Gas Hydrate Saturation Research Articles

Gas hydrate distribution and carbon sequestration through coupled microbial methanogenesis and silicate weathering in the Krishna–Godavari Basin, offshore India

The National Gas Hydrate Program Expedition 01 cored ten sites in the Krishna–Godavari basin, located on the southeastern margin of India. A comprehensive suite of pore water solute concentrations and isotope ratios were analyzed to investigate the distribution and concentration of gas hydrate along the margin, in situ diagenetic and metabolic reactions, and fluid migration and flow pathways. Gas hydrate was present at all of the sites cored, and in situ microbial methanogenesis leads to estimates of depth-integrated average gas hydrate saturations that are typically <5%. Deep-sourced fluid and gas migration produces gas hydrate saturations up to 68% along an isolated coarser-grained stratigraphic horizon at Site 15 and up to 41% within a fractured clay-dominated system at Site 10. Our results show that the CO2 produced through net microbial methanogenesis is effectively neutralized by silicate weathering throughout the sediment column drilled at each site (∼100–300 m), buffering the pore water pH and generating excess alkalinity via the same reaction sequence as continental silicate weathering. Most of the excess alkalinity produced by silicate weathering in the Krishna–Godavari basin is sequestered in Ca- and Fe-carbonates as a result of ubiquitous calcium release from weathering detrital silicates and dissolved Fe production within the methanogenic sediments. Formation of secondary hydrous silicates (e.g. smectite) related to incongruent primary silicate dissolution acts as a significant sink for pore water Mg, K, Li, Rb, and B. The consumption of methane through anaerobic oxidation of methane, sequestration of methane in gas hydrate, and sequestration of dissolved inorganic carbon in authigenic carbonates keeps methanogenesis as a thermodynamically feasible catabolic pathway. Our results combined with previous indications of silicate weathering in anoxic sediments in the Sea of Okhotsk, suggest that silicate weathering coupled to microbial methanogenesis should be occurring in continental margins worldwide, providing a net sink of atmospheric CO2.

New method for estimating gas hydrate saturation in the Shenhu area

Accurately predicting the saturation of the gas hydrate is one of the most critical issues for reserves calculation. At present, empirically geologic and physical constraints dependency equations have been developed to estimate hydrate saturation. However, the models derived from the equations are too simplistic to imitate the complicated submarine-sediments system. We have developed a new method for gas hydrate saturation predictions in the Shenhu area of the South China Sea. This method is different from traditional ones. It is dependent on a statistical estimator of data/samplings. The method solves the inverse problem for which a database consisting of pairs of inputs and corresponding outputs is required. This method uses radial-basis-function (RBF) interpolation to derive a nonlinear mapping function between P-wave velocity increment (database input) and gas hydrate saturation (database output). Such a mapping function is then used for predicting the output when input is known in other places. We defined the velocity increment as the amount of actual velocity deviated from background velocity. The parameters of the interpolating function were calibrated using the database. The test results of this method on well SH2 and well SH7 in the Shenhu area were quite satisfying. The RBF interpolation method is a fast and efficient way for gas hydrate saturation prediction, and it allowed us to bypass complicated physical models’ inference.

The impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments

AbstractGas hydrate and free gas accumulation in heterogeneous marine sediment is simulated using a two‐dimensional (2‐D) numerical model that accounts for mass transfer over geological timescales. The model extends a previously documented one‐dimensional (1‐D) model such that lateral variations in permeability (k) become important. Various simulations quantitatively demonstrate how focused fluid flow through high‐permeability zones affects local hydrate accumulation and saturation. Simulations that approximate a vertical fracture network isolated in a lower permeability shale (kfracture &gt;&gt; kshale) show that focused fluid flow through the gas hydrate stability zone (GHSZ) produces higher saturations of gas hydrate (25–70%) and free gas (30–60%) within the fracture network compared to surrounding shale. Simulations with a dipping, high‐permeability sand layer also result in elevated saturations of gas hydrate (60%) and free gas (40%) within the sand because of focused fluid flow through the GHSZ. Increased fluid flux, a deep methane source, or both together increase the effect of flow focusing upon hydrate and free gas distribution and enhance hydrate and free gas concentrations along the high‐permeability zones. Permeability anisotropy, with a vertical to horizontal permeability ratio on the order of 10−2, enhances transport of methane‐charged fluid to high‐permeability conduits. As a result, gas hydrate concentrations are enhanced within these high‐permeability zones. The dip angle of these high‐permeability structures affects hydrate distribution because the vertical component of fluid flux dominates focusing effects. Hydrate and free gas saturations can be characterized by a local Peclet number (localized, vertical, focused, and advective flux relative to diffusion) relative to the methane solubility gradient, somewhat analogous to such characterization in 1‐D systems. Even in lithologically complex systems, local hydrate and free gas saturations might be characterized by basic parameters (local flux and diffusivity).

Compressional and shear-wave velocities from gas hydrate bearing sediments: Examples from the India and Cascadia margins as well as Arctic permafrost regions

Shear wave velocity data have been acquired at several marine gas hydrate drilling expeditions, including the India National Gas Hydrate Program Expedition 1 (NGHP-01), the Ocean Drilling Program (ODP) Leg 204, and Integrated Ocean Drilling Program (IODP) Expedition 311 (X311). In this study we use data from these marine drilling expeditions to develop an understanding of general grain-size control on the P- and S-wave properties of sediments. A clear difference in the downhole trends of P-wave (Vp) and S-wave (Vs) velocity and the Vp/Vs ratio from all three marine regions was observed: the northern Cascadia margin (IODP X311) shows the highest P-wave and S-wave velocity values overall and those from the India margin (Expedition NGHP-01) are the lowest. The southern Cascadia margin (ODP Leg 204) appears to have similar low P-wave and S-wave velocity values as seen off India. S-wave velocity values increase relative to the sites off India, but they are not as high as those seen on the northern Cascadia margin. Such regional differences can be explained by the amount of silt/sand (or lack thereof) occurring at these sites, with northern Cascadia being the region of the highest silt/sand occurrences. This grain-size control on P-wave and S-wave velocity and associated mineral composition differences is amplified when compared to the Arctic permafrost environments, where gas hydrate predominantly occurs in sand- and silt-dominated formations. Using a cross-plot of gamma ray values versus the Vp/Vs ratio, we compare the marine gas hydrate occurrences in these regions: offshore eastern India margin, offshore Cascadia margin, the Ignik-Sikumi site in Alaska, and the Mallik 5L-38 site in the Mackenzie Delta. The log-data from the Arctic permafrost regions show a strongly linear Vp–Vs relationship, similar to the previously defined empirical relationships by Greenberg and Castagna (1992). P- and S-wave velocity data from the India margin and ODP Leg 204 deviate strongly from these linear trends, whereas data from IODP X311 plot closer to the trend of the Arctic data sets and previously published relationships. Three new linear relationships for different grain size marine sediment hosts are suggested:a)mud-dominated (Mahanadi Basin, ODP Leg 204 & NGHP-01-17): Vs = 1.5854 × Vp − 2.1649b)silty-mud (KG Basin): Vs = 0.8105 × Vp − 1.0223c)silty-sand (IODP X311): Vs = 0.5316 × Vp − 0.4916We investigate the relationship of gas hydrate saturation determined from electrical resistivity on the Vp/Vs ratio and found that the sand-dominated Arctic hosts show a clearly decreasing trend of Vp/Vs ratio with gas hydrate saturation. Though limited due to lower overall GH saturations, a similar trend is seen for sites from IODP X311 and at the ash-dominated NGHP-01-17 sediment in the Andaman Sea. Gas hydrate that occurs predominantly in fractured clay hosts show a different trend where the Vp/Vs ratio is much higher than at sand-dominated sites and remains constant or increases slightly with increasing gas hydrate saturation. This trend may be the result of anisotropy in fracture-dominated systems, where P- and S-wave velocities appear higher and Archie-based saturations of gas hydrate are overestimated. Gas hydrate concentrations were also estimated in these three marine settings and at Arctic sites using an effective medium model, combining P- and S-wave velocities as equally weighted constraints on the calculation. The effective medium approach generally overestimates S-wave velocity in high-porosity, clay-dominated sediments, but can be accurately used in sand-rich formations.

Comparison of methane mass balance and X-ray computed tomographic methods for calculation of gas hydrate content of pressure cores

Gas hydrate saturation was calculated for twelve pressure cores taken during National Gas Hydrate Program (NGHP) Expedition 01 in the Krishna–Godavari Basin, Bay of Bengal, at a location where thin gas hydrate veins were common. One of two methods were used to calculate gas hydrate saturation for each core: methane mass balance after depressurization and gas collection, considered the “gold standard” for measurement of gas hydrate saturation; or voxel intensity analysis (rather than binary segmentation) of X-ray computed tomographic (CT) reconstructions. Gas hydrate saturation in cores measured by methane mass balance was calculated to be 17.8%, 10.9%, 11.9%, 13.6%, 9.5%, 1.4%, and 0% by percent of core volume. Gas hydrate saturation in similar cores measured by intensity analysis of CT reconstructions was 17.2%, 9.1%, 6.7%, 7.8%, and 3.1% by percent of core volume. This quantitative CT intensity analysis contained systematic errors and therefore the calculated hydrate saturations are lower bounds. The systematic errors can be removed from the quantitative CT analysis by converting the CT intensities to real densities, though this was not possible for this study. All pressure core gas hydrate saturations were similar in magnitude to each other as well as to independent estimates of gas hydrate saturation from porewater freshening, and all variations in saturation could be explained by natural variation between samples. CT intensity (or preferably density) analysis of pressure cores showed promise for calculation of the saturation of vein hydrate in natural samples, allowing pressure cores to be used for further analyses under pressure after hydrate quantification. Theoretical examination of CT density analysis showed that this method would be unable to detect pore-filling gas hydrate; judicious examination of the results from CT density analysis versus other hydrate quantification methods on the same samples might allow quantification of pore-filling hydrate.

Anomalous porosity preservation and preferential accumulation of gas hydrate in the Andaman accretionary wedge, NGHP-01 site 17A

In addition to well established properties that control the presence or absence of the hydrate stability zone, such as pressure, temperature, and salinity, additional parameters appear to influence the concentration of gas hydrate in host sediments. The stratigraphic record at Site 17A in the Andaman Sea, eastern Indian Ocean, illustrates the need to better understand the role pore-scale phenomena play in the distribution and presence of marine gas hydrates in a variety of subsurface settings. In this paper we integrate field-generated datasets with newly acquired sedimentology, physical property, imaging and geochemical data with mineral saturation and ion activity products of key mineral phases such as amorphous silica and calcite, to document the presence and nature of secondary precipitates that contributed to anomalous porosity preservation at Site 17A in the Andaman Sea. This study demonstrates the importance of grain-scale subsurface heterogeneities in controlling the occurrence and distribution of concentrated gas hydrate accumulations in marine sediments, and document the importance that increased permeability and enhanced porosity play in supporting gas concentrations sufficient to support gas hydrate formation. The grain scale relationships between porosity, permeability, and gas hydrate saturation documented at Site 17A likely offer insights into what may control the occurrence and distribution of gas hydrate in other sedimentary settings.

Estimation of gas hydrate saturation in the Ulleung basin using seismic attributes and a neural network

Among the unconventional natural resources, gas hydrates have recently received much attention as a promising potential energy source. To develop gas hydrates, their distribution and saturation should be estimated, preferentially at the initial stage of development. In most cases, the distribution of gas hydrates can be identified by using seismic indicators including a bottom simulating reflector (BSR) and chimney/column structures, which indirectly determine the presence of gas hydrate. However, these indicators can be used only when they appear on a seismic image. Because the saturation of gas hydrate is generally calculated by using well logs, the information is limited to the well location. To overcome these limitations, seismic impedance inversion and neural network methods can be used. Seismic inversion enables the identification of a gas hydrate reservoir even if seismic indicators do not exist, and a neural network makes it possible to predict the gas hydrate saturation in a region of interest away from the wells by combining well logging data and other attributes extracted from the seismic data . In this study, to estimate the distribution and saturation of gas hydrates that are broadly distributed in the Ulleung basin of the East Sea, seismic inversions such as acoustic impedance (AI), shear impedance (SI), and elastic impedance (EI) were calculated, and then the seismic attributes (ratio of compressional wave velocity to shear wave velocity, Vp/Vs, and combinations of Lamé parameters, λρ and μρ) that have unique features in hydrated sediments were extracted. Gas-hydrate-bearing sediments displayed high AI, high SI, high EI (22.5°), low Vp/Vs ratio, high λρ, and high μρ compared the surrounding sediments. The sediments containing free gas displayed low AI, low SI, low EI (22.5°), high Vp/Vs ratio, low λρ, and low μρ due to the phase transition from gas hydrate to gas. By combining these findings, the distribution of gas hydrates was estimated even if seismic indicators were not present in the seismic profile. Using the extracted seismic attributes, as well as standard seismic attributes and three-phase Biot-type equation (TPBE)-derived saturation logs of gas hydrates at the wells which had a high correlation to the seismic attributes, the saturation of gas hydrates away from the wells could be estimated based on probabilistic neural network (PNN) predictions. To validate the predicted saturation, cross-validation of wells was undertaken. The average correlation coefficient between the predicted saturation and actual saturation logs at the UBGH-09 and UBGH2-10 wells was 82.6%. In addition, for the estimation of the saturation section of gas hydrate, a relatively high saturation region of gas hydrate corresponded well to the gas hydrate occurrence zone of each well. • Gas hydrate saturation is estimated in the Ulleung basin, East Sea, Korea. • Impedance inversion enables the distribution of a gas hydrate reservoir. • Saturation is predicted in a target region away from the wells by using PNN.

Gas hydrate saturation at Site C0002, IODP Expeditions 314 and 315, in the Kumano Basin, Nankai trough

AbstractThe degree of gas hydrate saturation at Integrated Ocean Drilling Program (IODP) Site C0002 in the Kumano Basin, Nankai Trough, was estimated from logging‐while‐drilling logs and core samples obtained during IODP Expeditions 314 and 315. Sediment porosity data necessary for the calculation of saturation were obtained from both core samples and density logs. Two forms of the Archie equation (‘quick‐look’ and ‘standard’) were used to calculate gas hydrate saturation from two types of electrical resistivity log data (ring resistivity and bit resistivity), and a three‐phase Biot‐type equation was used to calculate gas hydrate saturation from P‐wave velocity log data. The gas hydrate saturation baseline calculated from both resistivity logs ranges from 0% to 35%, and that calculated from the P‐wave velocity log ranges from 0% to 30%. High levels of gas hydrate saturation (&gt;60%) are present as spikes in the ring resistivity log and correspond to the presence of gas hydrate concentrations within sandy layers. At several depths, saturation values obtained from P‐wave velocity data are lower than those obtained from bit resistivity data; this discrepancy is related to the presence of free gas at these depths. Previous research has suggested that gas from deep levels in the Kumano Basin has migrated up‐dip towards the southern and seaward edge of the basin near Site C0002. The high saturation values and presence of free gas at site C0002 suggest that a large gas flux is flowing to the southern and seaward edge of the basin from a deeper and/or more landward part of the Kumano Basin, with the southern edge of the Kumano Basin (the location of site C0002) being the main area of fluid accumulation.

AVO analysis of BSR to assess free gas within fine-grained sediments in the Shenhu area, South China Sea

Gas hydrates have been identified from two-dimensional (2D) seismic data and logging data above bottom simulating reflector (BSR) during China's first gas hydrate drilling expedition in 2007. The multichannel reflection seismic data were processed to be preserved amplitudes for quantitatively analyzing amplitude variation with offset (AVO) at BSRs. Low P-wave velocity anomaly below BSR, coinciding with high amplitude reflections in 2D seismic data, indicates the presence of free gas. The absolute values of reflection coefficient versus incidence angles for BSR range from 0 to 0.12 at different CMPs near Site SH2. According to logging data and gas hydrate saturations estimated from resistivity of Site SH2, P-wave velocities calculated from effective media theory (EMT) fit the measured sonic velocities well and we choose EMT to calculate elastic velocities for AVO. The rock-physics modeling and AVO analysis were combined to quantitatively assess free gas saturations and distribution by the reflection coefficients variation of the BSRs in Shenhu area, South China Sea. AVO estimation indicates that free gas saturations immediately beneath BSRs may be about 0.2 % (uniform distribution) and up to about 10 % (patchy distribution) at Site SH2.

Modified effective medium model for gas hydrate bearing, clay-dominated sediments in the Krishna–Godavari Basin

During NGHP-Expedition-01, well logs were obtained for gas hydrate exploration in Krishna–Godavari (KG) offshore basin. These logs coupled with a suitable rock physics model can be used to understand the interaction between the sediment grains of unconsolidated marine sediments as well as with hydrate. In this paper, we study the friction-dependent effective medium model (EMM) to understand these grain interactions. The compressional (P) and shear (S) wave velocities of fluid saturated sediments are estimated using different friction parameters at Site NGHP-01-03, which represent the background fluid-saturated marine sediment, and are compared with the observed velocities derived from sonic logs. Our analysis shows that the shear velocity is overestimated for the Hertz–Mindlin contact theory [no-slip across the grain contact], but can be accurately estimated for the Walton's smooth contact model [zero friction across the grain contact]. It suggests that the background shear wave velocity need to be modeled without friction at the grain contact for unconsolidated marine sediments. Further, the friction-dependent EMM theory is tested at Site NGHP-01-07 which represents the load-bearing gas hydrate deposits in KG basin. The comparison between the gas hydrate saturations estimated from sonic and resistivity logs shows that saturations estimated from P-wave velocity match well with those estimated from resistivity and chloride anomaly and is largely independent of the frictional parameter. However, gas hydrate saturations estimated from shear wave velocity is overestimated in the absence of friction but agrees with the other estimates if an arbitrary small friction is included in the EMM.We further extended the friction-dependent EMM for multi-grain contact (clay + quartz + hydrate) in which the effective modulus of sediment matrix is estimated by accounting for all possible contact combinations among the grains like quartz–quartz (QQ), clay–clay (CC), clay–quartz (QC), quartz–hydrate (QH), clay–hydrate (CH), and hydrate–hydrate (HH). The gas hydrate saturations estimated from shear velocity assuming the same non-zero friction term are underestimated as compared to those estimated from P-wave velocity. Interestingly, the saturations estimated assuming zero-friction from both P- and S-wave velocities are comparable to each other and show a good match with those estimated from resistivity logs and chloride anomalies. The proposed EMM with zero friction and mixed grain contact is able to predict the velocities of fluid-saturated sediments as well as gas hydrate bearing sediments in KG offshore basin.

Reflectivity template, a quantitative intercept-gradient AVO analysis to study gas hydrate resources – A case study of Iranian deep sea sediments

Presence of gas hydrate and free gas in Iranian part of Makran accretionary prism changes the elastic properties of unconsolidated sediments and produces sharp bottom simulating reflectors (BSRs) which are observed on the 2-D seismic data. Different methods have been applied to estimate the gas hydrate and free gas saturations in marine sediments based on seismic measurements. Most of these methods are based on relating the elastic properties to the hydrate and free gas saturations and remotely estimating their concentration. In this regard, using the effective medium theory (EMT) which was developed for different modes of hydrate distribution is more considered among other rock physics theories. The main concern about saturation estimations based on EMT is that the velocities of the hydrate-bearing sediments primarily depend on how they are distributed within the pore space. Therefore, understanding the modes of hydrate distribution (at least cementing or non-cementing modes) is necessary to decrease the estimation uncertainties.The first intention of paper is to investigate amplitude variation versus offset (AVO) analysis of BSR to determine the hydrate distribution modes. The results from the probable saturation revealed that if the hydrate cements the sediment grains, BSR would show the AVO class IV and if hydrate does not cement the sediment grains, then BSR would show either the AVO class II or class III depending on the free gas saturation just beneath the BSR. The second intention of paper is to introduce some templates called reflectivity templates (RTs) for quantitative study of hydrate resources. These templates are provided based on the EMT to quantify the hydrate and free gas near the BSR. Validation of this approach by synthetic data showed that a reliable quantification could be achieved by intercept-gradient RTs, only if these attributes are determined with a high accuracy and good assumptions are made about the mineralogical composition and porosity of the unconsolidated host sediments. The results of this approach applied to a 2-D marine pre-stack time migrated seismic line showed that less than 10% of the gas hydrate accumulated near to the BSR in anticlinal-ridge type structure of Iranian deep sea sediments. The free gas saturation near to the BSR by assuming a homogeneous distribution was less than 3% and by assuming patchy distribution was about 3–10%.

Marine controlled source electromagnetic method used for the gas hydrate investigation in the offshore area of SW Taiwan

Bottom simulating reflectors (BSRs), high methane flux, shallow sulfide/methane interfaces, fluids venting from the seafloor, authigenic carbonates within sediments, methane reefs, and self-biomes are common seafloor features in the area off southwest Taiwan. The geophysical and geochemical signatures of these features suggest a high potential for gas hydrate (GH) reservoirs in the region. The BSRs are typically interpreted as the boundary between free gases and solid hydrate, whereas the upper reaches of the hydrate stability zone and the distribution of gas hydrate in shallow sediments are not well understood. This study shows the first results of a marine controlled-source electromagnetic survey, conducted in the offshore area of SW Taiwan in 2010. The survey aimed to provide electrical resistivity information of the shallow sediments. Three target areas were surveyed: (1) an area to the southeast of the Xiaoliuchiu Island (gas seep G96), (2) an area in the west of the Yung-An Ridge (YAR) and (3) an area in the northwest of the Good Weather Ridge (GWR). In total, fourteen survey lines with a total length of 72km were completed. Our preliminary results show that relatively high resistivity anomalies occur within pockmarks and at gas seepage sites. The apparent resistivity is estimated to be about 1Ohm-m higher than background in G96 and YAR sites, while an anomaly up to 2Ohm-m is found in the GWR. At gas seep site G96, the high resistivity anomaly may be due to the existence of authigenic carbonates; whereas, the high resistivity anomaly in the NW of the GWR site may also be due to the existence of gas hydrate in the shallow seabed. Based on the resistivity anomaly, the gas hydrate saturation is about 16% in the shallow sediments below the pockmark area in the northwest side of the GWR site.

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.

Imaging seismic velocities for hydrate-bearing sediments using converted waves near Yuan-An Ridge, off southwest Taiwan

Data from P-waves and from S-waves generated by P–S conversion on reflection from airgun shots recorded along four lines of ocean bottom seismometers were used to construct 2-D velocity sections near the Yuan-An Ridge, off southwest Taiwan. The locations of the ocean bottom seismometers were determined to high accuracy by an inversion based on the shot traveltimes. Traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P-wave (Vp) and S-wave (Vs) velocities of hydrate-bearing sediment layers. The inversion indicates a relatively high P-wave velocity beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. S-wave velocities of the sediments over the entire section, down to 400m below seafloor, range from 320 to 570ms−1. We suggested the lateral variation in Vp/Vs profiles in the hydrate-affected zones may be related to the migration conduit of gas-rich fluid and a characteristic of hydrate content. We model Vp using equations based on a modification of Wood’s equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ∼200m below the seafloor to 10–15% of pore space close to the bottom simulating reflector (BSR) in the survey area.

A kinetic model for the methane hydrate precipitated from venting gas at cold seep sites at Hydrate Ridge, Cascadia margin, Oregon

AbstractWe develop a kinetic model for hydrate crystallization from methane gas venting through shallow sediments at Hydrate Ridge on the Cascadia margin of Oregon that predicts how pore water chlorinity, temperature, and crystallized hydrate evolve after the onset of steady venting. Predictions are compared to observations at Ocean Drilling Program Site 1249. In the preferred model, calculated gas hydrate saturation and chloride concentrations reach those observed at depths less than 20 m below seafloor (bsf) under the southern summit of Hydrate Ridge in ~650 years, and the vertical water flux must be less than 50 kg/m2/yr. Hydrate accumulates more slowly between 20 m bsf and the base of the hydrate stability zone where there is no free gas, accumulating to observed levels of a few volume percent of hydrate in 105 to 106 years, depending on the water flux that is assumed through this zone. This dichotomy means that the presently observed gas venting must have been diverted to this area ~650 years ago, or be episodic and infrequent. If the gas venting for the last 650 years has been as observed today, the latent heat of hydrate precipitation in the upper 20 m of sediments would have caused the temperature to increase ~0.8°C at ~20 m bsf and ~0.2°C at ~100 m bsf. This would have caused a ~5 m rise in the elevation of the base of hydrate stability zone, and decreased the rate of hydrate crystallization from 1.5 kg CH4/m2/yr 650 years ago to 0.7 kg CH4/m2/yr today.

Anomalous wireline logging data caused by gas hydrate dissociation in the Shenhu area, northern slope of South China Sea

During the gas hydrate drilling expedition, GMGS-1 indicated that gas hydrate was discovered above the gas hydrate stability zone with a thickness of 10 meters at site SH3 in the Shenhu area, the northern slope of South China Sea. Gas hydrate saturations derived from observed pore-water chloride values in core samples reach the highest value of 26% in the pore space. Higher resistivity, lower P-wave velocity and slightly increasing density were identified. To explain the lower P-wave velocity, the Simplified Three-Phase Biot-type Equation (STPBE) was used to calculate the water-saturated P-wave velocity. At the depth of 195 m, the P-wave velocity from wireline logging is lower than that of the water-saturated velocity of the normal sediments. The Archie's parameters used to calculate gas hydrate saturation from resistivity were defined from the cross plot between density porosity and formation factor. Gas hydrate saturation estimated from resistivity using Archie equation with a = 1. 1 and m=2. 3 is about 5 similar to 20% of the pore space, with a maximum value of 26. 8%, which shows that the distribution of gas hydrate is heterogeneous in vertical. The dissociation of gas hydrate (the mixture of free gas and water) may be caused by drilling. Either the in-situ free gas or gas released from gas hydrate dissociation can cause the low well-log P-wave velocity. Because surface seismic data were acquired before drilling, synthetic seismograms generated using different P-wave velocities were compared with the seismic data, which can be used to identify the cause of the low P-wave velocity observed in the well log.

Depressurization experiment of pressure cores from the central Ulleung Basin, East Sea: Insights into gas chemistry

We performed depressurization experiments on pressure cores (PCs) recovered from Holes UBGH1-10B and UBGH2-11B, which were drilled during two separate expeditions in the Ulleung Basin (UBGH1 and UBGH2). Various lines of evidence, such as convex curvature in the P–V curves obtained by depressurization, as well as X-ray images and resistivity-at-bit (RAB) data, point to the presence of gas hydrates at these holes. We estimated gas hydrate saturation (Sh) at these holes to range from 5–23%, based on the amount of gas released from PCs. These Sh values are in agreement with those reported at other high gas flux settings.Molecular (C1/C2+) and isotopic (δ13CCH4, δDCH4, δ13CC2H6) indicators delineate a microbial source for the CH4 and C2H6. In Core UBGH1-10B-5R, the carbon isotopic fractionation factor between CO2 and CH4 (εc≈δ13CCO2−δ13CCH4) was observed to increase from 50 to 68 as pressure was reduced during our experiments. These changes illustrate the isotope fractionation that results from the preferential release of 13C depleted CO2 during the early stages of depressurization.The constant carbon isotope separation (∼10–20‰) between CO2 in void gases and in the dissolved inorganic carbon (DIC) pool shown in cores from the Ulleung Basin, has also been observed in gas hydrate-bearing sediments elsewhere. It is important to consider this effect when using carbon isotopic data in carbon cycling models, specifically when unraveling metabolic pathways of methane generation. In situations where partial CO2 degassing of a core is suspected, we recommend the use of the carbon isotope ratio of DIC instead of the gas measurements to best delineate the methane cycling pathways in marine systems.

Well Log Anomalies Caused by Gas Hydrate Dissociation in the Shenhu Area, Northern South China Sea

AbstractThe drilling GMGS‐1 reveals that gas hydrates are present above a stable zone with a thickness of 10 m at site SH3 in the Shenhu area, northern South China Sea. At this place, the gas hydrate saturation estimated from observed pore‐water chloride values is up to 26% in the pore space. Higher resistivity, lower P‐wave velocity and slightly increasing density characterize the gas hydrate‐bearing sediments. To analyze this low P‐wave velocity, the simplified three‐phase Biot‐type equation (STPBE) is used to calculate the water‐saturated P‐wave velocity. The results show that at the depth of 195 m, the P‐wave velocity from wireline logging is lower than that of the water‐saturated velocity of the normal sediments. The Archie's parameters used to calculate gas hydrate saturation from resistivity were defined from the cross plot between density porosity and formation factor. Gas hydrate saturation estimated from resistivity using the Archie equation with a=1.1 and m=2.3 is about 5%∼20% of the pore space, with a maximum value 26.8%, which shows that the distribution of gas hydrates is inhomogeneous in vertical direction. The dissociation of gas hydrates (the mixture of free gas and water) may be caused by drilling. Either the in‐situ free gas or gas released from gas hydrate dissociation can cause the low well‐log P‐wave velocity. Due to surface seismic data were acquired before drilling, synthetic seismograms generated using different P‐wave velocities are compared with the real seismic data, which can be used to identify the cause of the low P‐wave velocity observed in the well log.

울릉분지 시추공 분석 자료를 이용한 가스하이드레이트 함유층의 3차원 공간 물성 분포 추정

Drilling expedition #1 in 2007 and drilling expedition #2 in 2010 were performed for gas hydrate resources evaluation and optimal site selection of pilot test in Ulleung basin, East Sea, Korea. This study presents to build the 3D spatial distribution models using the estimated sedimentary facies, porosity, and gas hydrate saturation derived by well logs and core analysis data from UBGH1-4, UBGH1-9, UBGH1-10, UBGH1-14, UBGH2-2-1, UBGH2-2-2, UBGH2-6, UBGH2-9, UBGH2-10 and UBGH2-11. The objective of 3D spatial distribution modeling is to build a geological representation of the gas hydrate-bearing sediment that honors the heterogeneity in 3D grid scale. The facies modeling is populating sedimentary facies into a geological grid using sequential indicator simulation. The porosity and gas hydrate saturation modeling used sequential Gaussian simulation to populate properties stochastically into grid cells.

Measuring the in Situ Hydrate Saturation from γ-Ray Transmissivity Changes during Local Dissociation

The worldwide abundance of gas hydrates in areas with engineering activities highlights the need for an accurate understanding of how much hydrate the soil contains, before any subsequent assessments are made. For the currently deployed gas hydrate saturation measurement, techniques are based on absolute measurements, derived from either the elastic properties of the formation, its nuclear magnetic resonance, or its electrical resistivity. None of those methods is wholly satisfactory, because of either the required reference measurements or a high dependence upon empirical correlations. This study explores the possibility to determine the hydrate saturation from relative measurements, by deriving its initial value from changes in γ transmissivity during the dissociation process of the contained hydrate. A customized testing apparatus was built, in which hydrate-bearing sand samples of up to 40% pore space saturation were first formed and then tested. The γ transmission from a ⁶⁰Co source was measured by a scintillation counter, while the cylindrical samples were radially dissociated by heating from their axis. Seven independent dissociation tests with an initial hydrate saturation between 20 and 40% were conducted. The results show that the change in local density (γ transmissivity) is an exponential function of the initial hydrate volume in place.