Ulleung Basin Gas Hydrate Research Articles

Generation of Synthetic Compressional Wave Velocity Based on Deep Learning: A Case Study of Ulleung Basin Gas Hydrate in the Republic of Korea

This study proposes a deep-learning-based model to generate synthetic compressional wave velocity (Vp) from well-logging data with application to the Ulleung Basin Gas Hydrate (UBGH) in the East Sea, Republic of Korea. Because a bottom-simulating reflector (BSR) is a key indicator to define the presence of gas hydrate, this study generates the Vp for identifying the BSR by detecting the morphology of the hydrate in terms of the change in acoustic velocity. Conventional easy-to-acquire logging parameters, such as gamma-ray, neutron porosity, bulk density, and photoelectric absorption, were selected as model inputs based on a sensitivity analysis. Long short-term memory (LSTM) and an artificial neural network (ANN) were used to design an efficient learning-based predictive model with sensitivity analysis for hyperparameters. The LSTM model outperforms the ANN model by preserving the geological sequence of the well-logging data. Ten-fold cross-validation was conducted to verify the consistency of the LSTM model and yielded satisfactory results, with an average coefficient of determination greater than 0.8. These numerical results imply that generating synthetic well-logging via deep learning can accurately estimate missing well-logging data, contributing to the reservoir characterization of gas-hydrate-bearing sediments.

Diatom influence on the production characteristics of hydrate-bearing sediments: Examples from Ulleung Basin, offshore South Korea

The Ulleung Basin Gas Hydrate field expeditions in 2007 (UBGH1) and 2010 (UBGH2) sought to assess the Basin's gas hydrate resource potential. Coring operations in both expeditions recovered evidence of gas hydrate, primarily as fracture-filling (or vein type) morphologies in mainly silt-sized, fine-grained sediment, but also as pore-occupying hydrate in the coarser-grained layers of interbedded sand and fine-grained systems. A commonality across many of these occurrences is the presence of diatoms in the fine-grained sediment. Here we tested fine-grained sediment (median grain size <12.5 μm) associated with hydrate occurrences at four UBGH2 sites (UBGH2-2-2, UBGH2-3, UBGH2-6 and UBGH2-11) to investigate potential impacts of diatoms on efforts to extract methane from hydrate, or to tap hydrocarbon reservoirs beneath hydrate-bearing sediment. Two key considerations are: the extent to which diatoms control sediment mechanical properties, and the extent to which pore-water freshening, which occurs as gas hydrate breaks down during resource extraction, alters the diatom control on sediment mechanical properties. We conducted experiments to measure sediment index properties, sedimentation behavior and compressibility to address these considerations. We relied on scanning electron microscope (SEM) imagery and X-ray powder diffraction (XRD) to characterize the sediment mineralogy. Our high-level findings are that at the ∼20–45% (by volume) diatom concentrations observed at these UBGH2 sites, sediment compressibility increases with diatom content, but diatoms only appear to increase porosity and permeability at the highest diatom concentration (∼45%). Our measurements suggest in situ compression indices of 0.35–0.55 and permeabilities on the order of 0.01milliDarcies (1 × 10−17 m2) can be anticipated at these sites. Importantly, these properties are not expected to vary significantly upon pore water freshening that accompanies gas hydrate dissociation during production.

Potential freshening impacts on fines migration and pore-throat clogging during gas hydrate production: 2-D micromodel study with Diatomaceous UBGH2 sediments

Abstract The methane gas hydrate stored in natural sediments is considered a potential gas resource. Countries such as China, India, Japan, and Korea are interested in commercializing this resource, and offshore field pilot tests for gas production have been conducted using depressurization methods to destabilize gas hydrate and facilitate the migration of methane to the production well. However, fine-grained sediments (fines), which are present even in coarse-grained, gas hydrate-bearing sediments, can be resuspended in the production fluid, subsequently clogging pore throats in the formation and reducing the overall production efficiency. We conducted laboratory tests to evaluate the suspension and clogging potential of fines collected from the Ulleung Basin, East Sea, Korea during the 2010 Ulleung Basin Gas Hydrate Expedition 2 (UBGH2). Experimental results reveal that diatoms are prevalent in the sediment and largely control the suspension and clogging behavior. Fluid flow experiments in 2D micromodels show clogging occurs even when injecting the minimum sediment concentration (0.1 wt% in the fluid) through micromodels with pore-throat widths at the high end of the anticipated range for UBGH2 gas hydrate-bearing sands (100 μm). Mobile gas/fluid interfaces forming during gas hydrate dissociation accentuate clogging by concentrating and mobilizing fines. Sedimentation tests show pore-water freshening during dissociation is not anticipated to change the potential for diatoms to become entrained in the pore water flow, even for the observed gas hydrate saturations of ~80%. Muscovite and illite are also significant components of the tested sediment, however, and pore-water freshening increases their potential for resuspension and clogging. Overall, the resuspension and clogging potential of these fine sediments should increase as gas hydrate dissociation progresses in the thin, gas hydrate-bearing sands investigated in the Ulleung Basin.

Geomechanical, Hydraulic and Thermal Characteristics of Deep Oceanic Sandy Sediments Recovered during the Second Ulleung Basin Gas Hydrate Expedition

This study investigates the geomechanical, hydraulic and thermal characteristics of natural sandy sediments collected during the Ulleung Basin gas hydrate expedition 2, East Sea, offshore Korea. The studied sediment formation is considered as a potential target reservoir for natural gas production. The sediments contained silt, clay and sand fractions of 21%, 1.3% and 77.7%, respectively, as well as diatomaceous minerals with internal pores. The peak friction angle and critical state (or residual state) friction angle under drained conditions were ~26° and ~22°, respectively. There was minimal or no apparent cohesion intercept. Stress- and strain-dependent elastic moduli, such as tangential modulus and secant modulus, were identified. The sediment stiffness increased with increasing confining stress, but degraded with increasing strain regime. Variations in water permeability with water saturation were obtained by fitting experimental matric suction-water saturation data to the Maulem-van Genuchen model. A significant reduction in thermal conductivity (from ~1.4–1.6 to ~0.5–0.7 W·m−1·K−1) was observed when water saturation decreased from 100% to ~10%–20%. In addition, the electrical resistance increased quasi-linearly with decreasing water saturation. The geomechanical, hydraulic and thermal properties of the hydrate-free sediments reported herein can be used as the baseline when predicting properties and behavior of the sediments containing hydrates, and when the hydrates dissociate during gas production. The variations in thermal and hydraulic properties with changing water and gas saturation can be used to assess gas production rates from hydrate-bearing deposits. In addition, while depressurization of hydrate-bearing sediments inevitably causes deformation of sediments under drained conditions, the obtained strength and stiffness properties and stress-strain responses of the sedimentary formation under drained loading conditions can be effectively used to assess sediment responses to depressurization to ensure safe gas production operations in this potential target reservoir.

Core and sediment physical property correlation of the second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) results in the East Sea (Japan Sea)

We analyzed the data consist of core digital images and X-rays, core-logs, LWD (logging-while-drilling), and sediment grain-size from the second Ulleung Basin Gas Hydrate Expedition (UBGH2) in the East Sea. Core digital images and X-rays were spliced as a complete composite core in meters below seafloor (mbsf) for five sites; UBGH2-1_1 (Hole D), 2_1 (B), 2_2 (B), 2-6 (B) and 2-10 (C–D), and were correlated with the core-log and LWD measurements showing that possible gas hydrate bearing layers are between the depths of about 60–180 mbsf at these sites. Bulk densities generally increase with depth from 1.3 to 2.0 g/cm3 in LWD data, and from 1.1 to 1.8 g/cm3 onboard which measured lower than in-situ. Gas hydrate bearing sediments respond with an increase of LWD densities (1.4–1.6 g/cm3) and a decrease in core-logs (1.1–1.4 g/cm3). P-wave velocity values of LWD increase (1400 to 1700 m/s) with depth for non-reservoirs, and are high (1500 and 2000 m/s) within the gas hydrate bearing intervals depending on the hydrate saturations.Resistivity values logged onboard range from less than 1.0 to over 10.0 Ω-m, while LWD records are around 1.0 Ω-m and between 5.0 and 30.0 Ω-m in background sediments and possible gas hydrate reservoirs, respectively. High resistivity values were observed (5.0–30.0 Ω-m) within coarse-grained turbidites (mean grain-size between 2.9 and 5.1 ϕ; laminated sandy mud or muddy sands). Medium resistivities were observed (5.0 Ω-m) within the silt-dominant hemi-pelagic and turbiditic sediments (5.1–7.4 ϕ; crudely laminated, bioturbated, homogeneous sand, and disintegrated sand and sandy mud facies) bearing pore-filling gas hydrates, or disseminated gas hydrates either formed in pores or small fractures of fine-grained sediments. Core-log measurements are highly fluctuating and sensitive but mostly lower (e.g., density and resistivity) than LWD records.

동해 울릉분지 가스하이드레이트 퇴적층의 해리 유동 전산 분석 연구

국내 동해 울릉분지에는 막대한 부존량과 청정 에너지원으로 높은 잠재력을 지닌 가스하이드레이트가 부존되어 있으며, 2010년 울릉분지 2차 시추지역(UBGH2)에서 현장 생산 시험 후보지 선정을 위한 목적으로 시추작업이 수행되었다. 이 연구에서는 동해 울릉분지 현장 생산 시험 후보지역의 지질학적 모델을 바탕으로 전산 수치 해석을 이용하여 가스하이드레이트 퇴적층 내 해리 유동 및 산출성을 분석하였으며, 주요 생산 조건별 감압 생산 거동에 대한 민감도 분석을 수행하였다. 이를 통해 가스하이드레이트 현장 생산 시험 계획 수립 시 요구되는 예비 분석 자료를 도출하였다. In the Ulleung basin in the East Sea of Korea, it is assumed that huge amounts of gas hydrate(GH) are buried. In 2010, drilling operation was performed at the 2nd Ulleung Basin Gas Hydrate Drilling Expedition(UBGH2) to designate a site for field production test. In this study, based on the field production test site model reflecting geological properties, GH dissociation flow interpretation is analyzed and sensitivity analysis is performed to gain understanding of production behavior properties following bottomhole pressure(BHP) variation by using the numerical simulation. The results of this study provide a basis for the preliminary analysis of field production test site.

Towards quantifying the reaction network around the sulfate–methane-transition-zone in the Ulleung Basin, East Sea, with a kinetic modeling approach

We present a kinetic model based upon pore water data collected from eight sites drilled during the second Ulleung Basin gas hydrate drilling expedition (UBGH2) in 2010. Three sites were drilled at locations where acoustic chimneys were identified in seismic data, and the rest were drilled on non-chimney (i.e., background) environments. Our model, coupled a comprehensive compositional and isotopic data set, is used to illustrate the different biogeochemical processes at play in those two environments, in terms of reactions around the sulfate–methane-transition-zone (SMTZ). Organic matter decomposition is an important process for production of methane, dissolved inorganic carbon (DIC) and consumption of sulfate in the non-chimney sites, whereas anaerobic oxidation of methane (AOM) dominates both carbon and sulfur cycles in the chimney environment. Different sources of methane mediate AOM in the two settings. Internally produced methane through CO2 reduction (CR) and methanogenesis fuels AOM in the non-chimney sites, whereas AOM is sustained by methane from external sources in the chimney sites. We also simulate the system evolution from non-chimney to chimney conditions by increasing the bottom methane supply to a non-chimney setting. We show that the higher CH4 flux leads to a higher microbial activity of AOM, and more organic matter decomposition through methanogenesis. A higher methanogenesis rate and a smaller CR contribution relative to AOM in the chimney sites is responsible for the isotopically light DIC and heavy methane in this environment, relative to the non-chimney sites.

Scientific results of the Second Gas Hydrate Drilling Expedition in the Ulleung Basin (UBGH2)

As a part of Korean National Gas Hydrate Program, the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) was conducted from 9 July to 30 September, 2010 in the Ulleung Basin, East Sea, offshore Korea using the D/V Fugro Synergy. The UBGH2 was performed to understand the distribution of gas hydrates as required for a resource assessment and to find potential candidate sites suitable for a future offshore production test, especially targeting gas hydrate-bearing sand bodies in the basin. The UBGH2 sites were distributed across most of the basin and were selected to target mainly sand-rich turbidite deposits. The 84-day long expedition consisted of two phases. The first phase included logging-while-drilling/measurements-while-drilling (LWD/MWD) operations at 13 sites. During the second phase, sediment cores were collected from 18 holes at 10 of the 13 LWD/MWD sites. Wireline logging (WL) and vertical seismic profile (VSP) data were also acquired after coring operations at two of these 10 sites. In addition, seafloor visual observation, methane sensing, as well as push-coring and sampling using a Remotely Operated Vehicle (ROV) were conducted during both phases of the expedition. Recovered gas hydrates occurred either as pore-filling medium associated with discrete turbidite sand layers, or as fracture-filling veins and nodules in muddy sediments. Gas analyses indicated that the methane within the sampled gas hydrates is primarily of biogenic origin.This paper provides a summary of the operational and scientific results of the UBGH2 expedition as described in 24 papers that make up this special issue of the Journal of Marine and Petroleum Geology.

Occurrence and seismic characteristics of gas hydrate in the Ulleung Basin, East Sea

Multi-channel seismic reflection and well-log data from the Ulleung Basin, East Sea reveal several seismic signatures indicative of gas-hydrate occurrence in the Ulleung Basin that are associated with vertically and/or laterally stacked mass-transport complexes. The seismic indicators include (a) a bottom simulating reflector (BSR), (b) a seismic chimney, (c) high amplitude reflections within the gas hydrate stability zone (GHSZ), (d) acoustic blanking, (e) enhanced reflections below the BSR, and (f) seafloor gas-escape features. The BSR, associated with enhanced reflections below, is most commonly found over much of the basin indicating a physiochemical boundary of gas hydrates overlying free gas. Seismic chimneys are characterized by velocity pull-up and reduced reflectivity on the seismic sections, which appear to be caused by active migration of fluid gas vertically into the GHSZ. The logging data retrieved from the seismic chimneys showed elevated electrical resistivity (>80 Ohm-m) and P-wave velocity (>2000 m/s), indicating the presence of gas hydrate. Another seismic characteristic observed in gas hydrate bearing sediments is the strong amplitude reflections, defined by the relatively high reflectivity within the GHSZ. Acoustic blanking is likely to be the result of hydrate accumulation in the sediments causing a significant reduction of acoustic impedance contrast between sedimentary layers. Where the upward migrating gas seeps into the deep water column, seafloor pockmarks and mud mounds may be formed.Gas hydrate was recovered from the Ulleung Basin, East Sea in 2010 during the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) under the Korean National Gas Hydrate Program. Based on the results, gas-fluids migrate into the GHSZ through two distinct pathways: (1) structural conduits which include fault and fracture systems associated with seismic chimneys and (2) stratigraphic conduits associated with inclined turbidite/hemipelagic layers. Two types of gas-hydrate occurrence were identified in the basin: (1) a stratally-bound type (pore filling) within turbidite sand layers and (2) a locally concentrated type (massive, nodule or fracture filling) within upward-growing chimneys associated with near vertical faults. Relatively high concentrations of gas hydrate, however, tend to occur in localized seismic chimneys, rather than in the strata-related features. The successful recovery indicates that the Ulleung Basin provides favorable conditions for gas-hydrate formation.

Gas origin and migration in the Ulleung Basin, East Sea: Results from the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2)

We have investigated the gas chemistry of marine sediments (up to 250 m below seafloor, m bsf) recovered during the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) to identify and compare the gas origin and its migration at seismically non-chimney, truncated chimney and chimney sites in this basin. The molecular and isotopic data (C1/C2+ ratios > 300, δ13CCH4 < −65‰, and δDCH4 < −181‰) of the gases indicate that methane predominantly originates via microbial carbon dioxide reduction. The carbon isotopic fractionation (ɛc) between methane and carbon dioxide below 30 m bsf has a relatively constant value (ɛc = 66–68) at all sites, consistent with the microbial pathway. In contrary, samples shallower than 30 m bsf display significant differences with the seismic characteristics of each site. ɛc ranges from 65 to 81 in non-chimney sites but shows values as low as 45 in chimney sites, which is agreement with previously reported piston core data (Kim et al., 2012), and thought to reflect two-phase methane transport processes. In chimney sites, deep-sourced methane migrates independently of the carbon dioxide, which remains dissolved in the aqueous phase leading to the extremely low ɛc values observed in the upper 30 m bsf. Upward methane diffusion at non-chimney sites may also add a transport-induced fractionation as evidenced in mixing diagrams, but this process does not significantly affect the ɛc values.δ13CCO2 shows usually minimum values at the Sulfate–Methane Transition Zone (SMTZ) at all sites. However, in chimney sites, these values have a much more depleted signature, indicating faster rates of methane consumption by Anaerobic Oxidation of Methane (AOM) supported by the higher methane flux. Moreover, here the massive hydrate that forms in shallow sediments incorporates gases with a lower C1/C2 ratio (=higher C2/C1) than the surrounding sediment, as documented in hydrate-bound gas (BG) and void gas (VG) samples. Gas hydrate dissociation during the core recovery is released enriched ethane (C2H6) trapped in gas cage, which leads to have high C2H6 in VG samples.

Geochemical signature related to lipid biomarkers of ANMEs in gas hydrate-bearing sediments in the Ulleung Basin, East Sea (Korea)

The emission of methane as a greenhouse gas is controlled by the anaerobic oxidation of methane (AOM), which plays an important role in the biogeochemical methane cycle. During the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2), the distribution of lipid biomarkers and their compound-specific stable carbon isotope ratios related to methane were investigated in venting and non-venting sites (UBGH2-3, UBGH2-10) of gas-hydrate-bearing sediments in the Ulleung Basin. The objective of this study was to understand the microbial signatures related to methane cycling in organic-rich sediment in a marginal sea (East Sea/Japan Sea) of the western North Pacific. The concentrations of methane-related specific biomarkers (archaeol and sn-2-hydroxyarchaeol) at the sulphate–methane transition zone (SMTZ; sediment depth in UBGH2-3: 1–2 mbsf, in UBGH2-10: 6.8 mbsf) are typically higher than in other sediment sections and their δ 13C valuesare apparently depleted (−73.3‰ to −102.7‰) in the UBGH2-3 and UBGH2-10 study sites. However, the δ13C values of archaeol and sn-2-hydroxyarchaeol (between −59.6‰ and −66.5‰) are not depleted with the increased methane concentration in the sediments below the SMTZ in UBGH2-3, compared to the δ 13C values (about −60‰) of in situ methane. This suggests that methane production processes should be dominant in the deeper sediment sections (2.7–3.8 mbsf) rather than methane consumption by anaerobic methanotrophs (ANMEs) at the corresponding sediment depths. There were also higher δ 13C values (−47‰ to −32‰) for archaeol and sn-2-hydroxyarchaeol in the 3–6 mbsf sections at UBGH2-10, suggesting the prevalence of methanogenic activities. However, the δ13C values (−89.0‰ to −92.2‰) of archaeol and sn-2-hydroxyarchaeol were unexpectedly depleted in the deeper sediment section (5.2 mbsf) of the venting site (UBGH2-3), indicating that the past AOM occurred under low sulphate concentrations in the corresponding pore water. This study used the biomarker ratio (sn-2-hydroxyarchaeol/archaeol) of Archaea as a tool to demonstrate the different ANME communities, which was supported by 16S rRNA analysis in the sediments of venting and non-venting sites (UBGH2-3, UBGH2-10). Consequently, the biochemical signatures of methanotrophic and methanogenic activity were found at varying sediment depths at both sites.

Physical properties of sediments from the Ulleung Basin, East Sea: Results from Second Ulleung Basin Gas Hydrate Drilling Expedition, East Sea (Korea)

Physical properties, including basic index properties, undrained shear strength, and thermal conductivities were measured on conventional cores retrieved from the Ulleung Basin, during the second Ulleung Basin Gas Hydrate expedition (UBGH2). The UBGH2 logged 13 sites and cored 10 sites to locate potential sites for an offshore test production. A total of 211 conventional cores were recovered. Undrained shear strength, thermal conductivity, and index properties, including porosity and grain density, were measured onboard. Mineral composition analyses and grain size analyses were completed onshore after the expedition.The averaged porosity at each site ranges from 65% to 71%, the averaged grain density at each site ranges from 2.57 to 2.66 g/cm3, and the mean grain size mostly lies between 4 μm and 25 μm, with sparsely scattered coarse grained intervals. The relatively high porosity and low grain density are due to the large portion of diatomaceous sediments. The thermal conductivities average around 0.8 W/mK, and the low porosity and the abundance of clay mineral and OAPL-A may have caused the relatively low thermal conductivity in the Ulleung Basin.The geomechanical analyses revealed a few relevant findings important for sediment physical behaviors during gas hydrate production. The particle migration in coarse grained layers during production necessitates proper measure for sand productions while fine grained layers are mostly self-filtering. The vertical deformation estimated from the compression index suggested that the subsidence induced by the pore pressure change from the depressurization is much higher than those induced by the lost of gas hydrate particles from dissociation. Massive vertical deformation in fine-grained layers has been predicted to occur due to the pore pressure change from depressurization induced gas hydrate production. The drilling mud weight induced pressure window is mostly determined by the water depth in deep sea drilling, but caution should be taken in determination of fracture pressures in hydrate-bearing sediments, because gas hydrate saturations alter Poisson's ratio.

Improving gas hydrate saturation estimates using P-wave velocity log data by incorporating XRD-data for detailed matrix-mineralogy definition

Logging-while-drilling (LWD) and wire-line (WL) velocity data from the second Korean Ulleung Basin Gas Hydrate Drilling Expedition in 2010 (UBGH2) were used to predict gas hydrate saturations using the effective medium theory (EMT) at four representative sites: We used Sites UBGH2-10 and UBGH2-6 where gas hydrate occurs mostly within strata-bound sandy turbidite layers. Sites UBGH2-2_1 and UBGH2-2_2 are used to investigate a typical cold vent setting and fracture-dominated systems. We first use a new technique to correct the LWD velocity data within the shallower portions of the borehole (typically upper 60–100 m below seafloor, mbsf) where the nature of soft sediments often results in erroneous low velocity readings of the LWD tool around values between 1350 m/s and 1400 m/s. Using the Vertical Seismic Profile (VSP) data, a smooth background velocity trend is defined, onto which the LWD data is projected. After this correction, the LWD velocity data of all four sites were used to estimate gas hydrate saturation values using the EMT for a 2-phase mineral-assemblage of 90% clay and 10% quartz only, as well as a detailed mineralogy based on the core XRD data. Both these estimates are then compared to the estimates from the electrical resistivity data using an empirical Archie-analysis. The results from EMT and Archie are on average the same, when gas hydrate occurs as a pore-filling medium within thick sediment layers of homogenous nature, such as that encountered at Site UBGH2-10 where saturations are about 60% of the pore space in an interval of 124–128 mbsf. If gas hydrate occurs in thin sand layers where the thickness of the layer is at or smaller than the vertical resolution of the velocity logging tool, Archie-based results are higher than the EMT-based results due to the higher resolution of the resistivity logging-tool. In cold vent settings with fracture-dominated gas hydrate settings (such as that investigated at Sites UBGH2-2_1 and 2-2_2). Archie-based results are non-reliable and strongly over-estimated due to the anisotropic effect of near-vertical hydrate-filled fractures. In such cold vent settings, velocity-based techniques using the EMT are the preferred method to estimate the gas hydrate saturations. At Site UBGH2-2_1, hydrate saturations are on average 5–10% of the pore-space and even lower at Site UBGH2-2_2 (less than 5% on average).

Geotechnical and geophysical properties of deep marine fine-grained sediments recovered during the second Ulleung Basin Gas Hydrate expedition, East Sea, Korea

This study investigates the geotechnical and geophysical properties of minimally disturbed fine-grained marine sediments recovered during the second Ulleung Basin Gas Hydrate expedition (UBGH2), East Sea, Korea. Conventional laboratory geotechnical engineering test methods were used to characterize the sediments from a geotechnical engineering perspective. The sediments were mostly uncompacted and classified as highly plastic silty soils according to the Unified Soil Classification System (USCS); they exhibited high compressibility when subjected to an increase in effective stress (or a decrease in pore water pressure), low hydraulic conductivities, and low coefficient of consolidation (or pressure diffusion). It is expected that the application of a depressurization method for gas hydrate production at the investigated sites would cause a significant amount of settlement and compaction around the production hole. Because the shear wave velocity of gas hydrate-bearing sediments is much less sensitive to the stress state and is much higher than that of sediments without gas hydrates, it is possible to estimate the degree of hydrate saturation from shear wave velocity data measured in gas hydrate-bearing sediments. In addition, the prior presence of gas hydrates in the sediments was verified through a comparison of water content, compressibility, and electrical resistivity of the sediments.

Thermal conductivity of methane hydrate-bearing Ulleung Basin marine sediments: Laboratory testing and numerical evaluation

The thermal conductivity of gas hydrate-bearing sediments is a key parameter used to evaluate the heat transfer process relevant to phase transformation during gas hydrate production and to assess heat flux properties within the hydrate stability zone. Relatively few thermal conductivity values for in-situ hydrate-bearing sediments and laboratory hydrate-bearing sediments have been reported for under saturated conditions with vertical effective stress. In this study, we used sandy sediments obtained during the Ulleung Basin Gas Hydrate Expedition 2 (UBGH2) Expedition in Korea to synthesize methane hydrate-bearing sediments with saline pore fluid. The transient plane source method was implemented to measure the thermal conductivity of specimens with variable gas hydrate saturations. The measured thermal conductivity was ∼1.47 W/m/K and was independent of hydrate saturation. Results from additional tests of ice- and water-unsaturated sediments were also compared to our findings. Both geometric mean and 3D thermal network models corroborated the experimentally measured thermal conductivities assuming expected physical properties.

Characterization of gas hydrate reservoirs by integration of core and log data in the Ulleung Basin, East Sea

Examinations of core and well-log data from the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) drill sites suggest that Sites UBGH2-2_2 and UBGH2-6 have relatively good gas hydrate reservoir quality in terms of individual and total cumulative thicknesses of gas-hydrate-bearing sand (HYBS) beds. In both of the sites, core sediments are generally dominated by hemipelagic muds which are intercalated with turbidite sands. The turbidite sands are usually thin-to-medium bedded and mainly consist of well sorted coarse silt to fine sand. Anomalies in infrared core temperatures and porewater chlorinity data and pressure core measurements indicate that “gas hydrate occurrence zones” (GHOZ) are present about 68–155 mbsf at Site UBGH2-2_2 and 110–155 mbsf at Site UBGH2-6. In both the GHOZ, gas hydrates are preferentially associated with many of the turbidite sands as “pore-filling” type hydrates. The HYBS identified in the cores from Site UBGH2-6 are medium-to-thick bedded particularly in the lower part of the GHOZ and well coincident with significant high excursions in all of the resistivity, density, and velocity logs. Gas-hydrate saturations in the HYBS range from 12% to 79% with an average of 52% based on pore-water chlorinity. In contrast, the HYBS from Site UBGH2-2_2 are usually thin-bedded and show poor correlations with both of the resistivity and velocity logs owing to volume averaging effects of the logging tools on the thin HYBS beds. Gas-hydrate saturations in the HYBS range from 15% to 65% with an average of 37% based on pore-water chlorinity. In both of the sites, large fluctuations in biogenic opal contents have significant effects on the sediment physical properties, resulting in limited usage of gamma ray and density logs in discriminating sand reservoirs.

Gas hydrate occurrences and their relation to host sediment properties: Results from Second Ulleung Basin Gas Hydrate Drilling Expedition, East Sea

The Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) recovered various forms of gas-hydrate bearing sediments from 10 drill sites in the lower slope and basin floor of the Ulleung Basin. To characterize the gas-hydrate occurrences and the properties of the host sediments, whole-round core samples were taken from portions of recovered cores determined to be hydrate-bearing based on infrared (IR) scanning. These samples were further characterized by a variety of shipboard experiments such as imaging of the sediments with hand-held IR and visual cameras, measurements of pore water chlorinity within and around IR inferred cold regions in the core and grain-size analysis of pore-water squeeze cakes. Sediment compositions of selected samples were further characterized by X-ray diffraction and scanning electron microscopes during post-cruise analysis. The shipboard and post-cruise analysis results collectively indicate that the recovered gas hydrates mainly occur as 1) “pore-filling” type bounded by discrete silty sand to sandy silt layers, 2) “fracture-filling” veins and nodules, or 3) “disseminated” type in silt. In addition, minor but significant variation in gas hydrate concentrations were observed in diatomaceous silt where gas hydrates occur as “pore-filling” material in layers dominated by intact diatom frustules. Gas hydrate accumulations of “fracture-filling” type occur predominantly in regions where acoustic blanking features in the seismic record suggest gas migration from below the gas hydrate stability zone. Results from the UBGH2 core studies along with the analysis of similar samples from other expeditions, including those executed by the Ocean Drilling Program, the Integrated Ocean Drilling Program, and the First Ulleung Basin Gas Hydrate Drilling Expedition, greatly improved our understanding of lithologic controls on marine gas hydrate occurrences.

Stress-dependent and strength properties of gas hydrate-bearing marine sediments from the Ulleung Basin, East Sea, Korea

This paper presents a comprehensive laboratory experimental characterization of the host sediment from natural hydrate-bearing sediments recovered during the Ulleung Basin Gas Hydrate Expedition 2 (UBGH2) in 2010. The basic geotechnical properties are characterized and compared with previously reported data. The mineralogy and microstructure analyses confirm that the sediments mainly consist of illite, kaolinite, chlorite, calcite, and montmorillonite, with a high concentration of microfossils (diatoms). The presence of microfossils may affect many of the geotechnical properties, including plasticity, activity, specific surface, and grain size. The tested specimens have a relatively high initial void ratio, with a high compression index (e.g., Cc ranging from 0.6 to 1.0) and swelling index (e.g., Cs ranging from 0.1 to 0.25). The shear wave velocities measured during loading indicate a high sensitivity to the stress state, which demonstrates the critical role of effective stress in controlling the stiffness of the sediments. The peak friction angles are determined to be 17.6°–24.6° for clay and silt sediments, and 27.1° for high sand fraction specimens. The stress dependent and strength parameters obtained in this study may be very useful for performing stability analyses during hydrate dissociation and production.

Large-scale depositional characteristics of the Ulleung Basin and its impact on electrical resistivity and Archie-parameters for gas hydrate saturation estimates

Gas hydrate saturation estimates were obtained from an Archie-analysis of the Logging-While-Drilling (LWD) electrical resistivity logs under consideration of the regional geological framework of sediment deposition in the Ulleung Basin, East Sea, of Korea. Porosity was determined from the LWD bulk density log and core-derived values of grain density. In situ measurements of pore-fluid salinity as well as formation temperature define a background trend for pore-fluid resistivity at each drill site. The LWD data were used to define sets of empirical Archie-constants for different depth-intervals of the logged borehole at all sites drilled during the second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2). A clustering of data with distinctly different trend-lines is evident in the cross-plot of porosity and formation factor for all sites drilled during UBGH2. The reason for the clustering is related to the difference between hemipelagic sediments (mostly covering the top ∼100 mbsf) and mass-transport deposits (MTD) and/or the occurrence of biogenic opal. For sites located in the north-eastern portion of the Ulleung Basin a set of individual Archie-parameters for a shallow depth interval (hemipelagic) and a deeper MTD zone was achieved. The deeper zone shows typically higher resistivities for the same range of porosities seen in the upper zone, reflecting a shift in sediment properties. The presence of large amounts of biogenic opal (up to and often over 50% as defined by XRD data) was especially observed at Sites UBGH2-2_1 and UBGH2-2_2 (as well as UBGH1-9 from a previous drilling expedition in 2007). The boundary between these two zones can also easily be identified in gamma-ray logs, which also show unusually low readings in the opal-rich interval. Only by incorporating different Archie-parameters for the different zones a reasonable estimate of gas hydrate saturation was achieved that also matches results from other techniques such as pore-fluid freshening, velocity-based calculations, and pressure-core degassing experiments. Seismically, individual boundaries between zones were determined using a grid of regional 2D seismic data. Zoning from the Archie-analysis for sites in the south-western portion of the Ulleung Basin was also observed, but at these sites it is linked to individually stacked MTDs only and does not reflect a mineralogical occurrence of biogenic opal or hemipelagic sedimentation. The individual MTD events represent differently compacted material often associated with a strong decrease in porosity (and increase in density), warranting a separate set of empirical Archie-parameters.

Carbon cycling within the sulfate-methane-transition-zone in marine sediments from the Ulleung Basin

The significance of the various carbon cycling pathways in driving the sharp sulfate methane transition zone (SMTZ) observed at many locations along continental margins is still a topic of debate. Unraveling these processes is important to our understanding of the carbon cycle in general and to evaluate whether the location of this front can be used to infer present and past methane fluxes from deep reservoirs (e.g., gas hydrate). Here we report the pore water data from the second Ulleung Basin Gas Hydrate Expedition and on the results of a box model that balances solute fluxes among different carbon pools and satisfies the observed isotopic signatures. Our analysis identifies a secondary methanogenesis pathway within the SMTZ, whereby 25–35 % of the dissolved inorganic carbon (DIC) produced by the anaerobic oxidation of methane (AOM) is consumed by CO2 reduction (CR). To balance this DIC consumption, a comparable rate of organic matter degradation becomes necessary, which in turn consumes a significant amount of sulfate. The fraction of sulfate consumed by AOM ranges from 70 to 90 %. Whereas a simple mass balance would suggest a one to one relationship between sulfate and methane fluxes; our isotopic considerations show that methane flux estimates based solely on sulfate data may be in error by as much as 30 %. Furthermore, the carbon cycling within the SMTZ is fueled by a significant contribution (10–40 %) of methane produced by CR just below the SMTZ. Therefore sulfate gradient cannot necessarily be used to infer methane contributions from gas hydrate reservoirs that may lay tens to hundreds of meters below the SMTZ.