Hydrate Stability Zone Research Articles

Seafloor morphology and potential gas hydrate distribution in the offshore Niger Delta

Bottom simulating reflectors (BSRs) and seismic pipe features have been used as proxies for defining the distribution of gas hydrate sediments in the offshore Niger Delta. This is the most extensive mapping of gas hydrate sediments in the Delta as of today. The seismic data merge comes from multiple surveys acquired with different parameters and seismic resolutions over the course of decades of oil and gas exploration in the region. Indicated gas hydrate distribution generally follows the structural fabric of the Niger Delta with BSRs occurring along the apexes of the thrust-related ridges that have bathymetric relief on the seafloor. The presence of swarms of seismic pipe features landwards of BSR locations suggests hydrates occur beyond BSR locations. The potential gas hydrates sediment acreage in offshore Niger Delta is 17600 sq-km, representing 20% of the area with a thickness of the gas hydrate stability zone reaching 440 m in the more outboard regions of the Delta. Total gas hydrates sediment coverage likely exceeds this value as BSRs become indistinguishable from sediment strata in regions of flat dips. The presence of double BSRs further suggests the presence of thermogenic gas hydrates in the region and allows to extend the thickness of the potential hydrate zone to 550 m in the outboard regions of the Delta.

Subsea permafrost and associated methane hydrate stability zone: how long can they survive in the future?

Subsea permafrost and associated methane hydrate stability zone: how long can they survive in the future?

Submarine fluid flow system feeding methane emission in the northern South China Sea

AbstractSubmarine fluid flow system can transport methane into ocean. However, its evolution is not fully understood, particularly methane migration through the gas hydrate stability zone (GHSZ) in deep‐water settings. Here, we used 3D seismic and well‐logging data to show the currently active fluid flow system in the northern South China Sea. It was interpreted to have two parts and they together feed intermittent methane emission. Three gas clouds have been seismically imaged beneath the base of gas hydrate stability zone (BGHSZ) and a set of new faults can be identified within them. Twenty‐eight seismic pipes were found to penetrate three vertically stacked mass transport deposits (MTDs) above the gas clouds. Log‐seismic correlation shows that the seismic reflections in the pipe represent MTD sediment, bulk carbonate and gas hydrate‐ or free gas‐bearing sediments. We interpreted faults and pipes as the main migration conduits below and above the BGHSZ respectively. The MTD within the GHSZ could seal the underlying free gas transported by faults and thus overpressure built up at the base prior to the occurrences of the pipes and the fracturing through the overlying sedimentary succession. Subsequently, focused fluid flow entered the GHSZ, with the methane probably bypassing the GHSZ before pore clogging of gas hydrates occurred. Additionally, mapping of high‐amplitude reflections surrounding the upper portion of gas clouds reveals the relict free gas associated with three paleo‐GHSZ bases. Episodic emplacements of new MTDs repeatedly caused the upward shifts of the BGHSZ and the resultant gas hydrate dissociation, contributing to methane emission. We proposed that the occurrences of MTDs may facilitate methane emission by intermittently trapping methane and inducing gas hydrate dissociation in deep‐water settings.

Investigation of gas and gas hydrate accumulations along the continental margin of the Danube Delta (Romania and Bulgaria offshore) using seismic reflection data

In 2012, a comprehensive study of the Danube River’s submarine channels continental slope was conducted, employing multi-beam bathymetry and over 2300 km of high-resolution two- dimensional seismic reflection data. The investigation aimed to delve into the area's morphology, potential for gas hydrate presence, and the correlation between stratigraphic units and gas hydrates. Three distinct zones, revealed Bottom Simulating Reflectors (BSRs) indicating the base of gas hydrate accumulations in the seismic data. These BSR areas exhibited Type-1 reflections, characterized by continuous cuts across layers. Notably, five discrete levels of BSRs were detected, suggesting a consistent gas composition across them. The multiple BSR formations are attributed to higher sedimentation rates relative to gas hydrate dissolution rates. Mass transport deposits (MTDs) within the gas hydrate stability zone (6 in total) were identified; their highly consolidated nature could account for the absence of gas hydrates within them. Additionally, one MTD displayed elevated heat flow measurements, indicating a higher geothermal gradient, likely due to its relatively high thermal conductivity. This disparity in thermal properties explains the deeper-than-expected BSR in this specific region, as it forms at a lower temperature equilibrium level due to efficient heat conduction.

Dynamic evolution of gas hydrate systems affected by magmatism and Quaternary mass-transport deposits in the Qiongdongnan Basin, South China Sea, and implications for the global carbon cycle

Methane release from hydrate systems has long been considered as one of the important mechanisms for global carbon cycle perturbations and ocean acidification. Here, a Quaternary active gas hydrate system in Qiongdongnan Basin, South China Sea (SCS), has been analyzed by using seismic, logging while drilling (LWD), and core data. The results show that post-rift magmatism led to the reactivation of a magma diapir, and the faults and fractures related to its emplacement created efficient pathways for deep gas migration. The increase in overburden temperature and circulation of deep thermal fluids, especially in the center of the diapir area, thinned the gas hydrate stability zone (GHSZ). The accumulation of natural gas at the base of GHSZ and the thinning of GHSZ decreased the ability of hydrate bearing sediments to seal free gas and lead to hydraulic fracturing. The free gas entering the GHSZ was buried by mass transport deposits (MTDs) and then formed hydrates due to the overlying compaction and so further sealed the free gas under the GHSZ. This study indicates that two contrasting mechanisms are acting: (1) tectonic activity and magmatism favor the release of deep natural gas which may reach the ocean, (2) MTDs and the overlying compacted sections act as seals, inhibiting the release of such gas to the ocean. This study shows how the seal capacity of MTDs may reduce methane release from deep-seated reservoirs into the ocean. Given their widespread distribution along continental margins, we suggest that the impact of MTDs on vertical fluid flow, and potential consequences for ocean chemistry and the global carbon cycle, might be underestimated.

A Flat‐Lying Transitional Free Gas to Gas Hydrate System in a Sand Layer in the Qiongdongnan Basin of the South China Sea

AbstractMost marine gas hydrate systems follow a vertical pattern with hydrate overlying free gas. Here we document the discovery of a gas to hydrate system in a horizontal sand layer in the Qiongdongnan Basin of the South China Sea. Eight wells were drilled by the Guangzhou Marine Geological Survey in 2021–2022 to investigate the occurrence and mechanisms responsible for the formation of the system. We describe a free gas‐bearing sand reservoir at the center of the system sustained by advecting hot fluids and gas; away from the advecting zone, the cooler, surrounding sand reservoir is filled with hydrate. Observations at this site show that advective heat has a large control on hydrate formation in sands and may be a key mechanism which allows gas migration within the hydrate stability zone and the formation of high‐saturation hydrate in sand layers.

Characterising the effect of gas hydrate in pore pressure and stress using NGHP-02 downhole logs in Krishna Godavari offshore Basin, India

The present study demonstrates the impact of gas hydrate concentration and its morphology on pore pressure and sediment stress. We analyse the well-logging data of the 2nd Indian National Gas Hydrate Program (NGHP-02) in Area B of the Krishna Godavari Basin. Overpressure zones are delineated by observing porosity with depth and log inferred compaction trends. Eaton, Bower, Miller and porosity-based models are used for predicting pore pressure. The predicted pore pressures are ranging between ∼3 and 4.5 MPa, and ∼2.8 and 4.4 MPa respectively at Hole 02–22A and 02–23A, where the estimated fracture pressures and horizontal stresses are nearly equal to vertical stress at both holes. Within the gas hydrate stability zone (GHSZ), overpressure is not found; rather, our findings show that pore pressure falls with increasing gas hydrates saturation at both holes. This could be due to a decrease in water concentration during hydrate formation. Consequently, the effective stress of sediments increases, causing gas hydrate-filled vertical fractures to heave apart in the minimum horizontal stress directions. We observe overpressure below the gas hydrate layers at two holes, which is due to the load of the overlying sediments, low permeability created by the high concentration of gas hydrate at the bottom of GHSZ (about 40% and 80% at Hole 02–22A and 02–23A, respectively), and may be the microbial gas being produced at a faster rate than they are being expelled due to the low permeability. It is noticed that the strike directions of fractures vary with depth at both holes, which shows the anisotropic effects of stress conditions on gas hydrate-filled fractures during the time of formation.

Permafrost trapped natural gas in Svalbard, Norway

Permafrost is widespread in the High Arctic, including the Norwegian archipelago of Svalbard. The uppermost permafrost intervals have been well studied, but the processes at its base and the impacts of the underlying geology have been largely overlooked. More than a century of coal, hydrocarbon, and scientific drilling through the permafrost in Svalbard shows that accumulations of natural gas trapped at the base of permafrost are common. These accumulations exist in several stratigraphic intervals throughout Svalbard and show both thermogenic and biogenic origins. The gas, combined with the relatively young permafrost age, is evidence of ongoing gas migration throughout Svalbard. The accumulation sizes are uncertain, but one case demonstrably produced several million cubic metres of gas over 8 years. Heavier gas encountered in two boreholes on Hopen may be situated in the gas hydrate stability zone. While permafrost is demonstrably ice-saturated and acting as seal to gas in lowland areas, in the highlands permafrost is more complex and often dry and permeable. Svalbard shares a similar geological and glacial history with much of the Circum-Arctic, suggesting that sub-permafrost gas accumulations are regionally common. With permafrost thawing in the Arctic, there is a risk that the impacts of releasing of methane trapped beneath permafrost will lead to positive climatic feedback effects.

Long-distance migration and venting of methane from the base of the hydrate stability zone

Marine methane hydrate is an ice-like substance that is stable in sediment around marine continental margins where water depths are greater than ~450–700 m. The release of methane due to melting of hydrates is considered to be a mechanism for past global carbon-cycle perturbations and could exacerbate ongoing anthropogenic climate change. Increases in bottom-water temperature at the landward limit of marine hydrate around continental margins, where vulnerable hydrate exists at or below the seabed, cause methane to vent into the ocean. However, this setting represents only ~3.5% of the global hydrate reservoir. The potential for methane from hydrate in deeper water to reach the atmosphere was considered negligible. Here we use three-dimensional (3D) seismic imagery to show that, on the Mauritanian margin, methane migrated at least 40 km below the base of the hydrate stability zone and vented through 23 pockmarks at the shelf break, probably during warmer Quaternary interglacials. We demonstrate that, under suitable circumstances, some of the 96.5% of methane bound in deeper water distal hydrates can reach the seafloor and vent into the ocean beyond the landward limit of marine hydrate. This reservoir should therefore be considered for estimating climate change-induced methane release during a warming world.

Seismic characterisation of multiple BSRs in the Eastern Black Sea Basin

Long offset seismic reflection data reveal the presence of four Bottom Simulating Reflectors (BSR0-3) within folded sediments of the Tuapse Trough, along the NE margin of the Eastern Black Sea Basin (EBSB). Multiple BSRs are observed in other sites worldwide, however, their origin and formation mechanisms are still debated. Here, we investigate the formation mechanisms of the EBSB multiple BSRs based on their seismic character and on their physical properties derived from reflected and refracted arrival seismic velocities. Seismic reflection data are downward continued to enhance refracted arrivals. A 2D travel-time velocity model of the sub-seabed, using combined travel-times from non-downward-continued reflected and downward-continued refracted signals, shows variations in the physical properties at the BSRs and nearby sediments. The P-wave velocity (VP) increase of 1.55–1.72 km/s between the seafloor and BSR0 (258 mbsf) reflects normal compaction trends in sediments, whereas the VP of 1.75–1.83 km/s between BSR0 and BSR1 (360 mbsf) is higher than that expected for sediments at that depth. Beneath BSR1, a VP decrease from 1.83 km/s to 1.61 km/s occurs within a 70-80 m-thick layer including BSR2 (395 mbsf) and extending to BSR3 (438 mbsf). Beneath BSR3, VP increases. Based on an analytical model linking seismic velocity to physical properties, these VP trends can be explained by a gas hydrate saturation from 0 to 2% between the seafloor and BSR0, reaching 4 ± 2% just above BSR1. A free gas saturation of up to 20–25% is estimated within the low-velocity zone between BSR1 and BSR3. BSR1 likely represents the present-day base of the gas hydrate stability zone (BGHSZ), which aligns with the theoretical BGHSZ assuming a geothermal gradient of 26–30 °C/km. Based on seismic polarities and results from travel-time analysis and rock physics modelling, we suggest that hydrate dissociation and recycling processes may explain the negative polarity of BSR2 and BSR3, which are still visible due to the presence of relict gas, and inferred higher gas hydrate saturations close to the present-day base of the stability zone at BSR1. Also, structural and stratigraphic controls seem to have favoured focused free gas flow and hydrate formation at the top of an anticlinal structure, thus likely controlling multiple BSR generation in the EBSB.

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.

ИЗУЧЕНИЕ ВОЗМОЖНОСТИ ГИДРАТООБРАЗОВАНИЯ В ПЛАСТАХ СЕВЕРО-ЗАПАДНОЙ ЧАСТИ ИНДИГИРО-ЗЫРЯНСКОГО ПРОГИБА

The paper gives a brief description of geological section of the Indigirka-Zyryanka trough, promising for oil and gas, and the results of parametric drilling. Possibility study of hydrate formation in reservoir conditions of trough as one of the reasons for obtaining negative well test results was conducted. Performed calculations of the equilibrium conditions of hydrate formation have determined that the hydrate stability zone covers an insignificant interval of deposits 222 m thick below the bottom of permafrost rocks. It is concluded that the main reason for the insignificance of the hydrate formation zone is a relatively high geothermal gradient, which causes sufficiently high reservoir temperatures in the section. It is noted that favorable conditions for the formation of gas hydrates may arise in the presence of heavy differential pressure drawdown on the formation in the vicinity of borehole.

Low temperature and high pressure dramatically thicken the gas hydrate stability zone in rapidly formed sedimentary basins

Rapid sedimentation reduces the temperature and raises the pore pressure in sedimentary basins. During rapid sedimentation (>0.5 mm yr−1), cold sediment is buried fast and there is insufficient heat flow to keep the sediment at its steady state conductive equilibrium temperature. In addition, rapid deposition of low permeability mud results in overpressure due to the inability of the pore fluid to drain. It dramatically expands the thickness of the zone where hydrates are stable (the gas hydrate stability zone or GHSZ). We explore this effect with one-dimensional models. We then simulate the two-dimensional evolution of temperature and pressure of the Terrebonne Basin in the Gulf of Mexico. We use seismic, well data, and salt restoration to provide the initial and boundary conditions. We show that rapid burial reduces the geothermal gradient from ∼30 °C/km, which would be expected under equilibrium pressure and temperature, to as low as ∼10 °C/km; we also show that 25 MPa of overpressure is developed. This deepens the thickness of the GHSZ from ∼600 mbsf (its equilibrium depth) at the basin margin, to as much as 2000 mbsf basin-ward, in response to the increasing sedimentation rates. The model successfully simulates the deepening of the base of GHSZ that is interpreted from a bottom simulating reflection in the seismic data. The model and the observations suggest that the thickness of the GHSZ may be much thicker than commonly presumed and as a result the volume of carbon stored may be underestimated. Furthermore, the thickness of the hydrate stability will change significantly with time as sedimentation rate waxes and wanes implying that the hydrate stability zone is a dynamic component of the carbon cycle.

Submarine slope failures due to gas hydrate dissociation and degassing along the edge of gas hydrate stability zone in the Krishna Godavari basin

Submarine slope failures due to gas hydrate dissociation and degassing along the edge of gas hydrate stability zone in the Krishna Godavari basin

Experimental investigation on the stability of gas hydrates under near-wellbore conditions during CO2 injection for geologic carbon storage

Carbon capture and storage (CCS) is one of the key technologies in reducing the CO2 concentration in the atmosphere. The injection of CO2 into depleted reservoirs, saline aquifers and salt caverns to permanently store it in geologic formations is called geologic carbon storage (GCS).In this paper, the focus is set on the possible hydrate formation due to cooling while injecting CO2 in depleted gas reservoirs (DGR) for GCS, in which the reservoir pressure is so low that the pressure difference between injected fluid and reservoir at sand face can result in operational challenges. The temperature reduction due to abrupt pressure reduction near the wellbore, caused by the so-called Joule-Thompson (J-T) effect, plays a crucial role in the design of CO2 injection. Detection of hydrate in the early stage (equilibrium point) as well as the metastable zone lead to avoid interruption of the CO2 injection process due to plugging effects in the near wellbore area.Therefore, this study focused on the formation and equilibrium conditions as well as estimation of the metastable zone of CO2 hydrates formed in brine of different concentrations and at different CO2 phases. An experimental setup was build up and presented. Moreover, the main component of gas reservoirs, CH4, was taken into account and hydrates from mixed gas spheres were investigated. Also N2 and O2 were added to CO2 in small amounts because these gases can be introduced from the surface CO2 sequestration process as impurities. The studies were carried out in a temperature range of (266–284) K and in a pressure range of about (2.5–9.0) MPa adapted from the project field specifications. A good agreement of the equilibrium data with the predicted date from HYDOFF program by Sloan in all cases studied was found. The novelty of the present study is the investigation of the gas hydrate formation points (at cooling rates of 0.45 K/min) to estimate the metastable zone of gas hydrate. For all tests, the metastable range was quantified. Possible subcooling before gas hydrate formation was found to be between (1.4–9.0) K. This is an important outcome of the study as this implies that CO2 injection without gas hydrate forming seems to be still possible although operating conditions are already submerged into the GHSZ.Additionally, it was observed that the salinity of the water phase has significantly higher influence on the gas hydrate formation and dissociation than the components of gas mixture. Nevertheless, gas impurities (N2, O2) act as retardant in contrast to CH4 which doesn't affect the CO2 hydrate stability zone significantly.

In situ bottom sediment temperatures in the Siberian Arctic seas: Current state of subsea permafrost in the Kara sea vs laptev and East Siberian seas

Features of sediment temperature on the shelf and continental slope areas of the Russian Arctic seas and its physical properties are important for understanding the current state of subsea permafrost and the gas hydrates stability zone. New data are reported for the Kara Sea region where the bottom sediment temperatures are influenced by warming effects from great Siberian rivers and the Atlantic currents. The data collected during marine expeditions in 2019–2022 are combined with results of earlier marine studies, drilling operations, and geophysical surveys in the Laptev and East Siberian seas, in order to identify major trends of in situ temperature and properties distribution of bottom sediments in the Russian Arctic region.Most (85%) of bottom sediments in the Kara Sea shelf, as well as in the Laptev and East Siberian shelves, consist of water-saturated silty clay and silt with rather uniform particle size distribution. The obtained thermal conductivity and heat capacity values for the Kara Sea sediments agree with the values of 1.0 W/(m·K) and 2900 kJ/m3, respectively, obtained previously from other Arctic seas. Thermal conductivity becomes up to 40% higher depthward from 0 to 2 m subbottom depth, possibly, because of lower moisture content and porosity in more lithified sediments.The bottom sediment temperatures in the Arctic seas are distributed unevenly, especially in the Kara Sea shelf (from +5.0 °C in the west to −1.4 °C in the east), where the high sediment temperatures in the western and central parts of the Kara Sea being due to the effect of warm water inputs. The distribution of bottom sediment temperatures correlates well with distribution of relic subsea permafrost. Ice-bearing permafrost in the Siberian Arctic shelf extends from the shoreline till sea depths of 80–100 m, within the respective offshore distances of ∼800–1000 km in some areas, but permafrost remnants may exist locally at sea depths within 120 m. Buried 100–600 m thick continuous subsea permafrost may occur in the Kara, Laptev, and East Siberian shelves under unfrozen (cryotic) saline shallow sediments. However, subsea permafrost is discontinuous and sporadic at sea depths ∼70 m and more. Thus, the bottom sediment temperature features in the Arctic seas can be used as a proxy of subsea permafrost extent contenting intrapermafrost and subpermafrost gas and gas hydrate accumulations.

Diving deeper into seep distribution along the Cascadia convergent margin, United States

Previous margin-wide studies of methane seep distribution along the Cascadia Subduction Zone indicate peaks in seep density within the landward limit of the of gas hydrate stability zone (GHSZ; ≤500 m depth), suggesting a link between current ocean warming, acceleration of hydrate dissociated, and methane emissions. This inferred connection, however, may not account for regional geologic and/or structural complexities driving methane seepage. Expanding upon an existing seep database by adding new seep data, we conducted statistical and spatial analyses to determine margin-wide distribution trends and offer a tectonic framework for understanding the tendency toward non-normality and spatial clustering. We then highlight the role of local-scale drivers of seep formation in addition to the first-order tectonic framework, using systematic geologic/geomorphic characterization of seep emission sites in southern Cascadia and case studies using meta-attribute analysis of seismic reflection data. Seep distribution along the margin is non-random, but instead of clustering along the 500-m isobath, regions of high seep density occur in canyons and topographic highs. New findings from this study conclude that co-location of the outer arc high (OAH) and the landward limit of the GHSZ may explain high concentrations of seeps where deformation is the greatest and hydrates are unstable. Detailed analysis of the spatial relationships between seep sites and geologic-geomorphic features in southern Cascadia reveal a link between seeps and anticlines, with 52% of the seeps found in association with anticlines, 36% found at faults, 16% associated with canyons, and 11% at seafloor failure scarps. Given that a majority of anticlines are located along or seaward of the OAH in the actively deforming outer wedge, we suggest that the location of the OAH is a primary structural control on seep distribution. This scenario is supported by neural network analysis of multichannel seismic data revealing zones of probable fluid migration along vertical pipes, faults, and chimneys in the vicinity of active seep sites on anticlines. Determining linkages between seeps and submarine tectonic geomorphology is a crucial first step for understanding and forecasting the distribution of methane seepage, but also a necessity for evaluating causal relationships between ocean warming and gas hydrate stability.

MRI insight on multiphase flow in hydrate-bearing sediment and development mechanism of hydrate seal

Gas and water migration through the hydrate-bearing sediment are characteristic features in marine gas hydrate reservoirs worldwide. However, there are few experimental investigations on the effect of water-gas flow on the gas hydrate reservoir. In this study, gas-water migration in gas hydrate stability zone (GHSZ) was investigated visually employing a high-resolution magnetic resonance imaging (MRI) apparatus, and the formation of hydrate seal was experimentally investigated. Results revealed that normal flow of gas-water at the low flow rate of 1–0.25 mL/min will induce the hydrate reformation. Conversely, higher gas-water flow rates (at 2–0.5 and 4–1 mL/min) need higher reservoir pressure to induce the hydrate reformation. In addition, the hydrate reformation during the gas-water flow process produced the hydrate seal, which can withstand an over 9.0 MPa overpressure. This high overpressure provides the development condition for the underlying gas and/or water reservoir. A composite MRI image of the whole hydrate seal was obtained through the MRI. The pore difference between hydrate zone and coexistence zone produces a capillary sealing effect for hydrate seal. The hydrate saturation of hydrate seal was more than 51.6%, and the water saturation was more than 19.3%. However, the hydrate seal can be broken through when the overpressure exceeded the capillary pressure of the hydrate seal, which induced the sudden drop of reservoir pressure. This study provides a scientific explanation for the existence of high-pressure underlying gas below the hydrate layer and is significant for the safe exploitation of these common typical marine hydrate reservoirs.

Authigenic Carbonate Precipitation at Yam Seep Controlled by Continuous Fracturing and Uplifting of Four‐Way Closure Ridge Offshore SW Taiwan

AbstractHydrocarbon seeps are common manifestations of gas leakage from the seafloor. However, the fate of methane seepage within the gas hydrate stability zone at active margins is poorly constrained. This study presents a 40‐thousand‐year record of hydrocarbon seepage archived by a ∼5‐m long core composed of authigenic carbonate from the Yam Seep area, Four‐Way Closure Ridge off SW Taiwan. Different carbonate microfacies could be distinguished: Consolidated microcrystalline aragonite representing lithified host sediments intercalated by pure aragonite present in 10–50 cm thick intervals in the core. These aragonite intervals are interpreted as having precipitated within former fractures in the host rock. High resolution U‐Th dating of these aragonites is interpreted to record the minimum age of the opening of these fractures. The chronology of aragonite precipitation throughout the core suggests a record of continuous seepage from ∼41 to 2 ka that fluctuated in intensity over this time period. The chronology of putative fracturing events and observed carbonate precipitation suggest (a) an active period of fracturing and seepage from ∼37 to 27 ka, (b) a more quiescent period from ∼27 to 16 ka, followed by (c) another active period from ∼16 to 12 ka. A schematic model illustrates the evolution of carbonate formation within the core influenced by faulting, fracturing, erosion, gas hydrate accumulation, and aragonite precipitation and provides a unique 40,000‐year‐old record of methane seepage and crucial insights into the dynamics of long‐term seepage systems at active margins.

Characterization of a complex sand-rich gas hydrate reservoir system in the Indian marine continental margin with downhole log and seismic data

Logging-while-drilling (LWD) and coring data were acquired in Areas A, B, C and E during the National Gas Hydrate Program Expedition 02 (NGHP-02). At Sites NGHP-02-16, −17, −20, −23 and −24 of Area B in the Indian Krishna-Godavari Basin, the gas hydrate-bearing sand-rich reservoirs directly in contact with an underlying water-saturated sediment were revealed near the seismic-inferred bottom simulating reflector (BSR) along a large anticlinal structure. Lithofacies analysis of cores show that the lower gas hydrate-bearing reservoir section at these sites is an interbedded unit with thin fine-sand layers alternating with clay-rich layers. We reanalyzed the LWD data including resistivity, compressional-wave and shear-wave acoustic velocities to assess and compare the spatial variation of sand-hosted gas hydrate and possible related free-gas accumulations. The presence of free gas-bearing sediments in direct contact with gas hydrate-bearing sediments was confirmed at Sites NGHP-02-20 and −24 along the northeastern flank of the anticline. The sand-rich reservoirs associated with the Area B anticlinal feature reveal different pore-filling constituents and variable boundary conditions, including gas hydrate/water contacts and gas hydrate/free gas contacts. We infer that the sedimentary depositional history at Sites NGHP-02-20 and −24, may have resulted in an upward shift of the base of gas hydrate stability zone, that could have led to in situ gas hydrate dissociation and the accumulation of free gas and coexistence interval below the log-inferred gas hydrate-bearing reservoir sections. This study demonstrates that reservoir heterogeneity and fluid migration were major factors affecting the distribution of gas hydrate and free gas in the identified sand-rich reservoirs.