Hydrate Occurrence Research Articles

Pore-Water Geochemical Gradients of Sulfate, Calcium, Magnesium, and Iodide Correlated With Underlying Gas Hydrate Potential: A Case Study of the Shenhu Area, South China Sea

Geochemical profiles in pore water of marine sediments have been considered as, important indicators of gas hydrate occurrence. In the gas hydrate area around the world, the decrease of sulfate, calcium, and magnesium concentrations with depth mainly results directly or indirectly from the anaerobic oxidation of methane (AOM). The ubiquitous abnormally high concentration gradients of iodide in the research area reflect the large methane-generating potential of the area. Thus, we explore the feasibility of using gradients of sulfate, iodide, and authigenic carbonate precipitation as indicators for gas hydrate in the regional exploration of gas hydrate formation. We test the criterion in the gas hydrate zone in the South China Sea (SCS) for the recognition of gas hydrate by using the gradients of sulfate, calcium plus magnesium, and iodide. Contour maps of pore-water gradients from expeditions in the study area are used to correlate the key gradients to underlying gas hydrate occurrence. The results show that the largest potential gas hydrate indicated by the contour maps of the indicators are well consistent with the discovery of GMGS1 and GMGS3 expedition. It implies the possible applicability of this geochemical method in gas hydrate exploration. Also, we identify a promising area in the South China Sea for future gas hydrate investigations. It is the first collective application of the gradients of sulfate, calcium plus magnesium, and iodide to a gas hydrate terrane, especially to a large area of the SCS. We believe that the result of this research will benefit the future exploration of gas hydrate and will arouse a lot of interest from other researchers.

A unified effective medium modeling framework for quantitative characterization of hydrate reservoirs

Quantitative characterization of hydrate reservoirs is essential for evaluating hydrate resources and monitoring environmental hazards. Rock-physics models provide the theoretical basis for linking the hydrate saturation and distribution to the elastic properties. However, many of the existing models ignore the coupled effect of hydrate saturation and complex morphologies on the elastic behaviors and thereby fail to accurately describe hydrate reservoirs. To address these problems, the authors have proposed a unified effective medium modeling framework honoring multiple hydrate morphologies. In this framework, the manners in which the different types of hydrates interact with the sediment grains are characterized by different effective medium models. In addition to the hydrate saturation, the modeling results also reveal the effects of the cement factor, friction coefficient, incidence angle, lithology, and hydrate morphology on the elastic properties of the hydrate reservoirs. Then, five experimental data sets for synthetic methane and tetrahydrofuran hydrates formed under different conditions are used to validate the feasibility of the unified rock-physics modeling scheme. The good agreement between the theoretical predictions and laboratory data indicates that this method can effectively determine the hydrate occurrence mechanisms from the acoustic responses in different hydrate formation environments. On the basis of the constructed elastic rock-physics templates, multiple hydrate morphologies are determined with cluster analysis and then used for constraining the modeling scheme. Finally, the proposed modeling method is tested using sonic logs from Ignik Sikumi, North Slope of Alaska, and Hole U1518B, the northern Hikurangi margin, offshore New Zealand. The hydrate saturations estimated from the velocities are consistent with the interpretations based on resistivity and nuclear magnetic resonance logs, as well as chloride measurements, confirming the applicability and feasibility of our unified theoretical modeling framework.

The Thermal Effect of Submarine Mud Volcano Fluid and Its Influence on the Occurrence of Gas Hydrates

Mud volcanoes and other fluid seepage pathways usually transport sufficient gas for the formation of gas reservoirs and are beneficial to the accumulation of gas hydrate. On the other hand, the fluid thermal effects of mud volcanoes can constrain the occurrence of gas hydrates. Current field measurements indicate that fluid thermal anomalies impact the distribution of gas hydrates associated with mud volcanoes. However, due to the lack of quantitative analysis of the mud volcano fluid flow and thermal evolution, it is difficult to effectively reveal the occurrence of gas hydrates in mud volcano development areas and estimate their resource potential. This study took the Håkon Mosby Mud Volcano (HMMV) in the southwestern Barents Sea as the research object and comprehensively used seismic, well logging, drilling and heat flow survey data, combining the principles and methods of fluid dynamics and thermodynamics to study the fluid flow and heat transfer of a mud volcanic pathway. The space framework of the mud volcanic fluid temperature field thermal structure was established, the influence of the HMMV fluid thermal effect on gas hydrate occurrence was analyzed and the distribution and resource potential of gas hydrates in mud volcano development areas were revealed from the perspective of thermodynamics. This study provides a thermodynamic theoretical basis for gas hydrate accumulation research, exploration and exploitation under a fluid seepage tectonic environment.

Gas Hydrate Accumulation Related to Pockmarks and Faults in the Zhongjiannan Basin, South China Sea

The amplitude and coherence attributes of three-dimensional (3D) seismic data are used to confirm gas hydrate occurrence and to delineate its distribution in the Zhongjiannan basin, South China Sea. High amplitude anomalies (HAAs) are distributed above or below the regional base of gas hydrate stability zone (BGHSZ), which intersect with the bottom simulating reflectors (BSRs) or are interrupted by different types of pockmarks. The maximum amplitude attribute extracted along T1 (5.5 Ma) horizon is controlled by the widely distributed faults. The layer-bound polygonal faults (PFs) show networks of small normal faults, and the dominant orientations of PFs are similar or orthogonal to the regional tectonic faults, which provide the fluid migration pathways for gas and fluids to form HAAs. BSR shows the strong amplitude and continuous reflection where the faults or PFs can reach the BGHSZ without the influence of the pockmarks. Most of the pockmarks are related to the reactivation of faults and magmation, and some pockmarks are caused by the dissociation of gas hydrate. Around the matured pockmark, the BSR is discontinuous, and HAAs locally appear within the pockmarks. The inverted acoustic impedance profile shows obviously high values of HAAs except in pockmark zones. Partial HAAs occur above BGHSZ, and the continuity is interrupted by the pockmarks with only high values around the pockmarks. We propose that BSR, HAAs, pockmarks, and different types of faults are closely related to the occurrence and distribution of gas hydrates in the study area. This work allows us to understand the relationship between gas hydrate occurrence and accumulation with pockmarks, faults, and magmatic activities.

Gas Hydrate Dissociation Events During LGM and Their Potential Trigger of Submarine Landslides: Foraminifera and Geochemical Records From Two Cores in the Northern South China Sea

Although submarine slope failures and occurrence of gas hydrates are well known in the Dongsha area of the South China Sea the potential relationship between the aforementioned phenomena has not been clearly understood yet. Herein, we present carbon and oxygen isotope compositions of benthic foraminifera and sulfur isotopic composition of chromium reducible sulfur (CRS; δ34SCRS) from two cores from the Dongsha slope, aiming at identifying gas hydrate dissociation events in geological history. The geochemical data indicated that a large amount of gas hydrate dissociated at the beginning of the Last Glacial Maximum (LGM). Meanwhile, disturbances in the sedimentary strata revealed that a submarine landslide occurred at the end of the Last Glacial Maximum. Moreover, the associated abrupt increase of benthic foraminifera abundance implies that the submarine landslide was probably caused by an intense methane releasing from gas hydrate dissociation. A smaller scale submarine landslide related to gas hydrate dissociation was also recorded in core 973-5, retrieved from the flat area at the base of the slope.

Evolution of hydrate habit and formation properties evolution during hydrate phase transition in fractured-porous medium

Natural gas hydrate, as an efficient and clean energy resource, are naturally distributed in porous and fractured-porous medium. With the most recent development of advanced micro-scale imaging techniques, hydrate habits evolution, hydrate occurrences, and pore structure evolution as well as seepage characteristics during hydrate phase transition in porous hydrate-bearing sediments have been studied extensively at pore scale. However, there are few studies on gas hydrates in fractured-porous sediment. In this work, xenon hydrate phase transition experiment by excess-gas method is carried out in a fractured sandstone core with in-situ micro computed tomography (micro-CT) scanning to explore the evolution of hydrate habits and physical parameters of the host sediment. The results indicate that hydrate-bearing sediment is a dynamic equilibrium system as hydrate synthesis and decomposition occur simultaneously at each moment of hydrate phase transition induced by pressure change. The hydrate occurrences in fractured hydrate reservoirs include contiguous-sheet, clustered and isolated, which are slightly different from that of porous hydrate formation; and the contiguous-sheet hydrate is the occurrence that dominantly determines the seepage characteristics of fractured hydrate-bearing sediments. In addition, the logic diagram for hydrate growth paths in fractured-porous medium is presented for the first time. These findings are significant for detailed understanding of pore-scale hydrate distribution throughout phase transition process and provide theoretical basis for precise modeling of permeability in host sediments.

Research Progress on Global Marine Gas Hydrate Resistivity Logging and Electrical Property Experiments

Natural gas hydrate is widely spread in marine environments around the world. It has great energy potential due to its high methane gas content. High-precision exploration and evaluation of marine gas hydrate still face great challenges as it is affected by the complex reservoir control mechanisms and distribution characteristics. Resistivity is widely used in geophysical logging and theoretical research on gas hydrate-bearing reservoirs by utilizing the high sensitivity electrical response. In this paper, based on the examination of the global marine gas hydrate occurrences, resistivity logging results are summarized. Then the key remaining gas hydrate resistivity experimental concerns are reviewed. In summary, resistivity properties are a reliable means to derive the gas hydrate reservoir characteristics, despite the effect induced by the anisotropic properties of hydrate reservoirs and drilling technology. The overall resistivity change associated with the occurrence of pore filling gas hydrate in reservoirs are relatively small, and the specific value is affected by sediment lithology and hydrate saturation. On the other hand, fracture filling hydrate reservoirs have strong anisotropy, and massive hydrate occurrences (i.e., layers of gas hydrate with no sediment) section shows very high resistivity variation. Clay minerals are an important factor restricting the accurate estimation of gas hydrate saturations from in situ resistivity measurements. Many experimental studies have proposed the correction of Archie empirical formula, but widely representative models have not yet been developed. It is worth noting that more complex resistivity measurements may be able to provide additional electrical response information on various gas hydrate systems.

Double bottom simulating reflectors and tentative interpretation with implications for the dynamic accumulation of gas hydrates in the northern slope of the Qiongdongnan Basin, South China Sea

The origin of double bottom simulating reflectors (BSRs) and their relationship with gas hydrate systems are of great significance to the exploration of gas hydrate resources in submarine settings. Based on seismic data, this paper presents the first identified double BSRs in the northern slope of the Qiongdongnan Basin (QDNB), northern South China Sea. The imaged double BSRs are located in submarine ridges associated with a canyon system. These two BSRs (BSR1 and BSR2) are characterized by two subparallel high-amplitude reflections that mimic the seafloor’s distribution but have the opposite polarity. Patches of enhanced reflections (ERs) with high acoustic impedance indicate the occurrence of gas hydrate above BSR1 and between BSR1 and BSR2. The ERs indicate that the free gas terminates against BSR2 and is located beneath BSR2. The velocity spectrum of the strata shows that the interval velocity above the BSR1 and between BSR1 and BSR2 reaches as high as ∼2450 m/s, suggesting the presence of gas hydrates. The numerical modelling results of the gas hydrate stability zone (GHSZ) show that the geothermal gradient of the study area is 31.3–48.9 °C/km. Three potential formation mechanisms for these double BSRs are proposed: (1) variations in the P-T conditions in the GHSZ caused by erosion and redeposition associated with the submarine canyon; (2) precipitation of gas hydrates with different structures due to a supply of both thermogenic gas and biogenic gas; and (3) a relic paleo-BSR formed by the upward migration of a geothermal fluid through possible gas chimneys resulting in gas hydrate dissociation. BSR1 is correlated with the BGHSZ of structure I (SI) gas hydrates, and BSR2 may be correlated with the BGHSZ of structure II (SII) gas hydrates. Therefore, SI and SII gas hydrates may coexist in the study area, and SII gas hydrates and free gas may be present between BSR1 and BSR2. The occurrence of double BSRs suggests the dynamic accumulation of gas hydrates and free gas due to the erosion and redeposition of a submarine canyon, which shifted the BGHSZ and resulted in the differential distribution of the gas hydrates. A model illustrating the migration and accumulation of the hydrocarbons and gas hydrates associated with the occurrence of double BSRs is proposed, providing a reference for future gas hydrate exploration in the deepwater QDNB.

Occurrence of high-saturation gas hydrate in a fault-compartmentalized anticline and the importance of seal, Green Canyon, abyssal northern Gulf of Mexico

Occurrence of high-saturation gas hydrate in a fault-compartmentalized anticline and the importance of seal, Green Canyon, abyssal northern Gulf of Mexico

Mineralogy and pore characteristics of marine gas hydrate-bearing sediments in the northern South China Sea

Properties of host sediments are critical to understanding the formation and occurrence of gas hydrates in nature. This study investigates the mineralogy and pore characteristics of marine gas hydrate-bearing sediments (GHBS) within and below two high-saturation zones cored in drilling site W01 in the Qiongdongnan Basin in the South China Sea. The study involves an original application of high-resolution SEM imaging and automated mineralogy to fields-of-view (FOVs) large enough to undertake quantitative analyses at micro- to nano-scales. The samples are mainly fine-grained (<63 μm) and mineralogically dominated by illite, montmorillonite and quartz, significantly higher in clay mineral content than GHBS cored elsewhere in the South China Sea. Porosities determined from thin sections are from 27.6% to 33.0% and the pores are dominated by diameters of 0.1–1 μm. A sample from the upper high-saturation GHBS is rich in clay minerals and clay-sized grains with pores mainly 0.1–1 μm in size, while a sample from the lower zone of high-saturation GHBS has less clay minerals, more sands, and pore diameters of >1 μm are as important as those 0.1–1 μm in size. A third sample from an underlying zone has similar mineral compositions, grain-size and pore-size distributions to those in the uppermost high-saturation hydrate bearing zone. The largest pores are due to microfossil cavities, but their contribution to total porosity is <5%, suggesting their role in fine-grained sediments should not be overestimated. It is inferred that grain size and pore size, affected by variations in mineral content, are important factors influencing differences in hydrate accumulation. The geological source of gas and its migration are also factors determining the distribution of gas hydrate in the study area. Our approach using large FOVs to obtain quantitative micro- and nano-scale data from SEM images facilitates correlation to centimeter- and meter-scale information from well logs and seismic data, and offers opportunities to obtain new insights into the in-situ occurrence and accumulation of gas hydrate and its effects on physical properties of gas hydrate reservoirs.

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

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

Seismic characterization and modelling of the gas hydrate system in the northern Bay of Bengal, offshore Bangladesh

The offshore Bangladesh includes the northern Bengal fan, where sediment supply from the Ganges and Brahmaputra rivers has resulted in the accumulation of up to 20 km of shallow-marine, fluvio-deltaic and slope sediments that have accumulated during rapid tectonic subsidence since the late Miocene. The high sedimentation rates, along with high organic matter content, make this area favorable for the formation of natural gas from both microbial and thermogenic sources. Here we use multichannel seismic reflection profiles and modelling of the gas hydrate stability zone (GHSZ) to present the first evidence for the occurrence of natural gas hydrate in the offshore Bangladesh. First, we analyze the sediments of the shelf and slope areas, which are characterized by downslope sediment transport features and by the presence, in places, of faults/fractures as well as widely distributed amplitude anomalies and seismic facies that we relate to the presence of gas. A high-amplitude reversed polarity reflection of variable continuity that mimics the seafloor and cross-cut stratigraphy is interpreted as a Bottom Simulating Reflector (BSR). The BSR is observed in several areas that are predominantly located in the E-SE of the study area, in water depths of 1300–1900 m and at depths below seafloor of 250–440 m. Sediments above BSR locations generally show higher seismic interval velocities reaching values of ∼1920–1940 m/s, which are consistent with the presence of gas hydrate in shallow marine sediments. Furthermore, the BSR lies at approximately the same depth as the theoretical base of the gas (methane) hydrate stability zone (BGHSZ), calculated assuming a 3.5 % wt pore water salinity and using existing geothermal gradient and seafloor temperature data from the study region. However, in places, the BSR lies deeper or shallower than the base of the modelled BGHSZ. These discrepancies include areas where faults/fractures and seismic evidence linked to fluid flow from deeper reservoirs reach the GHSZ disrupting its stratigraphic continuity. At these locations, we suggest that faults/fractures act as fluid migration pathways causing localized heat-flow perturbations and/or changes in the hydrate-forming gas composition both likely affecting the depth of the GHSZ. Our results provide the first evidence of the gas hydrate potential in the offshore Bangladesh and should drive future research and data acquisition aiming to understand the composition, saturation and thickness of the gas hydrate-bearing sediments in this region.

Seismic analysis of the geological occurrence of gas hydrate in the Colombian Caribbean offshore

The bottom-simulating reflector (BSR) recognized in seismic sections represents the division between free gas and gas hydrate. This reflector has been identified in the permafrost and several petroliferous marine basins worldwide, including the Colombia offshore basins. Using high quality three-dimensional (3D) seismic data from the Sinú offshore basin (southern Caribbean), the BSR was exceptionally well-imaged within two geomorphological settings, including the Southern Caribbean Deformed Belt and the Colombia Basin, indicating possible gas hydrate presence in the study area. The BSR occurrence was mapped and categorized following existing nomenclature developed off Taiwan into three (3) categories: Basin-type, Ridge-type and Submarine Canyon-type. The BSR occurrence was dominated by the Ridge type category that was mainly found in the accretionary prism within modern and ancient anticlinal structures, followed by the Basin-type BSR found mainly at the Colombia Basin margin and a few intraslope sub-basins. The Submarine Canyon-type was found on small segments of the main drainage routes that cut through anticlinal structures. BSR distribution and characterization was also correlated with a seabed geomorphologic map, including thrusts with bathymetric expression, revealing that the BSR typology map closely resembled the N–S trend of subsurface structures in the south, as well as their NE-SW trend in the central portion of the accretionary prism. We suggest the dimensions (width and depth) of the seafloor features does influence BSR occurrence, tending to parallel major anticlines, and wider/deeper portions of intraslope sub-basins and submarine canyons, while omitting small-scale features.

Organic geochemical characteristics of sediments in the natural gas hydrate geo-system in Shenhu area, South China Sea

The Shenhu area of the South China Sea is one of the leading areas for natural gas hydrate investigation and exploitation in China. In this study, total organic carbon (TOC) content, total nitrogen (TN) content, and gas chromatographic-mass spectrometric (GC–MS) analysis of the soluble organic matter of sediments from sites W01B, W02B, and W03B were completed to understand the characteristics of organic matter in the sedimentary system with gas hydrates and provide theoretical support for further exploration in this area. Source indicators such as δ13Corg, TAR, and C21-/C21+ revealed that sites W01B and W02B have similar provenance characteristics in vertical profiles, with an increase of terrestrial source in the hydrate occurrence layers, while site W03B shows a gradual increase in marine source contribution as depth decreases. The asymmetric erosion and lateral deposition of the channel-levee depositional system are considered the controlling factors for the discrepancies in provenance indexes among the three sites. Analysis of the different source indicators revealed that the different kinds of soluble organic matter were subjected to varying degrees of biodegradation, and the distribution characteristics of n-alkanes were preferentially altered. Furthermore, it is found that biodegradations at sites W01B and W02B were more pronounced, particularly in hydrate occurrence layers. All of the microbial degradation indexes increase simultaneously and are consistent with the vertical distribution of hydrates, showing a similar triangular distribution trend. The increase in squalane, a characteristic compound representing methanogens, indicates that microbial degradation and methanogenesis are synchronized in the hydrate occurrence horizon, providing sufficient microbial gases for gas hydrates formation.

A Numerical Model for Determining Deep Methane Flux Linked to the Free Gas Zone: Application to the Ocean Drilling Program Site 995 and Implications for Regional Deep Methane Flux at the Blake Ridge

The lack of the quantification of deep dissolved methane flux prevents us from accurately understanding hydrate accumulation and distribution at a given geologic setting where vertically upward methane advection dominates the hydrate system. The upward deep methane flux was usually applied as an assumed value in many previous studies. Considering the deep methane flux changes the methane concentration in the pore water and further affects the phase transfer between the gas and aqueous phases depending on the in situ methane concentration, we link gas bubbles distribution to deep dissolved methane flux. Here, we constructed a numerical model to quantify the dissolved methane flux from depth based on the parameters related to gas bubble distribution, including the residual gas saturation in sediments and the free gas zone (FGZ) thickness. We then applied our model to ODP Site 995 at the Blake Ridge where methane was sourced from deep layers. Our model results predict an upward deep methane flux of 0.0231 mol/m2/a and the occurrence of another gas interval in deeper sediments, which are consistent with seismic data. We further explored the influence of upward methane flux on hydrate accumulation and found that the thin hydrate occurrence zone at nearby Site 994 likely resulted from a small deep methane flux. Combined with the previous conclusion of high deep methane flux at Site 997, we showed that along the Blake Ridge drilling transect the estimated deep methane fluxes decrease with increasing distance from the crest of the ridge. This approach for quantifying deep methane flux is complementary to the current hydrate accumulation model and provides new insights into the regional methane flux estimation at the Blake Ridge.

Geological, geophysical, and geochemical characteristics of deep-routed fluid seepage and its indication of gas hydrate occurrence in the Beikang Basin, Southern South China Sea

Deep-routed fluid seepage conduits are widely distributed in the Southern South China Sea (SSCS), and gas hydrates have been discovered inside these conduits through drilling and logging. Due to a lack of integrated geological, geophysical, and geochemical data, deep-routed fluid seepage conduits and their relationship to gas hydrate occurrence are still unclear. In this study, a comprehensive investigation of fluid seepage was conducted through systematic analysis of high-resolution 2-D seismic data, multibeam bathymetry data, headspace gas and carbon isotope components, authigenic carbonates, pore water, biomarkers, and hydrate phase equilibrium simulation. Multiple types of deep-routed conduits—which typically manifested as mud volcanoes, diapirs, fluid-escape pipes, and paleo-uplift associated faults—were identified. These manifestations were generally accompanied by Bottom Simulating Reflectors (BSRs), blanking zones, locally enhanced reflections, "pulled-up" reflections, pockmarks, mounds, and gas plumes, which indicated fluid seepage and possible gas hydrate accumulation. The Sulfate Methane Transition Zone (SMTZ) in the fluid seepage sites occurred at relatively shallow depths of 325–660 cmbsf (centimeter below the seafloor), suggesting a high methane flux and sufficient gas supply. The carbon isotope composition of the methane (−92.6‰ < δ13C < −50.2‰) and the detection of ethane, propane, and n-butane, as well as multiple geological configuration biomarkers, all indicate that deep-sourced hydrocarbons and thermogenic gas have migrated into the shallow layer through deep-routed conduits. Several geological and geochemical anomalies, including gas expansion, gas bubbling in piston core sediments, negative pore water chloride anomalies, and δ18O enrichment (3.8‰ < δ18O < 4.1‰) in the cold seep carbonates, provide evidence of hydrate dissociation during core recovery. The deep-routed fluid seepage and the accompanying hydrocarbon gas supply have a significant influence on the potential thermogenic gas-sourced hydrate accumulation and evolution. Overall, the co-existence of fracture-filling hydrates and pore-filling hydrates formed from the same supply of thermogenic gas and biogenic gas was inferred through comprehensive study and comparison with adjacent hydrate drilling areas. This article is the first systematic study of the characteristics of hydrate accumulation under the fluid seepage background in the Beikang Basin, and it provides a favorable reference for revealing the relationship between deep-sourced fluid seepage and hydrate formation. However, more exploration work needs to be conducted to confirm the above assumptions.

Numerical prediction of phase equilibrium conditions of gas hydrates up to 200 MPa in the presence of sodium and calcium chloride solutions

The occurrence of natural gas hydrates has both beneficial and negative practical implications. Nowadays, the most numerical studies in gas hydrates formation field were limited to a pressure of 74 MPa. However, these components are abundantly formed at permafrost and under the seafloor in which the pressure exceeds 100 MPa. In this study, the prediction of equilibrium conditions of gas hydrates was investigated numerically in pure gas and gas mixture up to 200 MPa. The inhibition of hydrates formation was examined by using NaCl and CaCl2 at different molar fractions. The Lee-Kesler-Plocker equation of state was selected as the most adequate, to predict gas hydrate stability limits, compared to other numerical approaches. The modeling results indicate that our model is in good agreement with experimental findings up to 200 MPa, but due to the lack of data in literature the maximum pressure compared was 163.25 MPa for gas mixture in pure water. The salt inhibition investigation also shows a good accuracy of our model at molar fractions less than 0.05 in both pure gas and gas mixture. It can be concluded that this model permits to study the stability limits of hydrates in wide range of gas composition and chloride salt solutions.

Gas Hydrate Saturation Estimates, Gas Hydrate Occurrence, and Reservoir Characteristics Based on Well Log Data from the Hydrate-01 Stratigraphic Test Well, Alaska North Slope

The Hydrate-01 Stratigraphic Test Well was drilled at the Kuparuk 7-11-12 site on the Alaska North Slope in December 2018. Sonic log data provide compressional (P) and shear (S) slowness from which we determine gas hydrate saturation (Sgh) estimates using effective medium theory. The sonic Sgh estimates compare favorably with Sgh estimated from resistivity and nuclear magnetic resonance (NMR) logs, showing that gas hydrate occupies up to approximately 90% of the pore space in the target reservoir sands. The informally named B1 sand (2294 feet below mean sea level) shows lower VP/VS ratios than the D1 sand (2770 feet below mean sea level), with the lower part of the B1 sand showing lower VP/VS ratios than the upper part of the B1 sand. This corresponds to a stiffer, or more “cemented”, behavior for the lower B1 sand and less cemented behavior for the D1 sand. This trend could be due to differences in the reservoirs themselves or in the gas hydrate morphology or to both factors. We observe that the presence of gas hydrate in the upper B1 sand has greater impact on hydraulic permeability (measurements suggest a greater difference between intrinsic and effective permeability) than in the D1 sand, possibly related to gas hydrate morphology but more likely due simply to higher gas hydrate saturations in the upper B1 sand. Analyses of Sgh relative to porosity, shale fraction, and intrinsic permeability show that reservoir quality (as represented by these three metrics) exerts control on gas hydrate saturation. Grain size and mineralogy data show somewhat smaller grains and better sorting in the D1 reservoir relative to the upper B1 reservoir and smaller grains and greater clay fraction in the lower B1 reservoir relative to the other two reservoir zones. Together, these data suggest that reservoir characteristics play a role in the observed VP/VS patterns, but gas hydrate morphology (possibly varying with saturation) must also be considered.

Hydrate formation and dissociation characteristics in clayey silt sediment

The geological occurrence of natural gas hydrates in clayey silt sediments has been extensively confirmed. Compared with sandy sediments, clayey silt is characterized by a large specific surface area, high capillary pressure, and high bound water content, which leads to distinct characteristics of hydrate formation and dissociation. The formation and dissociation of methane hydrates in clayey silt sediments with illite or montmorillonite were experimentally investigated. It was determined that an increase in the water content of silt and illite-silt sediments from ~9.47% to ~28.41% caused a decrease in the water-to-hydrate conversion ratio owing to the formation of a hydrate film between water and CH4 gas. However, in montmorillonite-silt sediments, the water-to-hydrate conversion rate increased between 9.47% and 18.94% of water saturation and decreased between 18.94% and 28.41% of water saturation under the combined effects of weakly bound water content and capillary pressure. In the hydrate dissociation experiment, the temperature of sediments with ~9.47% water content decreased more, which was caused by the smaller specific heat capacity of sediments with lower water content. Meanwhile, the temperature drops below the freezing point during the hydrate dissociation process, which is caused by the strong inhibition of hydrate phase equilibrium by montmorillonite. This study demonstrates the effects of clay type and water content on hydrate formation and dissociation, which is helpful for understanding the production behavior of clayey silt hydrate reservoirs.

Formation and occurrence characteristics of methane hydrate in the complex system of sodium dodecyl sulfate and porous media

ABSTRACT In recent years, porous media have attracted a lot of attention due to the enhanced kinetics of hydrate formation. However, how porous media promote hydrate formation is still an open topic. In this study, two different porous media, activated alumina and glass beads (average particle size 1, 3, 5 mm), were mixed with sodium dodecyl sulfate (SDS) to promote methane hydrate formation (275.15 K, 6 MPa). The smaller the particle size is, the better the promoting effect of porous media is, corresponding to faster formation kinetics and higher gas consumption. The average hydrate formation rate (2.3 mmol/min) and T90 (136 min) in the 1 mm activated alumina system was 2.6 and 0.4 times higher than the average hydrate formation rate (0.9 mmol/min) and T90 (349 min) in the 3 mm activated alumina system, respectively. Better performance of activated alumina particles than that of glass beads because of its rich micropores and unique electric double layer distribution in solution. The gas storage density of hydrate in the 1 mm alumina system can reach 129.13 Vg/Vh, while the highest gas storage density of hydrate in the 1 mm glass bead system was 122.35 Vg/Vh, which was similar to that of hydrate in the 3 mm activated alumina system (123.63 Vg/Vh). The gas consumption of the system was not affected by mass transfer but was controlled by particle size and solution volume with the presence of SDS. Under supersaturated and saturated conditions, it is found that the porous media bed and hydrate layer moved upward under the action of capillary force and methane gas driving force. This work provides a reference for the formation and occurrence of hydrates with the presence of porous media and surfactants.