheni South landslide. The 3D data show a variety of fluid migration indicators, free gas accumulations and manifestations of the base of gas hydrate stability in the pre-slide sedimentary units and the lower unit of the landslide system. The data also show that the landslide system is composed of an upper and lower unit that are separated by an intra-debris negative-polarity reflection. Free gas accumulations directly beneath the landslide units suggest that the debris acts as a boundary for rising fluids and only few migration pathways to the intra-debris reflector are observed in the distal parts of the landslide. Deformation within the landslide's debris is focused in the upper landslide unit, and we interpret the intra-debris reflector as a basal shear zone or ‘glide plane’ upon which the debris has been remobilized. The origin of the intra-debris reflector is unclear, but we suggest it could be a relatively coarse-grained horizon that would be prone to fluid flow focusing and the development of excess fluid pressure. Our seismic study provides one of the most detailed examples of a subaqueous landslide system and reveals insights into the fluid flow system and potential basal shear zone development of the Tuaheni Landslide Complex.

Gas Hydrate Formation Amid Submarine Canyon Incision: Investigations From New Zealand's Hikurangi Subduction Margin

AbstractWe investigate gas hydrate system dynamics beneath a submarine canyon on New Zealand's Hikurangi subduction margin using seismic reflection data and petroleum systems modeling. High seismic velocities just above the base of gas hydrate stability (BGHS) indicate that concentrated gas hydrates exist beneath the canyon. Two‐dimensional gas hydrate formation modeling shows how the process of canyon incision at this location alters the distribution and concentration of gas hydrate. The key modeling result is that free gas is trapped beneath the gas hydrate layer and then “captured” into a concentrated gas hydrate deposit as a result of a downward‐shift in the BGHS driven by canyon incision. Our study thus provides new insight into the functioning of this process. From our data, we also conceptualize two other models to describe how canyons could significantly change gas hydrate distribution and concentration. One scenario is related to deflection of fluid flow pathways from over‐pressured regions at the BGHS toward the canyon, and the other is based on relationships between simultaneous seafloor uplift and canyon incision. The relationships and processes described are of global relevance because of considerations of gas hydrate as an energy resource and the influence of both submarine canyons and gas hydrate systems on seafloor biodiversity.

WITHDRAWN: Three-dimensional seismic evidence of extensive gas hydrate deposits linked with large free gas columns suggests long-range advection mechanisms

The mechanisms associated with the accumulation of natural gas hydrates in coarse-grained sediments are largely unconstrained. An improved understanding of such processes is crucial to efficiently prospect for hydrate resources. Therefore, we use three-dimensional seismic imaging to reveal amplitude patterns reflecting extensive hydrate deposits along dipping layers in the deepwater frontal anticlines of the Guajira Basin (offshore Northern Colombia). These gas hydrate-bearing sediments have a seismic character that is highly diagnostic for elevated saturations (>50%) in sand-rich lithologies, being expressed as segments of enhanced stratigraphic reflections, comparable in amplitude but opposite in polarity to the reflections observed in the underlying free gas zone. Observations in seismic sections and amplitude maps of a bottom-simulating reflection and key horizons indicate that the distribution of hydrates and free gas is largely affected by the structural setting of the study area, where hydrates and free gas further have markedly diverse extents in different anticlines. We recognise a link between hydrates and free gas, such that the most extensive hydrate accumulations and the largest (up to 570 m) free gas columns occur along the same dipping horizons, cross-cutting the base of gas hydrate stability. These observations suggest that a long-range advection of free gas operates with varying effectiveness in these structures. We hypothesise that these hydrate deposits form by overpressure-controlled pulses of free gas invasion into the gas hydrate stability zone along dipping stratal conduits. This process implies that hydrate-bearing sediments constitute an efficient but dynamic membrane seal for large buoyant free gas columns. This coupling between free gas and hydrate accumulations represents an extraordinary example of hydrate distribution governed by advective processes illustrated in three dimensions.

Gas hydrate formation in compressional, extensional and un-faulted structural settings – Examples from New Zealand's Hikurangi margin

We investigate gas hydrate formation processes in compressional, extensional and un-faulted settings on New Zealand's Hikurangi margin using seismic reflection data. The compressional setting is characterized by a prominent subduction wedge thrust fault that terminates beneath the base of gas hydrate stability, as determined from a bottom-simulating reflection (BSR). The thrust is surrounded by steeply dipping strata that cross the BSR at a high angle. Above the BSR, these strata are associated with a high velocity anomaly that is likely indicative of relatively concentrated, and broadly distributed, gas hydrates. The un-faulted setting—sedimentary infill of a slope basin on the landward side of a prominent thrust ridge—is characterized by a strong BSR, a thick underlying free gas zone, and short positive polarity reflection segments that extend upward from the BSR. We interpret the short reflection segments as the manifestation of gas hydrates within relatively coarse-grained sediments. The extensional setting is a localized, shallow response to flexural bending of strata within an anticline. Gas has accumulated beneath the BSR in the apex of folding. A high-velocity zone directly above the BSR is probably mostly lithologically-derived, and only partly related to gas hydrates. Although each setting shows evidence for focused gas migration into the gas hydrate stability zone, we interpret that the compressional tectonic setting is most likely to contain concentrated gas hydrates over a broad region. Indeed, it is the only setting associated with a deep-reaching fault, meaning it is the most likely of the three settings to have thermogenic gas contributing to hydrate formation. Our results highlight the importance of anisotropic permeability in layered sediments and the role this plays in directing sub-surface fluid flow, and ultimately in the distribution of gas hydrate. Each of the three settings we describe would warrant further investigation in any future consideration of gas hydrates as an energy resource on the Hikurangi margin.

Observed correlation between the depth to base and top of gas hydrate occurrence from review of global drilling data

AbstractA global inventory of data from gas hydrate drilling expeditions is used to develop relationships between the base of structure I gas hydrate stability, top of gas hydrate occurrence, sulfate‐methane transition depth, pressure (water depth), and geothermal gradients. The motivation of this study is to provide first‐order estimates of the top of gas hydrate occurrence and associated thickness of the gas hydrate occurrence zone for climate‐change scenarios, global carbon budget analyses, or gas hydrate resource assessments. Results from publically available drilling campaigns (21 expeditions and 52 drill sites) off Cascadia, Blake Ridge, India, Korea, South China Sea, Japan, Chile, Peru, Costa Rica, Gulf of Mexico, and Borneo reveal a first‐order linear relationship between the depth to the top and base of gas hydrate occurrence. The reason for these nearly linear relationships is believed to be the strong pressure and temperature dependence of methane solubility in the absence of large difference in thermal gradients between the various sites assessed. In addition, a statistically robust relationship was defined between the thickness of the gas hydrate occurrence zone and the base of gas hydrate stability (in meters below seafloor). The relationship developed is able to predict the depth of the top of gas hydrate occurrence zone using observed depths of the base of gas hydrate stability within less than 50 m at most locations examined in this study. No clear correlation of the depth to the top and base of gas hydrate occurrences with geothermal gradient and sulfate‐methane transition depth was identified.

Potential for gas hydrate formation at the northwest New Zealand shelf margin - New insights from seismic reflection data and petroleum systems modelling

In this study we provide evidence for methane hydrates in the Taranaki Basin, occurring a considerable distance from New Zealand's convergent margins, where they are well documented. We describe and reconstruct a unique example of gas migration and leakage at the edge of the continental shelf, linking shallow gas hydrate occurrence to a deeper petroleum system. The Taranaki Basin is a well investigated petroleum province with numerous fields producing oil and gas. Industry standard seismic reflection data show amplitude anomalies that are here interpreted as discontinuous BSRs, locally mimicking the channelized sea-floor and pinching out up-slope. Strong reverse polarity anomalies indicate the presence of gas pockets and gas-charged sediments. PetroMod™ petroleum systems modelling predicts that the gas is sourced from elevated microbial gas generation in the thick slope sediment succession with additional migration of thermogenic gas from buried Cretaceous petroleum source rocks. Cretaceous–Paleogene extensional faults underneath the present-day slope are interpreted to provide pathways for focussed gas migration and leakage, which may explain two dry petroleum wells drilled at the Taranaki shelf margin. PetroMod™ modelling predicts concentrated gas hydrate formation on the Taranaki continental slope consistent with the anomalies observed in the seismic data. We propose that a semi-continuous hydrate layer is present in the down-dip wall of incised canyons. Canyon incision is interpreted to cause the base of gas hydrate stability to bulge downward and thereby trap gas migrating up-slope in permeable beds due to the permeability decrease caused by hydrate formation in the pore space. Elsewhere, hydrate occurrence is likely patchy and may be controlled by focussed leakage of thermogenic gas. The proposed presence of hydrates in slope sediments in Taranaki Basin likely affects the stability of the Taranaki shelf margin. While hydrate presence can be a drilling hazard for oil and gas exploration, the proposed presence of gas hydrates opens up a new frontier for exploration of hydrates as an energy source.

Simulation of gas hydrate dissociation caused by repeated tectonic uplift events

AbstractGas hydrate dissociation by tectonic uplift is often used to explain geologic and geophysical phenomena, such as hydrate accumulation probably caused by hydrate recycling and the occurrence of double bottom‐simulating reflectors in tectonically active areas. However, little is known of gas hydrate dissociation resulting from tectonic uplift. This study investigates gas hydrate dissociation in marine sediments caused by repeated tectonic uplift events using a numerical model incorporating the latent heat of gas hydrate dissociation. The simulations showed that tectonic uplift causes upward movement of some depth interval of hydrate‐bearing sediment immediately above the base of gas hydrate stability (BGHS) to the gas hydrate instability zone because the sediment initially maintains its temperature: in that interval, gas hydrate dissociates while absorbing heat; consequently, the temperature of the interval decreases to that of the hydrate stability boundary at that depth. Until the next uplift event, endothermic gas hydrate dissociation proceeds at the BGHS using heat mainly supplied from the sediment around the BGHS, lowering the temperature of that sediment. The cumulative effects of these two endothermic gas hydrate dissociations caused by repeated uplift events lower the sediment temperature around the BGHS, suggesting that in a marine area in which sediment with a highly concentrated hydrate‐bearing layer just above the BGHS has been frequently uplifted, the endothermic gas hydrate dissociation produces a gradual decrease in thermal gradient from the seafloor to the BGHS. Sensitivity analysis for model parameters showed that water depth, amount of uplift, gas hydrate saturation, and basal heat flow strongly influence the gas hydrate dissociation rate and sediment temperature around the BGHS.

Gas migration through Opouawe Bank at the Hikurangi margin offshore New Zealand

This study presents 2D seismic reflection data, seismic velocity analysis, as well as geochemical and isotopic porewater compositions from Opouawe Bank on New Zealand’s Hikurangi subduction margin, providing evidence for essentially pure methane gas seepage. The combination of geochemical information and seismic reflection images is an effective way to investigate the nature of gas migration beneath the seafloor, and to distinguish between water advection and gas ascent. The maximum source depth of the methane that migrates to the seep sites on Opouawe Bank is 1,500–2,100 m below seafloor, generated by low-temperature degradation of organic matter via microbial CO2 reduction. Seismic velocity analysis enabled identifying a zone of gas accumulation underneath the base of gas hydrate stability (BGHS) below the bank. Besides structurally controlled gas migration along conduits, gas migration also takes place along dipping strata across the BGHS. Gas migration on Opouawe Bank is influenced by anticlinal focusing and by several focusing levels within the gas hydrate stability zone.

Possible link between weak bottom simulating reflections and gas hydrate systems in fractures and macropores of fine-grained sediments: Results from the Hikurangi Margin, New Zealand

Small amounts of free gas in interstitial sediment pores are known to significantly lower compressional (P-) wave velocity (Vp). This effect, combined with moderately elevated Vp from the presence of gas hydrates, is usually thought to be the cause for the often observed strong negative reflection coefficients of bottom simulating reflections (BSRs) at the base of gas hydrate stability (BGHS). At several locations however, weak BSRs have been observed, which are difficult to reconcile with a presence of gas in sediment pores. We here present a rock physics model for weak BSRs on the Hikurangi Margin east of New Zealand. Thin sections of a fine-grained mudstone sample from a submarine outcrop in the vicinity of a weak BSR show macroscopic porosity in the form of fractures and intrafossil macropores. We apply the Kuster-Toksöz theory to predict seismic velocities for a rock with water-saturated interstitial micropores and gas or hydrates in macroscopic pore space simulating fractures or compliant macropores. We match field observations of a weak BSR with a reflection coefficient of −0.016 with two end-member models; (1) rocks with gas hydrate-filled voids with a concentration of <4% of bulk sediment overlying water-filled voids, or (2) fully gas-saturated voids at a concentration of <2% beneath water-filled voids. A natural system is likely to consist of a combination of these end-members and of macroporosity filled with a mixture of water and gas or hydrate. Our results suggest weak BSRs may be caused by gas hydrate systems in fractures and macropores of fine-grained sediments with fully water-saturated interstitial pore space. Gas may be supplied into the macroscopic pore space by diffusion-driven short-range migration of methane generated within the gas hydrate stability field or, our favoured model based on additional geologic considerations, long-range advective migration from deeper sources along fractures.