Base Of The Gas Hydrate Stability Zone Research Articles

Dynamic accumulation of a high-grade gas hydrate system: insights from the trial production gas hydrate reservoir in the Shenhu area, northern South China Sea

The ultimate enrichment level and quantity of gas hydrate resources are influenced by the dynamic process of accumulation and preservation. High-resolution 3-D seismic data, logging while drilling (LWD), pressured coring, and in situ testing were used to characterize the dynamic accumulation and preservation of the trial production high-grade gas hydrate reservoir (HGGHR) in the Shenhu area. Through seismic variance analysis and ant-tracking, we found that newly identified mud diapir-associated faults with three development stages controlled the migration and accumulation of gas hydrate and shifted the base of the gas hydrate stability zone (BGHSZ), resulting in dynamic accumulation and dissociation of gas hydrates. The recognized double bottom simulating reflectors (BSRs) were concluded to have been formed due to the shift of the BGHSZ caused by the variational equilibrium conditions. The interval between the double BSRs was inferred to be a disequilibrium zone where gas recycling occurred, contributing to the coexistence of gas hydrates and free gas and the dynamic formation of the HGGHR. Multiple gliding faults formed within the GHSZ in the late period have altered the HGGHR and control the present thickness and distribution of the gas hydrates and free gas in the hanging wall and footwall. Under the influence of geothermal fluids and the fault system associated with the mud diapir, the HGGHR experienced dynamic accumulation with three stages, including early accumulation, medium-term adjustment, and late alteration and preservation. We conclude that four factors affected the formation, distribution, and occurrence of the HGGHR: the geothermal fluids accompanying the deep mud diapir below the reservoir, the dual supply of thermogenic gas and biogenic gas, the recycling of hydrate gas beneath the BGHSZ, and the post-gas hydrate faults developed within the GHSZ. A geological model illustrating the dynamic formation of the trial production HGGHR was proposed, providing a reference for future exploration of HGGHRs with a great production potential in deepwater settings.

Can Glacial Sea‐Level Drop‐Induced Gas Hydrate Dissociation Cause Submarine Landslides?

AbstractWe conducted two‐dimensional numerical simulations to investigate the mechanisms underlying the strong spatiotemporal correlation observed between submarine landslides and gas hydrate dissociation due to glacial sea‐level drops. Our results suggest that potential plastic deformation or slip could occur at localized and small scales in the shallow‐water portion of the gas hydrate stability zone (GHSZ). This shallow‐water portion of the GHSZ typically lies within the area enclosed by three points: the BGHSZ–seafloor intersection, the seafloor at ∼600 m below sea level (mbsl), and the base of the GHSZ (BGHSZ) at ∼1,050 mbsl in low‐latitude regions. The deep BGHSZ (>1,050 mbsl) could not slip; therefore, the entire BGHSZ was not a complete slip surface. Glacial hydrate dissociation alone is unlikely to cause large‐scale submarine landslides. Observed deep‐water (much greater than 600 mbsl) turbidites containing geochemical evidence of glacial hydrate dissociation potentially formed from erosion or detachment in the GHSZ pinch‐out zone.

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.

Sedimentary deformation relating to episodic seepage in the last 1.2 million years: a multi-scale seismic study from the Vestnesa Ridge, eastern Fram Strait

Seafloor hydrocarbon seepage is a natural fluid release process that occurs worldwide on continental shelves, slopes, and in deep oceanic basins. The Vestnesa sedimentary ridge in the eastern Fram Strait hosts a deep-water gas hydrate system that became charged with hydrocarbons ∼2.7 Ma and has experienced episodic seepage along the entire ridge until a few thousand years ago, when seepage activity apparently ceased in the west but persisted in the east. Although it has been documented that faults and fractures play a key role in feeding the seeps with thermogenic gases, the mechanisms controlling seepage periodicity remain poorly understood. Here we integrate high-resolution P-cable 3D seismic and Chirp data to investigate the spatial and temporal evolution of high-resolution fractures and fluid flow features in the west of the Vestnesa Ridge. We characterize sediment deformation using a fracture density seismic attribute workflow revealing two highly deformed stratigraphic intervals and associated small-scale pockmarks (<20 m diameter). Chronostratigraphic constraints from the region show that these two highly deformed intervals are influenced by at least three major climatic and oceanic events during the last 1.2 million years: the Mid-Pleistocene Transition (∼1.25–0.7 Ma), the penultimate deglaciation (∼130 ka) and the last deglaciation (Heinrich Stadial 1: ∼16 ka). These periods of deformation appear associated with seismic anomalies potentially correlated with buried methane-derived authigenic carbonate and have been sensitive to shifts in the boundary of the free gas-gas hydrate interface. Our results show shifts (up to ∼30 m) in the depth of the base of the gas hydrate stability zone (GHSZ) associated with major changes in ocean bottom water temperatures. This ocean-driven effect on the base of the GHSZ since the Last Glacial Maximum coincides with the already highly deformed Mid-Pleistocene Transition sedimentary interval and likely enhanced deformation and gas leakage along the ridge. Our results have implications for understanding how glacial cycles impact fracture formation and associated seepage activity.

Salt Diapir‐Driven Recycling of Gas Hydrate

AbstractBy harnessing both hypothetical, synthetic basin and gas hydrate (GH) system models and real‐world models of well‐studied salt diapir‐associated GH sites at Green Canyon (Gulf of Mexico) and Blake Ridge (U.S. Atlantic coast), we propose and demonstrate salt movement (and in particular, diapirism) to be a new mechanism for the recycling of marine GH. At Green Canyon, for example, we show that by considering this newly proposed diapir‐driven recycling mechanism in conjunction with previously proposed lithological control on sandy‐reservoir‐hosted hydrate at the base of the GH stability zone (BGHSZ; ∼bottom‐simulating reflector, BSR), modeled GH saturations match drilling data. Overall, salt diapir movement‐induced GH recycling provides a temperature‐driven mechanism by which GH saturations at the BGHSZ may reach >90 vol. % and by which GH volumes near and free gas volumes beneath the BGHSZ may be increased significantly through time. Interestingly, comparison of salt diapir‐driven recycling and sediment burial‐driven recycling scenarios suggests notably higher rates of recycling via diapir‐driven versus burial‐driven processes. Our results suggest that GH and associated free gas accumulations above salt diapir crests represent particularly attractive targets for unconventional and conventional hydrocarbon resource exploration and for scientific and academic drilling expeditions aimed at exploiting GH systems. Salt basins containing GH systems—including passive margin basins of the Gulf of Mexico, southeastern Brazil, and southwestern Africa—are therefore compelling localities for studying salt‐driven GH recycling and for salt diapir‐associated natural gas exploration.

Dissolution migration of gas, a mechanism to enrich ethane near the BSR and increase upwardly C1/C2 ratios in the hydrate-occurring zones: Insight from pore-scale experimental observation

Most experiments show that gas hydrates are often enriched in C2+ gases relative to the feeding gas source because of fractionation during hydrate crystallization directly from free gases and dissolved gases. However, sediments below and near the base of the gas hydrate stability zone (BGHSZ) in many ocean drilling program (ODP)/International Ocean Discovery Program (IODP) sites are relatively enriched in C2+ hydrocarbon gases, compared with the hydrate-occurring zone above. It is still unclear what kind of process causes the abrupt decreases in C1/C2+ ratios with the depth in headspace gas in sediments around seismic bottom-simulating reflector (BSR) and increasing upward C1/C2 ratios in the hydrate-occurring zone. To test the “dissolution/migration mechanism” and its links to the enrichment of ethane near the BSR and increasing upward C1/C2 ratios in the hydrate-occurring zone, we performed a series of pore-scale experimental observations, simulating the gas dissolution–migration–hydration processes, and investigated the effects of the composition of feeding gases and temperatures on the composition of the hydrate grown under the dissolution–migration mechanism. Hydrates are grown from aqueous fluids supplied by the migration of gases dissolved from the capillary-trapped free gas in a capillary high-pressure optical cell, with different supplying gases (90 mol% CH4 + 10 mol% C2H6, 80 mol% CH4 + 20 mol% C2H6) and a geothermal gradient (temperature from 278.15 to 293.15 K). The gas hydrate structure and composition were determined by quantitative Raman spectroscopy. Our study indicated that (1) under the dissolution–migration–hydration processes, the mole fraction of C2H6 in hydrates is depleted compared with gas sources, which confirms that the dissolution–migration of gases is a mechanism to enrich ethane near the BSR; (2) the proportion of C2H6 in structure I (sI) or structure II (sII) hydrates decreases with decreasing temperature, and decreasing temperature enlarges the difference of diffusion coefficient between methane and ethane and enhances the gas fractionation during migration, which could cause the increase upwardly C1/C2 ratios in the hydrate-occurring zone. A simplified geological model was proposed to explain the variability of hydrate composition with depth in the hydrate-occurring zone and the fractionation of gases near the BSR.

Impact of Gas Saturation and Gas Column Height at the Base of the Gas Hydrate Stability Zone on Fracturing and Seepage at Vestnesa Ridge, West-Svalbard Margin

The Vestnesa Ridge, located off the west Svalbard margin, is a >60 km long ridge consisting of fine-grained sediments that host a deep-marine gas hydrate and associated seepage system. Geological and geophysical observations indicate the predominance of vertical fluid expulsion through fractures with pockmarks expressed on the seafloor along the entire ridge. However, despite the apparent evidence for an extended free gas zone (FGZ) below the base of the gas hydrate stability zone (BGHSZ), present-day seafloor seepage has been confirmed only on the eastern half of the sedimentary ridge. In this study, we combine the relationships between aqueous phase pressure, capillary pressure, sediment clay fraction, porosity, and total stress to simulate how much gas is required to open preexisting fractures from the BGHSZ towards the seafloor. Data from four specific sites with different lithology and pressure regime along the ridge are used to constrain the simulations. Results demonstrate that fracturing is favored from the FGZ (with gas saturations < 0.1 and gas column heights < 15 m) towards the seafloor. Neglecting the capillary pressure overpredicts the size of the gas column by up to 10 times, leading to erroneous maximum gas vent volume predictions and associated ocean biosphere consequences. Further parametric analyses indicate that variations in the regional stress regime have the potential to modify the fracture criterion, thus driving the differences in venting across the ridge. Our results are in line with independent geophysical observations and petroleum system modeling in the study area, adding confidence to the proposed approach and highlighting the importance of the capillary pressure influence on gas pressure.

Influence of seafloor topography on gas hydrate occurrence across a submarine canyon-incised continental slope in the northern margin of the south china sea

In a submarine canyon-incised continental slope, the observed bottom simulation reflection (BSR) frequently appears not to match with the predicted base of the gas hydrate stability zone (BGHSZ). However, the mechanism behind this discrepancy is often debated due to the lack of in-situ borehole measurements. In the Shenhu area, a submarine canyon region on the northern continental slope of the South China Sea, a suite of datasets, including 3D seismic data, well logs and in-situ borehole temperature measurements enable us to study the gas hydrate stability zone. In-situ borehole temperature measurements reveal a nonlinear geothermal gradient within shallow 200 m-thick marine sediments. The geothermal gradient fit with the uppermost sediment interval (<50 m) neglected produces a better match between the depths of predicted BGHSZ and observed BSR. The gas hydrate stability zone (GHSZ) modeling using in-situ borehole measurement information at Site W07 shows that the predicted BGHSZ agrees well with the imaged BSR located atop an inter-canyon ridge, demonstrating that the gas hydrate system is in a steady state. Two-dimensional GHSZ modeling along seismic profiles transecting the ridge shows that the observed BSRs along the western margin of the ridge are deeper than the predicted BGHSZ. We interpret this apparent discrepancy to be attributed to topographic effects. Two scales of topographic effects, i.e. an alternating distribution of canyons and inter-canyon ridges, and localized crests and troughs over the crest of inter-canyon ridges, are revealed to influence gas hydrate system in the study area. The curved BGHSZ resulted from the topographic effect could facilitate the concentration of gas hydrate by enlarging the accumulation space and sealing free gas immediately below BGHSZ.

In-situ borehole temperature measurements confirm dynamics of the gas hydrate stability zone at the upper Danube deep sea fan, Black Sea

Coring, geophysical logging, and in-situ temperature measurements were performed with the MARUM-MeBo200 seafloor rig to characterize gas hydrate occurrences in sediments of the Danube deep sea fan, off Romania, Black Sea. The new drilling data showed no evidence for significant gas hydrate saturations within the sediments but the presence of free gas at the depth of the bottom-simulating reflector (BSR). In-situ temperature and core-derived geochemical data suggest that the current base of the gas hydrate stability zone (BGHSZ) is ∼20 m shallower than the BSR. Investigation of the seismic data around the drill sites shows several locations where free gas previously trapped at a former BGHSZ migrated upwards forming a new reflection above the BSR. This shows that the gas hydrate system in the Danube deep sea fan is still responding to climate changes initiated at the end of the last glacial maximum.

Thermal Characterization of Pockmarks Across Vestnesa and Svyatogor Ridges, Offshore Svalbard

AbstractThe Svalbard margin represents one of the northernmost gas hydrate provinces worldwide. Vestnesa Ridge (VR) and Svyatogor Ridge (SR) west of Svalbard are two prominent sediment drifts showing abundant pockmarks and sites of seismic chimney structures. Some of these sites at VR are associated with active gas venting and were the focus of drilling and coring with the seafloor‐deployed MARUM‐MeBo70 rig. Understanding the nature of fluid migration and gas hydrate distribution requires (among other parameters) knowledge of the thermal regime and in situ gas and pore fluid composition. In situ temperature data were obtained downhole at a reference site at VR defining a geothermal gradient of ~78 mK m−1 (heat flow ~95 mW m−2). Additional heat probe data were obtained to describe the thermal regime of the pockmarks. The highest heat flow values were systematically seen within pockmark depressions and were uncorrelated to gas venting occurrences. Heat flow within pockmarks is typically ~20 mW m−2 higher than outside pockmarks. Using the downhole temperature data and gas compositions from drilling we model the regional base of the gas hydrate stability zone (BGHSZ). Thermal modeling including topographic effects suggest a BGHSZ up to 40 m deeper than estimated from seismic data. Uncertainties in sediment properties (velocity and thermal conductivity) are only partially explaining the mismatch. Capillary effects due to small sediment grain sizes may shift the free gas occurrence above the equilibrium BGHSZ. Changes in gas composition or pore fluid salinity at greater depth may also explain the discrepancy in observed and modeled BGHSZ.

Detection of Gas Hydrates in Faults Using Azimuthal Seismic Velocity Analysis, Vestnesa Ridge, W‐Svalbard Margin

AbstractJoint analysis of electrical resistivity and seismic velocity data is primarily used to detect the presence of gas hydrate‐filled faults and fractures. In this study, we present a novel approach to infer the occurrence of structurally controlled gas hydrate accumulations using azimuthal seismic velocity analysis. We perform this analysis using ocean‐bottom seismic data at two sites on Vestnesa Ridge, W‐Svalbard Margin. Previous geophysical studies inferred the presence of gas hydrates at shallow depths (up to ~190–195 m below the seafloor) in marine sediments of Vestnesa Ridge. We analyze azimuthal P‐wave seismic velocities in relation with steeply dipping near‐surface faults to study structural controls on gas hydrate distribution. This unique analysis documents directional changes in seismic velocities along and across faults. P‐wave velocities are elevated and reduced by ~0.06–0.08 km/s in azimuths where the raypath plane lies along the fault plane in the gas hydrate stability zone (GHSZ) and below the base of the GHSZ, respectively. The resulting velocities can be explained with the presence of gas hydrate‐ and free gas‐filled faults above and below the base of the GHSZ, respectively. Moreover, the occurrence of elevated and reduced (&gt;0.05 km/s) seismic velocities in groups of azimuths bounded by faults suggests compartmentalization of gas hydrates and free gas by fault planes. Results from gas hydrate saturation modeling suggest that these observed changes in seismic velocities with azimuth can be due to gas hydrate saturated faults of thickness greater than 20 cm and considerably smaller than 300 cm.

The Mechanism of Methane Gas Migration Through the Gas Hydrate Stability Zone: Insights From Numerical Simulations

AbstractFree gas migration through the gas hydrate stability zone (GHSZ) and subsequent gas seepage at the seabed are characteristic features in marine gas hydrate provinces worldwide. The biogenic or thermogenic gas is typically transported along faults from deeper sediment strata to the GHSZ. Several mechanisms have been proposed to explain free gas transport through the GHSZ. While inhibition of hydrate formation by elevated salinities and temperatures have been addressed previously in studies simulating unfocused, area‐wide upward advection of gas, which is not adequately supported by field observations, the role of focused gas flow through chimney‐like structures has been underappreciated in this context. Our simulations suggest that gas migration through the GHSZ is, fundamentally, a result of methane gas supply in excess of its consumption by hydrate formation. The required high gas flux is driven by local overpressure, built up from gas accumulating below the base of the GHSZ that fractures the overburden when exceeding a critical pressure, thereby creating the chimney‐like migration pathway. Initially rapid hydrate formation raises the temperature in the chimney structure, thereby facilitating further gas transport through the GHSZ. As a consequence, high hydrate saturations form preferentially close to the seafloor, where temperatures drop to bottom water values, producing a prominent subsurface salinity peak. Over time, hydrates form at a lower rate throughout the chimney structure, while initial temperature elevation and salinity peak dissipate. Thus, our simulations suggest that the near‐surface salinity peak and elevated temperatures are a result of transient high‐flux gas migration through the GHSZ.

Geophysical characterisation of active thermogenic oil seeps in the salt province of the lower Congo basin part I: Detailed study of one oil-seeping site

Abstract We report the geophysical characterisation of natural oil seep sites through a combination of sea surface evidence of oil leakage from spatial imagery with a large collection of seafloor and subsurface geophysical data. This paper provides a detailed characterisation of one selected active seep site and identifies possible specific feature of oil seep sites. The oil seep is a complex-shaped feature on the seafloor consisting of a cluster of heterometric pockmarks inside a main depression area and peripheral metre-scale seafloor mounds. A strong deformation related to salt tectonics controls the location of the seafloor source by fracturing the overburden. The associated thermal anomaly induces a vertical modification position of the base of the gas hydrate stability zone (BGHSZ) that is used as a fluid migration route towards the crest of the diapir. The combination of local depressions and seafloor amplitude anomalies linked with vertical high-amplitude pipes rooted on the BGHSZ suggests a focused fluid flow towards the seafloor. In peripheral areas, the seafloor mounds are linked by shallow faults to buried high amplitude patches on sub-bottom profiler sections. The combination of restricted-size seafloor mounds with a progressive deepening of the high amplitude from the seafloor suggests a substantial decrease of the hydrocarbon flow towards peripheral areas. The proximity of actively oil-supplying seafloor depressions and seafloor mounds shows that the hydrocarbon flow rapidly decreases laterally. The thermogenic seep site is affected by two consistent and sub-parallel reflections with negative polarity. The first is interpreted as the methane-related BGHSZ, the second could correspond to the base of a thermogenic BGHSZ produced by a mixture of heavier gas. The seafloor roughness and double BSR appear to be specific features of oil seep sites. The geophysical features revealed at a localised study area will be extrapolated towards a larger province for relevance validation.

Numerical simulations of depressurization-induced gas production from an interbedded turbidite gas hydrate-bearing sedimentary section in the offshore India: Site NGHP-02-16 (Area-B)

Abstract The recent National Gas Hydrate Program Expedition 02 (NGHP-02) identified the existence of gas hydrate-bearing sand reservoirs at a number of sites in the offshore of India including Site NGHP-02-16 in Area-B of the Krishna-Godavari Basin. The architecture of that gas hydrate accumulation is characterized by thin, gas hydrate-bearing, high quality sand layers interbedded with mud layers within a turbidite sedimentary interval. The lowest gas hydrate-bearing layer contacting a thinly-interbedded saline aquifer designates the base of the gas hydrate stability zone (BGHSZ). The proximity of the BGHSZ and the average temperature around 20 °C make the reservoir a favorable target for hydrate destabilization by means of the depressurization method. The results of the reservoir simulations indicate high gas production potential from this marine gas hydrate deposit with manageable concomitant water production using a well completion design that hydraulically isolates layers with water-saturated sands. Using a detailed geological input model, the predicted cumulative gas rates reach ∼3.1 × 104 m3/day (∼1.1 MMscf/day) after 90 days of continuous depressurization and demonstrate sustained production rates ∼3.0 × 104 m3/day (∼1.0 MMscf/day) after 5 years of production. The interbedded nature of this gas hydrate occurrence promotes the development of horizontal dissociation interfaces between gas hydrate-bearing sand and mud layers. As a result, non-uniform gas production along the horizontal interfaces becomes a primary determinant of reservoir performance. Simulation cases have been executed to determine the impact of the uncertainty in in situ reservoir permeability and the manner in which intrinsic permeability dynamically changes during dissociation in response to the imposed effective stress increase. The cases based on low initial effective permeability and high sensitivity of compaction to stress result in the least favorable production predictions.

Mapping the resistivity structure of Walker Ridge 313 in the Gulf of Mexico using the marine CSEM method

A marine controlled source electromagnetic (CSEM) campaign was carried out in the Gulf of Mexico to further develop marine electromagnetic techniques in order to aid the detection and mapping of gas hydrate deposits. Marine CSEM methods are used to obtain an electrical resistivity structure of the subsurface which can indicate the type of substance filling the pore space, such as gas hydrates which are more resistive. Results from the Walker Ridge 313 study (WR 313) are presented in this paper and compared with the Gulf of Mexico Gas Hydrate Joint Industry Project II (JIP2) logging while drilling (LWD) results and available seismic data. The hydrate, known to exist within sheeted sand deposits, is mapped as a resistive region in the two dimensional (2D) CSEM inversion models. This is consistent with the JIP2 LWD resistivity results. CSEM inversions that use seismic horizons provide more realistic results compared to the unconstrained inversions by providing sharp boundaries and architectural control on the location of the resistive and conductive regions in the CSEM model. The seismic horizons include: 1) the base of the gas hydrate stability zone (BGHSZ), 2) the top of salt, and 3) the top and bottom of a fine grained marine mud interval with near vertical hydrate filled fractures, to constrain the CSEM inversion model. The top of salt provides improved location for brines, water saturated salt, and resistive salt. Inversions of the CSEM data map the occurrence of a ‘halo’ of conductive brines above salt. The use of the BGHSZ as a constraint on the inversion helps distinguish between free gas and gas hydrate as well as gas hydrate and water saturated sediments.

Mapping gas hydrate and fluid flow indicators and modeling gas hydrate stability zone (GHSZ) in the Ulleung Basin, East (Japan) Sea: Potential linkage between the occurrence of mass failures and gas hydrate dissociation

The Ulleung Basin, East (Japan) Sea, is well-known for the occurrence of submarine slope failures along its entire margins and associated mass-transport deposits (MTDs). Previous studies postulated that gas hydrates which broadly exist in the basin could be related with the failure process. In this study, we identified various features of slope failures on the margins, such as landslide scars, slide/slump bodies, glide planes and MTDs, from a regional multi-channel seismic dataset. Seismic indicators of gas hydrates and associated gas/fluid flow, such as the bottom-simulating reflector (BSR), seismic chimneys, pockmarks, and reflection anomalies, were re-compiled. The gas hydrate occurrence zone (GHOZ) within the slope sediments was defined from the BSR distribution. The BSR is more pronounced along the southwestern slope. Its minimal depth is about 100 m below seafloor (mbsf) at about 300 m below sea-level (mbsl). Gas/fluid flow and seepage structures were present on the seismic data as columnar acoustic-blanking zones varying in width and height from tens to hundreds of meters. They were classified into: (a) buried seismic chimneys (BSC), (b) chimneys with a mound (SCM), and (c) chimneys with a depression/pockmark (SCD) on the seafloor. Reflection anomalies, i.e., enhanced reflections below the BSR and hyperbolic reflections which could indicate the presence of gas, together with pockmarks which are not associated with seismic chimneys, and SCDs are predominant in the western-southwestern margin, while the BSR, BSCs and SCMs are widely distributed in the southern and southwestern margins. Calculation of the present-day gas-hydrate stability zone (GHSZ) shows that the base of the GHSZ (BGHSZ) pinches out at water depths ranging between 180 and 260 mbsl. The occurrence of the uppermost landslide scars which is below about 190 mbsl is close to the range of the GHSZ pinch-out. The depths of the BSR are typically greater than the depths of the BGHSZ on the basin margins which may imply that the GHOZ is not stable. Close correlation between the spatial distribution of landslides, seismic features of free gas, gas/fluid flow and expulsion and the GHSZ may suggest that excess pore-pressure caused by gas hydrate dissociation could have had a role in slope failures.

Assessment of gas hydrate saturation in marine sediments from resistivity and compressional-wave velocity log measurements in the Mahanadi Basin, India

Gas hydrate was recovered in the Mahanadi Basin along the eastern continental margin of India during the India National Gas Hydrate Program (NGHP) Expedition-01 in 2006. Infrared imaging of recovered core confirmed gas hydrate occurs predominantly in discrete layers. Pore-water chemistry, electrical resistivity and acoustic velocity down-hole logs were used to estimate gas hydrate saturations at three of the sites in the Mahanadi Basin: Sites NGHP-01-08, -09, and -19. Gas hydrate saturation estimated from pore-water chloride concentrations shows values up to ∼10% of the pore space at ∼200 m below seafloor just above the base of the gas hydrate stability zone (BGHSZ). Gas hydrate saturations estimated from electrical resistivity and acoustic velocity logs using standard relations and modeling approaches are comparable to each other and saturations are ∼10–15% of the pore space. Seismic reflection data were also analyzed for the evidence of gas hydrate, and a bottom-simulating reflector (BSR) was imaged along the seismic profiles in the study area. The depth of the BSR is varying from ∼200 m to ∼300 m below seafloor depending on water depth in the Mahanadi Basin. The occurrence of gas hydrate was observed to be associated with deep-water channel and levee complexes (especially at Site NGHP-01-19) based on the regional seismic data. But the cored/logged section at each site lacked any significant sand fraction, which does not allow for higher gas hydrate saturations. As identified from seismic time-slice data, all sites drilled in the Mahanadi Basin are within the steeper slope region of the channel system and any sand bypassed this region. Significant sand deposition would occur further down slope where typical fan-type deposits can be inferred from the seismic data and thus higher accumulations of gas hydrate would be expected.

Seismic facies analyses as aid in regional gas hydrate assessments. Part-II: Prediction of reservoir properties, gas hydrate petroleum system analysis, and Monte Carlo simulation

A new strategy to assess occurrences of marine gas hydrate has been implemented for the study area of the Ulleung Basin, East Sea, where two comprehensive gas hydrate drilling programs completed logging and coring at 18 sites. In this study we introduce multi-attribute analyses using five drill sites along a regional 2D seismic line to predict physical properties of P-wave velocity, density, and porosity required for gas hydrate saturation calculations. The five well-sites allow a reasonable cross-validation of the final multi-attribute-based prediction and to derive statistical parameters used in a Monte Carlo simulation of total gas hydrate volume in the study area. A similar analysis was completed for a 3D seismic volume around Site UBGH1-4. The crucial boundaries of the top of gas hydrate occurrence zone (TGHOZ) and base of the gas hydrate stability zone (BGHSZ) that are part of the gas hydrate petroleum system were calculated using constraints on geothermal gradient, pore-fluid salinity, type of hydrocarbon gases present, sedimentation rates, and total organic carbon content based on results from the two drilling expeditions carried out in the Ulleung Basin as well as other regional studies. Gas hydrate saturations were determined following the effective medium theory and using the sedimentological data obtained at the drill sites to define required mineral compositions. Seismic facies classification allowed the definition of occurrence of certain sediment types along the seismic data used in this study, and physical properties from the multi-attribute analyses and mineral compositions were assigned to each facies class. Statistical parameters of mean and standard deviation were estimated for each input parameter (P-wave velocity, bulk density, mineral compositions, grain density, density-porosity, TGHOZ, BGHSZ) in the gas hydrate saturation calculations based on individual uncertainties in the measurements. In the final step, a Monte Carlo simulation is used to calculate the total amount of gas hydrate (in m3) present in the study region for the two example data set of a 2D seismic line and one 3D seismic volume. The Monte Carlo simulation draws on the various statistical distributions of the input parameters and we have implemented 150,000 simulation runs in this study. The final histogram distributions of total volume of gas hydrate (in m3) allow determination of the mean, median, and mode values, as well as the 95% and 5% probability thresholds for gas hydrate volumes being present in the two study regions, though they are not meant as estimates of possible commercial amounts of gas associated with gas hydrates in the Ulleung Basin.

Estimation of the gas-hydrate resource volume in a small area of the Ulleung Basin, East Sea using seismic inversion and multi-attribute transform techniques

We estimated the volume of gas-hydrate and in-place gas in a small (37 km × 58 km) area of the Ulleung Basin, East Sea from a dense grid of 2-D seismic and logging-while-drilling (LWD) data, using seismic inversion and multi-attribute transform techniques. Multi-attribute transform finds the relationship between any measured or calculated logs and the combination of the seismic attributes and the acoustic impedance computed from inversion. We assumed that the bottom-simulating reflector marks the base of the gas-hydrate stability zone (GHSZ). First, the pore-space gas-hydrate saturation at the wells was estimated from the simplified three-phase Biot-type equation. Then, the porosity and the pore-space gas-hydrate saturation along the seismic lines were predicted from multi-attribute transform. The GHSZ was divided into ten layers of the equal time thickness. The time thickness of each layer was converted into depth, using the time–depth relationship constructed from seismic-to-well tie at the wells, and gridded at 500-m cell size. The average porosity and pore-space gas-hydrate saturation were computed for each layer and multiplied to obtain the average total gas-hydrate saturation which was gridded with the same cell size as the thickness grid. Thus, each 2-D cell is represented by a rock volume and an average total gas-hydrate saturation. The gas-hydrate volume for each cell was computed by the multiplication of the cell rock volume and the average total gas-hydrate saturation. Finally, the total gas-hydrate volume was computed by summing the gas-hydrate volumes of all cells. The estimated gas-hydrate and in-place gas volumes in the study area are about 3.43 × 109 m3 (1.21 × 1011 ft3) and about 4.50 × 1011 m3 (1.59 × 1013 ft3), respectively. The more conservative estimates, excluding the top three layers that comprise about 50 m of the near-seafloor sediments where the LWD data are often unreliable, are 3.03 × 109 m3 (1.07 × 1011 ft3) for gas hydrate and about 3.97 × 1011 m3 (1.40 × 1013 ft3) for gas.

IODP Expedition 328: Early Results of Cascadia Subduction Zone ACORK Observatory

Abstract. Integrated Ocean Drilling Program (IODP) Expedition 328 was devoted to the installation of an Advanced CORK (Circulation Obviation Retrofit Kit) in the Cascadia subduction zone accretionary prism to observe the physical state and properties of the formation as they are influenced by long-term and episodic deformation and by gas hydrate accumulation. Pressures are monitored at four levels on the outside of a standard 10 3/4-inch casing string, two above and two below the base of the gas-hydrate stability zone at 230 mbsf (m below seafloor). The casing was sealed at the bottom, leaving the inside open down to 302 mbsf for installation of a tilt meter, seismometer, and thermistor cable (scheduled for 2013). The initial data, recovered in July 2011, document an initially smooth recovery from the drilling perturbation followed by what may be a sequence of hole-collapse events. Pressure at the deepest screen is roughly 40 kPa above hydrostatic; higher pressures (80 kPa) are observed at the two screens close to the level of hydrate stability. Tidal variations at the deepest screen are in phase with ocean tides, and define a loading efficiency of 0.6, which is reasonable in light of the consolidation state of the for-mation (porosity ~0.5). Tidal signals near the level of gas hydrate stability display large phase lags, probably as a consequence of hydraulic diffusion stimulated by the large contrast in interstitial fluid compressibility at the gas-hydrate boundary. The degree of isolation among the screens, the anticipated good coupling, and the estimated strain-to-pressure conversion efficiency (~5 kPa μstrain−1) indicate that this installation will serve well to host a variety of hydrologic, seismic, and geodynamic experiments. doi: 10.2204/iodp.sd.13.02.2011