Hydrate Accumulations Research Articles

Hydrocarbon gas seepage along the Gydratny Fault (Lake Baikal)

The Gydratny Fault, extending in SW-NE over 60 km, is situated in the central basin of Lake Baikal. This fault governs a distribution of hydrocarbon gas discharge zones on the lake bottom, many of them are hydrate-bearing. During the Class@Baikal-2019 expedition gas samples were obtained from bottom sediments along the fault zone, molecular and isotopic properties of the gases were studied. It is concluded that the fault zone is permeable for hydrocarbons and serves as an important fluid flow conduit from deep sources to the surface. The most intensive gas seepage was determined along the central and north-eastern segments of the fault zone where it is associated with the MSU hydrate-bearing structure and the Novosibirsk and Ukhan mud volcanoes. Within these structures the highest concentrations of methane and C2+ components were detected, the heaviest isotopic composition of methane carbon (–57‰ VPDB) were identified and near-surface gas hydrate accumulations were observed. The south-western segment of the fault is characterized by the lowest concentrations of gases in sediments with light carbon isotopic composition of methane (–76‰ VPDB).A dominant gas migration mechanism (advection or diffusion) in the upper part of the basin sedimentary section conditions the registered concentrations of methane and its homologues in sediments and their carbon isotopic composition. Diffusion and dispersion of migrated thermogenic gases and significant portion of microbial methane in seeping gases are characteristic for the areas where the fault is buried under relatively thick modern sediments. Focused gas migration followed by its focused discharge are specific for the areas where the fault propagates to the lake floor and is well-expressed in a bottom relief. Two models of hydrocarbon gases migration to the surface explaining the observed gas-geochemical characteristics of bottom sediments are described and discussed. Obtained results can help in enhancing interpretation of data of gas geochemical prospecting for offshore oil and gas.

Experimental study on the thermodynamic performance of a novel tetrabutylammonium bromide hydrate cold storage system

Clathrate hydrates with huge phase change latent heat are rapidly becoming a key means of improving and replacing current cold storage technologies. Efficient hydrate cold storage systems play a pivotal role in achieving carbon peaks. This study proposes a novel hydrate cold storage system, with a hydrate-on-coil component and an internal circulating gas disturbance device- and investigates the cold storage characteristics. In this study, comparative analyses of the tetrabutylammonium bromide (TBAB) hydrate cooling rate, cooling capacity, and energy storage ratio outside the hydrate-on-coil were conducted for various values of the coolant inlet temperature, flow rate, and gas disturbance, and the in situ formation morphology of TBAB hydrates outside the hydrate-on-coil were recorded. The results indicated that internal circulating gas disturbance achieved the double promotion effect of enhanced heat transfer and promoted hydrate phase change, which led to a significant increase in cooling capacity. Gas disturbance boosted the cooling capacity by approximately 20% to more than 80% of the theoretical cooling capacity. A theoretical cooling capacity of 86% was achieved at a storage temperature of 1 °C. Notably, gas disturbance could reduce the cooling capacity due to the increase in coolant temperature and the decrease in flow rate. The hydrate packing factor reached 79.5%, and the system energy storage efficiency was maintained at over 85%. Furthermore, the form of hydrate accumulation outside the hydrate-on-coil was influenced by natural convection and bubble uplift (forced convection). These two types of heat transfer affected the hydrate accumulation pattern and cold storage process. The insights gained from this study may provide important experimental support and a reference for the application and improvement of cold storage technology.

Study on the Formation Characteristics of CO2 Hydrate and the Rheological Properties of Slurry in a Flow System Containing Surfactants.

Adding low dosage hydrate inhibitors to the hydrate systems makes the generated hydrate particles more uniformly dispersed in the liquid phase, which can significantly reduce the hydrate accumulation and blockage in oil and gas pipelines. The effect of surfactant hydrophile–lipophilic balance (HLB) values on hydrate flow characteristics was studied with a flow loop. The experimental results showed that there was a critical HLB value. When the HLB value was 4.3–9.2, it had an inhibitory effect on the hydrate induction time, and when the HLB value was greater than 10.2, it had a promoting effect. The hydrate volume fraction increased gradually with the increase in the HLB value, while the slurry apparent viscosity decreased with the increase in the HLB value. It was also found that different types of surfactants all showed the effects of anti-agglomerant and dispersion, which can obviously improve the flow of the hydrate slurry. Finally, the analyzed results showed that the hydrate slurry exhibited shear-thinning behaviors that can be identified as a pseudoplastic fluid based on the Herschel–Bulkley rheological model, and the functional relationship between the rheological index and the solid phase hydrate volume fraction was obtained using the fitting method. This study can provide a reference for the preparation of high-efficiency hydrate anti-agglomerants.

Migration and accumulation characteristics of natural gas hydrates in the uplifts and their slope zones in the Qiongdongnan Basin, China

Various factors controlling the accumulation of natural gas hydrates (NGHs) form various enrichment and accumulation modes through organic combination. This study mainly analyzes the geological and geophysical characteristics of the NGHs occurrence in the uplifts and their slope zones within the deep-water area in the Qiongdongnan (QDN) Basin (also referred to as the study area). Furthermore, it investigates the dominant governing factors and models of NGHs migration and accumulation in the study area. The results are as follows. (1) The uplifts and their slope zones in the study area lie in the dominant pressure-relief direction of fluids in central hydrocarbon-rich sags in the area, which provide sufficient gas sources for the NGHs accumulation and enrichment through pathways such as gas chimneys and faults. (2) The top and flanks of gas chimneys below the bottom simulating reflectors (BSRs) show high-amplitude seismic reflections and pronounced transverse charging of free gas, indicating the occurrence of a large amount of gas accumulation at the heights of the uplifts. (3) Chimneys, faults, and high-porosity and high-permeability strata, which connect the gas hydrate temperature-pressure stability zones (GHSZs) with thermogenic gas and biogenic gas, form the main hydrate migration system. (4) The reservoir system in the study area comprises sedimentary interlayers consisting of mass transport deposits (MTDs) and turbidites. In addition, the reservoir system has developed fissure- and pore-filling types of hydrates in the pathways. The above well-matched controlling factors of hydrate accumulation enable the uplifts and their slope zones in the study area to become the favorable targets of NGHs exploration.

Geochemistry and sources of hydrate-bound gas in the Shenhu area, northern south China sea: Insights from drilling and gas hydrate production tests

Research on the origin and source of hydrate-bound gas and its relationship to deep conventional oil and gas accumulation in a basin is critical to understanding the accumulation mechanism of gas hydrates and to resource evaluation of gas hydrate accumulation. In this study, the hydrate-bound gas obtained via pressure coring and the production gas recovered during a production test on a gas hydrate reservoir in the Shenhu area offshore of Southern China were tested and discussed. The geochemical analysis results indicate that methane is the predominant gas, and heavier hydrocarbons (C2+) are also present but in low concentrations. The molecular compositions of the hydrate gas recovered from two production test sites are similar to those of the hydrate-bound gas acquired via pressure coring. In addition to the isotopic composition of the methane, the carbon and hydrogen isotopes of the C2+ hydrocarbons were obtained for the first time. The δ 13 C isotopes of the methane range from −66.6‰ to −46.2‰, indicating that the hydrate-bound gases have a mixed origin, containing both biogenic and thermogenic gases. The plot of δ 13 C 1 versus δ 13 C 2 suggests that the biogenic and thermogenic hydrocarbons were derived from marine organic matter and terrestrial organic matter, respectively. The isotopic characterization of the hydrate-bound gas reveals that the thermogenic hydrate gas contains both humic-type gas and sapropel-type gas, but the sapropel-type gas is predominant. The source rocks of the thermogenic hydrate gas are interpreted to be both the gas-prone coal measure strata of the Enping Formation and the oil-prone medium-deep lacustrine strata of the Wenchang Formation, the latter of which contributed more to the hydrocarbon supply of the gas hydrates. In addition, the maturity of the source rocks of the thermogenic hydrate gas may be lower than that of the deeply buried conventional hydrocarbons discovered in the Baiyun Sag-Panyu Low Uplift area. The seismic profile crossing the shallow gas hydrate accumulations and deep conventional gas reservoirs clearly shows that the proven LW3-1 thermogenic gas reservoir communicated with the gas hydrate stability zone through vertical migration pathways formed by high-angle faults and gas chimneys. This indicates that there was a cogenetic relationship between the thermogenic hydrate gas and the deep conventional hydrocarbon reservoir, which was supplied the thermogenic gas derived from both the Wenchang and Enping formations. The major implications of this finding are that it confirms, rather than theorizes, the identity of the hydrocarbon source rocks of the thermogenic hydrate gas in the Shenhu area for the first time, and it demonstrates the coupling relationship between the shallow gas hydrate accumulations and the deep conventional reservoirs proposed in previous studies. The mechanisms of hydrocarbon migration and gas hydrate accumulation in the production test gas hydrate reservoirs in the Shenhu area were also identified, providing a valuable reference for subsequent exploration and for the potential exploitation of gas hydrate reservoirs in the South China Sea. • The δ 13 C and δD isotopes of the heavier hydrocarbons (C2+) in the Shenhu area are obtained for the first time. • The origin of the hydrate gas obtained from the second trial production well in the Shenhu area is confirmed. • The hydrocarbon source rocks of the thermogenic hydrate gas in the Shenhu area are identified. • Coupled paragenetic relationship between shallow gas hydrate and deep petroleum system is revealed.

Effects of Differential Tectonic Activities on Overpressure Evolution in the Deep Water Area of the Qiongdongnan Basin: Implications for Natural Gas Hydrate Accumulation

Effects of Differential Tectonic Activities on Overpressure Evolution in the Deep Water Area of the Qiongdongnan Basin: Implications for Natural Gas Hydrate Accumulation

A 3D basin modeling study of the factors controlling gas hydrate accumulation in the Shenhu Area of the South China Sea

Great advancement has been made on natural gas hydrates exploration and test production in the northern South China Sea. However, there remains a lot of key questions yet to be resolved, particularly about the mechanisms and the controls of gas hydrates enrichment. Numerical simulaution would play signficant role in addressing these questions. This study focused on the gas hydrate exploration in the Shenhu Area, Northern South China Sea. Based on the newly obtained borehole and multichannel reflection seismic data, the authors conducted an integrated 3D basin modeling study on gas hydrate. The results indicate that the Shenhu Area has favorable conditions for gas hydrate accumulation, such as temperature, pressure, hydrocarbon source, and tectonic setting. Gas hydrates are most concentrated in the Late Miocene strata, particularly in the structual highs between the Baiyun Sag and the Liwan Sag, and area to the south of it . It also proved the existence of overpressure in the main sag of source rocks, which was subject to compaction disequilibrium and hydrocarbon generation. It also shown that the regional fault activity is not conducive to gas hydrate accumulation due to excess gas seepage. The authors conjecture that fault activity may slightly weaken overpressure for the positive effect of hydrocarbon expulsion and areas lacking regional fault activity have better potential.

Primary deposition and early diagenetic effects on the high saturation accumulation of gas hydrate in a silt dominated reservoir in the Gulf of Mexico

On continental margins, high saturation gas hydrate systems (>60% pore volume) are common in canyon and channel environments within the gas hydrate stability zone, where reservoirs are dominated by coarse-grained, high porosity sand deposits. Recent studies, including the results presented here, suggest that rapidly deposited, silt-dominated channel-levee environments can also host high saturation gas hydrate accumulations. Here we present several sedimentological data sets, including sediment composition, biostratigraphic age from calcareous nannofossils, grain size, total organic carbon (TOC), C/N elemental ratio, δ13C-TOC, CaCO3, total sulfur (TS), and δ34S-TS from sediments collected with pressure cores from a gas hydrate rich, turbidite channel-levee system in the Gulf of Mexico during the 2017 UT-GOM2-1 Hydrate Pressure Coring Expedition. Our results indicate the reservoir is composed of three main lithofacies, which have distinct sediment grain size distributions (type A-silty clay to clayey silt, type B-clayey silt, and type C-sandy silt to silty sand) that are characteristic of variable turbidity current energy regimes within a Pleistocene (< 0.91 Ma) channel-levee environment. We document that the TOC in the sediments of the reservoir is terrestrial in origin and contained within the fine fraction of each lithofacies, while the CaCO3 fraction is composed of primarily reworked grains, including Cretaceous calcareous nannofossils, and part of the detrital load. The lack of biogenic grains within the finest grained sediment intervals throughout the reservoir suggests interevent hemipelagic sediments are not preserved, resulting in a reservoir sequence of silt dominated, stacked turbidites. We observe two zones of enhanced TS at the top and bottom of the reservoir that correspond with enriched bulk sediment δ34S, indicating stalled or slowly advancing paleo-sulfate-methane transition zone (SMTZ) positions likely driven by relative decreases in sedimentation rate. Despite these two diagenetic zones, the low abundance of diagenetic precipitates throughout the reservoir allowed the primary porosity to remain largely intact, thus better preserving primary porosity for subsequent pore-filling gas hydrate. In canyon, channel, and levee environments, early diagenesis may be regulated via sedimentation rates, where high rates result in rapid progression through the SMTZ and minimal diagenetic mineralization and low rates result in the stalling of the SMTZ, enhancing diagenetic mineralization. Here, we observed some enhanced pyritization to implicate potential sedimentation rate changes, but not enough to consume primary porosity, resulting in a high saturation gas hydrate reservoir. These results emphasize the important implications of sedimentary processes, sedimentation rates, and early diagenesis on the distribution of gas hydrate in marine sediments along continental margins.

Dissociation of gas hydrates by hydrocarbon migration and accumulation-derived slope failures: An example from the South China Sea

The mechanism of slope failure associated with overpressure that is caused by hydrocarbon migration and accumulation remains unclear. High-resolution seismic data and gas hydrate drilling data collected from the Shenhu gas hydrate field (site SH5) offer a valuable opportunity to study the relations between submarine slope failure and hydrocarbon accumulation and flow that is associated with a ∼2 km-diameter gas chimney developed beneath site SH5 where none gas hydrates had been recovered by drilling and sampling despite the presence of distinct bottom simulating reflectors (BSRs) and favorable gas hydrate indication. The mechanism of submarine slope failure resulted from buoyancy extrusion and seepage-derived deformation which were caused by overpressure from a ∼1100 m-high gas column in a gas chimney was studied via numerical simulation. The ∼9.55 MPa overpressure caused by hydrocarbons that migrated through the gas chimney and then accumulated beneath subsurface gas hydrate-bearing impermeable sediments. This may have resulted in a submarine slope failure, which disequilibrated the gas hydrate-bearing zone and completely decomposed the gas hydrate once precipitated at site SH5. Before the gas hydrate decomposition, the largely impermeable sediments overlying the gas chimney may have undergone a major upward deformation due to the buoyancy extrusion of the overpressure in the gas chimney, and slope failure was initiated from plastic strain of the sediments and reduced internal strength. Slope failure subsequently resulted in partial gas hydrate decomposition and sediment permeability increase. The pressurized gas in the gas chimney may have diffused into the overlying sediments controlled by seepage-derived deformation, causing an effective stress reduction at the base of the sediments and significant plastic deformation. This may have formed a new cycle of submarine slope failure and finally the total gas hydrate dissociation. The modeling results of buoyancy extrusion and seepage-derived deformation of the overpressure in the gas chimney would provide new understanding in the development of submarine slope failure and the link between slope failure and gas hydrate accumulation and dissociation.

Experimental and Modeling Study of Kinetics for Hydrate Decomposition Induced by Depressurization in a Porous Medium

The hydrate decomposition kinetics is a key factor for the gas production from hydrate-saturated porous media. Meanwhile, it is also related to other factors. Among them, the permeability and hydrate dissociation surface area on hydrate dissociation kinetics have been studied experimentally and numerically in this work. First, the permeability to water was experimentally determined at different hydrate saturations (0%, 10%, 17%, 21%, 34%, 40.5%, and 48.75%) in hydrate-bearing porous media. By the comparison of permeability results from the experimental measurements and theoretical calculations with the empirical permeability models, it was found that, for the lower hydrate saturations (less than 40%), the experimental results of water permeability are closer to the predicted values of the grain-coating permeability model, whereas, for the hydrate saturation above 40%, the tendencies of hydrate accumulation in porous media are quite consistent with the pore-filling hydrate habits. A developed two-dimensional core-scale numerical code, which incorporates the models for permeability and hydrate dissociation surface area along with the hydrate accumulation habits in porous media, was used to investigate the kinetics of hydrate dissociation by depressurization, and a “shrinking-core” hydrate dissociation driven by the radial heat transfer was found in the numerical simulations of hydrate dissociation induced by depressurization in core-scale porous media. The numerical results indicate that the gas production from hydrates in porous media has a strong dependence on the permeability and hydrate dissociation surface area. Meanwhile, the simulation shows that the controlling factor for the dissociation kinetics of hydrate switches from permeability to hydrate dissociation surface area depending on the hydrate saturation and hydrate accumulation habits in porous media.

Integrated signatures of secondary microbial gas within gas hydrate reservoirs: A case study in the Shenhu area, northern South China Sea

A series of highly saturated pore-filled gas hydrate deposits were recently discovered in the Shenhu area of northern South China Sea. However, the main hydrocarbon sources of the gas hydrates remain controversial. The main hypotheses include a shallowly buried microbial gas source or a deeply buried thermogenic hydrocarbon source. In this study, an integrated geochemical-microbial-geological analysis of gas hydrate and sediment samples from sites SC1 and SC2 in the Shenhu area was conducted to investigate the possible hydrocarbon sources, and to identify the signatures of the secondary microbial gases in the gas hydrate reservoirs. The results show that the gas hydrates from sites SC1 and SC2 exhibit the geochemical characteristics of secondary microbial gas (C1/(C2+C3) >100, −50.2‰<δ13C–C1<−47.4‰), and the propane (C3) component and liquid thermogenic hydrocarbons (C15+) have been severely biodegraded (a strongly elevated δ13C–C3 value and a high abundance of 25-norhopanes). In addition, we discovered that the in-situ Quaternary marine sediments exhibit relatively low generation potential of primary microbial gas because of the extremely low organic matter richness (total organic matter content (TOC) < 0.3%) and low in-situ temperatures (5–18 °C). Furthermore, a relatively high abundance of methanogens (∼7%–15%) were detected within the gas hydrate reservoir, which may be attributed to the methane generation process of hydrocarbon biodegradation. Moreover, the geological-geophysical interpretations indicate that the distributions of the gas hydrate accumulations and the shallow gas pools are strictly controlled by the effective faults connecting them with the deeply buried thermogenic source rocks. These lines of evidence suggest a significant contribution of secondary microbial methane to sites SC1 and SC2, which further indicates that the main contribution was from deeply buried thermogenic hydrocarbon sources. Our results suggest that the secondary microbial methane within gas hydrates is an important terminal product that has been converted from thermogenic hydrocarbons via microbial degradation, which helps explain why the geochemistry of most of the hydrate-bound gas indicates a microbial origin even though the geological evidence indicates that the gas is mainly sourced from deeply buried thermogenic hydrocarbons.

Effect of hydrophilic silica nanoparticles on hydrate formation during methane gas migration in a simulated wellbore

Natural gas hydrates are mostly formed in low-permeability and fractured muddy sedimentary formations. Adding suitable nanoparticles to the drilling fluid system can improve its filtrate resistance and fracture plugging, and effectively weaken the invasion of drilling fluid into the reservoir. However, it is likely that nanoparticles promote hydrate formation and accumulation in wellbores which will induce accidents. Therefore, this study investigated the effect of hydrophilic silica nanoparticles with particle sizes of 30 nm, 60 nm, and 80 nm and concentrations of 0.5–4.0 wt% on hydrate formation during upward migration of methane gas using a dynamic simulation system for hydrate formation in a wellbore. The experimental results show that under the condition of methane gas migration, hydrophilic silica nanoparticles inhibit hydrate formation. The inhibition effect increased with the growth in the particle size under a constant concentration, whereas it first increased and then decreased with increasing nanoparticle concentration under a constant particle size. The strongest inhibition effect was observed at a hydrophilic silica nanoparticle concentration of 2.0 wt%. The influence of hydrophilic silica nanoparticles on hydrate formation may be mainly determined by their hydrophilic properties, heat and mass transfer, and gas migration in the wellbore. Our research indicates that hydrophilic silica nanoparticles can be added to hydrate drilling fluid systems if their concentration can be properly controlled.

Characterization of bacterial and archaeal community structure in deep subsurface sediments in the Shenhu area, northern South China Sea

Gas hydrate-bearing sediments provide an extreme and unique habitat for bacteria and archaea, but the relationship between the microbial community and gas hydrate formation is poorly known. In the Shenhu area of the northern South China Sea, it was inferred that the gas source is mixed biogenic-thermogenic, and contradictions exist between relatively low total organic carbon content and high saturation of gas hydrates. The synthesized geophysical and geological evidence showed that deep thermogenic fluid could migrate upward through migrating pathways. In this study, samples were collected from Shenhu area from 224 to 271 mbsf (meters below the seafloor), which covered gas hydrate-bearing sediments (GHBS), free gas-bearing sediments (FGBS), and non-gas hydrate-bearing sediments (NGHBS), and 16S rRNA gene sequencing was used to characterize bacterial and archaeal community structure. The results revealed significant differences of microbial community structure among the three sediment types as indicated by alpha (richness and diversity indices) and beta (Bray-Curtis-based non-metric multidimensional scaling) diversity analyses as well as community composition analyses. In bacterial community, abundant genera included Pseudomonas, Halomonas, Alcanivorax, Thiobacillus, Marinobacter, Rhodococcus, and Dietzia, some of which were identified as putative hydrocarbon degraders. Most abundant archaeal genera were potential methanogenic archaea, and the dominant genera between GHBS and FGBS differed. Multiple types of methanogenesis were noted, and the hydrogenotrophic type was inferred to be predominant. Deviations of temperature of methanogens from the geothermal gradient of the study area again suggested the migration of organic matters. Based on the characterization of bacterial and archaeal community structure in the deep subsurface sediments, we proposed a geological model for the gas hydrate formation and accumulation of the gas hydrate reservoir, emphasizing the possibility of microbially-mediated secondary microbial gas formation from deep thermogenic fluid. This study characterized bacterial and archaeal community structure in deep subsurface sediments, which can provide insight into the potential roles of bacteria and archaea including hydrocarbon degradation and methanogenesis in gas hydrate formation and help reevaluate the contribution of deep thermogenic fluid to gas hydrate formation in the Shenhu area of the northern continental slope of the South China Sea.

Gas hydrate accumulation in shelf break setting: Example from the Qiongdongnan Basin in the northern slope of the South China Sea

Based on interpretation and inversion of high‐resolution seismic data, a previously undocumented occurrence of pore‐filled gas hydrates that are associated with a submarine canyon system was discovered in the shelf break of the north‐east Qiongdongnan Basin (QDNB) in the northern South China Sea (SCS). Continuous bottom simulating reflections (BSRs) were recognized in the submarine striking NW‐SE ridges along the shelf break. The two‐way travel times (TWTs) of the BSRs are about 200–250 ms below the sea floor and the burial depths are mapped at ~180–225 mbsf after time‐depth conversion. The distribution of the enhanced reflections (ERs) and acoustic blanking zone overlying the BSRs indicate possible pore‐filled gas hydrate enrichment. ERs have been commonly observed immediately below the BSRs, indicating the accumulation of free gas, which exhibits distinct amplitude versus offset (AVO) anomalies. The acoustic impedance inversion results show that the gas hydrate saturation is 20–50%, and the thickness of the gas hydrate‐bearing layers is usually ~10–40 m, with a maximum thickness of up to ~90 m in local regions. Large‐scale listric faults originating from the deep sag connect the hydrocarbon source region and the shallow gas hydrate stability zone (GHSZ) and act as migration pathways for biogenic gas and potentially thermogenic gas. In addition, hydrocarbons may also be transported into the GHSZ along the sidewalls of the canyon. Canyon incision is believed to cause the increase in the burial depth of the BSRs on the flank of the ridge toward the base of the channel and thereby to trap the hydrocarbons migrating along the sidewall in permeable beds due to the decrease in permeability caused by the gas hydrate accumulation, resulting in the occurrence of ERs immediately below the BSRs. It is speculated that in the channel‐levee system, the coarse‐grained sediments should be deposited within the GHSZ, and the reservoir's physical properties are conducive to the wide distribution and enrichment of pore‐filled gas hydrates in the shelf break of the northern QDNB.

Gas hydrate stability zone in Muri coalfield, Qinghai Province, China

ABSTRACTThe gas hydrate stability zone (GHSZ) is the essential condition for gas hydrate accumulation, which is controlled by three main factors: gas component, geothermal gradient and permafrost thickness. Based on the gas component of hydrate samples from drilling in Muri coalfield, the gas hydrate phase equilibrium curve was calculated using Sloan's natural gas hydrate phase equilibrium procedure (CSMHYD) program. Through temperature data processing of coalfield boreholes, some important data such as thickness of permafrost and geothermal gradient were obtained. The GHSZ parameters of a single borehole were calculated by programming based on the above basic data. The average thickness of GHSZ of 85 boreholes in Muri coalfield amounted to approximately 1000 m, indicating very broad space for gas hydrate occurrence. The isogram of GHSZ bottom depth drawn from single borehole data in Muri coalfield demonstrated the regional distribution characteristics of GHSZ, and identified three favourable areas of gas hydrate occurrence where the bottom of GHSZ had a burial depth &gt;1500 m – namely, the southern part of Juhugeng Mining Area, the middle part of Duosuogongma Mining Area and the eastern part of Xuehuoli Mining Area.

Occurrence of Tankhoy field coals in South Baikal bottom sediments

The purpose of the study is to describe the first finds of coal-bearing clays and coals in the bottom sediments of the southern basin of Lake Baikal and compare them with terrestrial coal-bearing deposits of the Tankhoy field. Comparative analysis of the lithological composition and colour of bottom sediments and terrestrial sections, as well as the concentration of organic carbon and conducted palynological analysis allowed their correlation. At the lake’s depth of 900 m the authors discovered a coal-bearing strata in situ (st 56), which later was stratigraphically correlated with the terrestrial coalbearing part of the Tankhoy suite. The fragments of coal found in bottom sediments basically along the entire Tankhoy field, especially bedrock coals on the underwater slope in South Baikal up to 1300 m deep prove the distribution of the coal-bearing part of the Tankhoy suite in the sublacustrine part of the lake throughout the entire slope (from 5 to 10 km offshore) and confirm the distribution area of the Tankhoy paleolake over a significant area of the contour of modern southern basin of Lake Baikal. The finds of coal-bearing strata on these and other various sub-bottom depths, i.e. under various pressure and temperature conditions, suggest that coals themselves and coal-bearing mudstones may be a generation facility of secondary microbial methane. This should be taken into account when searching for gas hydrocarbon and gas hydrate accumulations as well as assessing methane cycles in Lake Baikal.

Gas Hydrate Accumulation and Occurrence Associated with Cold Seep Systems in the Northern South China Sea: An Overview

Studying deep-water cold seep systems is of great significance to gas hydrate exploration due to their close relationship. Various cold seep systems and related gas hydrate accumulations have been discovered in the northern South China Sea in the past three decades. Based on high-resolution seismic data, subbottom profiles, in situ submergence observations, deep drilling and coring, and hydrate gas geochemical analyses, the geological and geophysical characteristics of these cold seep systems and their associated gas hydrate accumulations in the Qiongdongnan Basin, the Shenhu area, the Dongsha area, and the Taixinan Basin have been investigated. Cold seep systems are present in diverse stages of evolution and exhibit various seabed microgeomorphic, geological, and geochemical features. Active cold seep systems with a large amount of gas leakage, gas plumes, and microbial communities and inactive cold seep systems with authigenic carbonate pavements are related to the variable intensity of the gas-bearing fluid, which is usually derived from the deep strata through mud diapirs, mud volcanoes, gas chimneys, and faults. Gas hydrates are usually precipitated in cold seep vents and deeper vertical fluid migration pathways, indicating that deep gas-bearing fluid activities control the formation and accumulation of gas hydrates. The hydrocarbons collected from cold seep systems and their associated gas hydrate reservoirs are generally mixtures of biogenic gas and thermogenic gas, the origin of which is generally consistent with that of deep conventional gas. We also discuss the paragenetic relationship between the gas-bearing fluid and the seafloor morphology of cold seeps and the deep-shallow coupling of gas hydrates, cold seeps, and deep petroleum reservoirs. It is reasonable to conclude that the deep petroleum systems and gas-bearing fluid activity jointly control the development of cold seep systems and the accumulation of gas hydrates in the northern South China Sea. Therefore, the favorable areas for conventional oil and gas enrichment are also prospective areas for exploring active cold seeps and gas hydrates.

Quaternary deep-water sedimentary characteristics and their relationship with the gas hydrate accumulations in the Qiongdongnan Basin, Northwest South China Sea

Recent geological investigations and offshore drillings have revealed the great prospects of the gas hydrate resources in the Quaternary sediments of the Qiongdongnan Basin (QDNB). Pore-filling gas hydrates were first discovered in the sand layers during the 2019 gas hydrate drilling expedition (GMGS6) in the QDNB. Logging data, 2D/3D seismic data, cores, and grain size analysis were used to study the sedimentary facies and their relationships with the gas hydrates. It was found that the Quaternary sediments are hemipelagic deposits, mass transport deposits (MTDs), and channel-levee facies. The MTDs are characterized by vertical superposition and transverse migration. Both fracture-filling (in vein, massive, and nodular occurrences) and pore-filling (disseminated) gas hydrates were identified. The fracture-filling gas hydrates mainly occur in the muds. The pore-filling gas hydrates mainly occur in the sand layers of the channel-levee facies, which can provide reservoir spaces for the gas hydrates. It is speculated that the MTDs could be a favorable seal for the accumulation of gas hydrates. Multi-stage gas charging may have caused the multi-layer distribution of the gas hydrates. Under the premise of sufficient gas sources, the combination of channel-levee facies and overlying MTDs is conducive to the enrichment and preservation of pore-filling gas hydrates, which could be an exploration target. This study provides a reference for gas hydrate prediction and exploration in the QDNB and other prospecting areas with similar geological settings around the world.

Modeling Growth Kinetics of Methane Hydrate in Stirred Tank Batch Reactors

Hydrate formation could be looked upon as multicomponent and multiphase reaction which is heavily dependent on mass transfer and heat transfer limitations even under favorable thermodynamic conditions. Gas uptake measurement is one of the easiest ways to understand the kinetics of hydrate growth. In a typical gas uptake measurement, one could easily observe three phases of hydrate formation: in phase-I, hydrate forming gas dissolves in the liquid phase which leads to hydrate nucleation; in phase-II, fast hydrate growth is observed; and in phase-III, hydrate grows slowly for relatively longer time periods. In a batch reactor, a slow down in hydrate growth rate as seen in phase-III is either due to a drop in the pressure of the reactor during hydrate growth and/or reduced mass transfer due to hydrate accumulation at the interface. In this work, a model is developed to predict phase-II events. The model proposed is based on an earlier model available in the literature which captured the intrinsic kinetics of gas hydrate growth for a semibatch reactor. Model discussed in the current study works for batch and semibatch reactor, it captures the kinetics for different stirrer speeds, water to gas ratios and different thermodynamic conditions. Experimental validation was done in a batch reactor at 274 K and 6 MPa with methane as the hydrate forming gas. A batch reactor with two different stirrer arrangements, different water-to-gas ratios, and different stirrer speeds were considered, and the mass transfer limitations for both the reactor configurations were studied. Further, a comparison study with the existing model and the modified model (current study) showed that the current model can be extended to other reactor types.

Doubly substituted isotopologues of methane hydrate (13CH3D and 12CH2D2): Implications for methane clumped isotope effects, source apportionments and global hydrate reservoirs

Recently developed methane clumped isotope techniques provides fresh and novel insights into methane biogeochemistry, which have been unobtainable through other techniques such as conventional stable isotope determinations and molecular composition measurements of hydrocarbons. Nonetheless, the governing processes and mechanisms which control the clumped isotope signatures (Δ13CH3D and Δ12CH2D2) of natural methane samples remain an active area of investigation. Here, we present paired clumped isotope measurements in methane hydrate, which is a major methane reservoir widely distributed along the continental margins and which plays an important role in the global carbon cycle and climate system. Our study aims to shed new light into the fundamental processes of methane clumped isotope effects and their potential to answer fundamental questions regarding the source and migration of methane found in naturally occurring gas hydrate accumulations.Gas hydrate samples were recovered from five shallow marine sediment sites on the eastern margin of the Japan Sea and most of them present Δ13CH3D and Δ12CH2D2 temperatures ranging from ∼15 to ∼170 °C that apparently match expected methane formation temperatures. The distribution of these clumped isotope signatures along the equilibrium line is best explained by the mixing effect of equilibrated thermogenic methane formed at temperatures of 165 ± 15 °C and biogenic methane equilibrated at 1–2 °C, which may result from slow methanogenesis, anaerobic oxidation of methane (AOM), or their combination. The influences of gas migration/diffusion, hydrate formation and dissociation on Δ13CH3D and Δ12CH2D2 values are insignificant. By combining clumped isotope results with other traditional approaches, a thermogenic and two microbial end-members as well as their isotopic compositions were identified and the relative contribution of each end-member was also quantified. The results not only demonstrate the applicability of methane clumped isotope data to identify potential end-members in natural methane samples, but also reveal that more conventional carbon isotope approaches may significantly underestimate the fraction of thermogenic methane present in global gas hydrate reservoirs. Improvements in the accuracy of source apportionment enable us to better understand the formation history and mechanisms of gas hydrate accumulation, as well as the role played by gas hydrate dissociation in past geological events. The estimated formation temperatures of thermogenic end-member can be further applied in reconstruction of the paleo geothermal gradient at the time when the thermogenic methane was formed at marine sedimentary environment.