Hydrate Accumulations Research Articles

Compression behaviour of hydrate bearing carbonate sand - fines mixtures

High gas hydrate content has been found in fine-grained sediments containing substantial amount of foraminifera in the South China sea. One of the possible hydrate accumulation habits is filling the intra- particle voids in the foraminifera. To understand the effects of this hydrate accumulation habit on the compression behaviour of the fine-grained sediment, two series of isotropic compression tests were conducted. Due to high intra-particle porosity, carbonate sand (CS) was mixed with the fines to mimic the hydrate formation in the intra-particle voids in the laboratory prepared soil specimens. The compression test results revealed that the mixtures of fines with as high as 40% CS content can exhibit the transitional behaviour such that non-convergent compression lines are observed at the high stress level. It is evident that breakage of CS grains is negligible in these mixtures. Hence, the initial fabrics are not erased under the high stress level resulting in non-convergent compression lines. The compression curves of the hydrate bearing CS-fines mixtures can be classified into three different stress regimes. There is no significant difference in the compressibility of the soil mixtures with and without hydrate in the low stress regime. As the stress increases further reaching the medium stress regime, the hydrate reduces the compressibility of the soil mixtures with increasing hydrate saturation. Upon reaching the high stress regime, the bond breakage at the inter-particle contacts becomes significant leading to the convergence of compression curves between the hydrate bearing and host soil mixtures. It is also found that a new effective hydrate saturation, representing the amount of hydrate in the inter-particle voids, is better correlated to the compressibility of the hydrate bearing soil mixtures.

Diapir structure and its constraint on gas hydrate accumulation in the Makran accretionary prism, offshore Pakistan

ABSTRACT The Makran accretionary prism is located at the junction of the Eurasian Plate, Arabian Plate and Indian Plate and is rich in natural gas hydrate (NGH) resources. It consists of a narrow continental shelf, a broad continental slope, and a deformation front. The continental slope can be further divided into the upper slope, middle slope, and lower slope. There are three types of diapir structure in the accretionary prism, namely mud diapir, mud volcano, and gas chimney. (1) The mud diapirs can be grouped into two types, namely the ones with low arching amplitude and weak-medium activity energy and the ones with high arching amplitude and medium-strong activity energy. The mud diapirs increase from offshore areas towards onshore areas in general, while the ones favorable for the formation of NGH are mainly distributed on the middle slope in the central and western parts of the accretionary prism. (2) The mud volcanoes are mainly concentrated along the anticline ridges in the southern part of the lower slope and the deformation front. (3) The gas chimneys can be grouped into three types, which are located in piggyback basins, active anticline ridges, and inactive anticline ridges, respectively. They are mainly distributed on the middle slope in the central and western parts of the accretionary prism and most of them are accompanied with thrust faults. The gas chimneys located at different tectonic locations started to be active at different time and pierced different horizons. The mud diapirs, mud volcanoes, and gas chimneys and thrust faults serve as the main pathways of gas migration, and thus are the important factors that control the formation, accumulation, and distribution of NGH in the Makran accretionary prism. Mud diapir/gas chimney type hydrate develop in the middle slope, mud volcano type hydrate develop in the southern lower slope and the deformation front, and stepped accretionary prism type hydrate develop on the central and northern lower slope. The middle slope, lower slope and deformation front in the central and western parts of the Makran accretionary prism jointly constitute the NGH prospect area.

РУДНЫЕ, НЕФТЕГАЗОВЫЕ РАЙОНЫ ЮЖНО-ОХОТСКОЙ ПРОВИНЦИИ И ГЛУБИННАЯ ГЕОДИНАМИКА

In the South Okhotsk Sea province – on the islands of Sakhalin, Kunashir, Iturup, Urup and surrounding sea areas – many occurrences of rare, noble metal and other mineralizations as well as of oil-and-gas fields, gas hydrate accumulations, and isolated areas of active emission of water-hydrocarbon gases are known. Occurrences and deposits of solid, liquid and gaseous mineral resources are controlled by hidden deep fault transform zones: Nosappu (Tuscarora), Iturup, and Urup. These long-lived extended (more than 1000 km) zones are distinguished at the N-W Pacific megaplate margin near the S-E flank of the Kuril-Kamchatka trogue. Using the seismotomographic methods we have established their extension to the west from the seismic focal zone in the oceanic slab that subducted into the transition zone of the mantle. In the areas of strike-slip extension the faults accounted for the active formation of the drainage channels for the penetration of the sea water in the lithosphere with the following serpentinization of its ultramafites, and for decompressional generation of ascending mantle-derived abiogenic fluid flows. The latter penetrated from the underslab asthenosphere in the oversubduction mantle wedge and beneath the lithospheric mantle, where they accounted for the development of the processes of metasomatism. The subsequent migration of flows initiated the creation of primary magma reservoirs in the lower parts of the continental lithosphere, and intermediate and peripheral chambers in the Earth’s crust. The injection of melts from the chambers in the consolidated Earth's crust led to the formation of abyssal, hypabyssal intrusive massifs, arch-dome uplifts and magmatogenic-ore (ore-magmatic) systems predominantly among the rocks of the pre-Pliocene basement. The concentration of oil and gas accumulations mainly from the mantle-derived abiogenic hydrocarbons containing mercury, gold, rhenium, and PGE in the Cenozoic sedimentary basins amidst the reservoirs under the impermeable beds also resulted from deep under- and overslab fluid flows.

Heterogeneity properties of methane hydrate formation in a pilot-scale hydrate simulator

The accumulation and distribution features of gas hydrates are the significant issues affecting the hydrate resource assessment, the hydrate dissociation behaviors, and the risk evaluation of hydrate exploitation. This work aims to investigate the methane hydrate formation process by numerical simulation based on the experimental data in a pilot-scale hydrate simulator. Methane hydrate is formed by multi-step water injection method. Simulation results of the evolutions of the temperature, the system pressure, the mass of formed hydrate, and the remaining mass of methane gas are in good agreement with those of the experiment. The spatial distributions of the gas, water, and hydrate are all found to be very heterogeneous in the vessel during the whole simulation period. The free methane gas tends to accumulate in the pores near the upper boundary because of buoyancy and diffusion effects, while the liquid water shows an opposite trend under the gravity and capillary effects. Finally, the formed hydrates are found to be mainly accumulated in the upper area of the reactor due to the more favorable gas-water contact conditions, and the hydrate saturation decreases from the roof to the bottom layer by layer. In addition, the inhomogeneity of methane hydrate is more pronounced along the vertical direction than the horizontal one during the formation process. Generally, the heterogeneous formation of gas hydrate is dominated by the migration processes of gas and water in porous media. Moreover, the selected kinetic model is also important for the description of the heterogeneous hydrate formation behavior.

Geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the first offshore gas-hydrate production test region in the Shenhu area, Northern South China Sea

Abstract Integrated three-dimensional seismic, logging, sediment cores, and geochemical testing data were collected from Guangzhou Marine Geological Survey 3 and 4 hydrate drilling expeditions and used in this study for a comprehensive investigation of the geological and geophysical features and accumulation mechanism of hydrates in the first offshore gas-hydrate production test region (GHPTR) in the Shenhu area of the South China Sea. Seismic signatures indicative of disseminated hydrates and free gas include the bottom simulating reflector (BSR), gas chimney, and mud diapir associated with enhanced seismic reflections, acoustic blanking, masking, and chaotic appearance have been observed. The acoustic travel-time responses, density, and compensated neutron three porosity log analysis, high-precision grid tomography inversion analysis, and constrained sparse spike inversion confirm the presence of free gas below the gas-hydrate-bearing zone (GHBZ). Free-gas-bearing zones have significantly different p-wave impedances and low-velocity anomalies than the overlying GHBZ and surrounding strata. These anomalous zones are controlled by the structural attitude of the reservoir strata, which are characterized as inter-bedded stratigraphic units. Variations in the type and geological characteristics of the hydrocarbon migration pathways were observed between sites W18 and W19 on the western ridge and sites W11 and W17 on the eastern ridge in the GMGS study area. The efficiency of gas migration in the western ridge may be higher than that in the eastern ridge, resulting in variations in hydrate gas types, thickness of the GHBZ, and gas migration flux and accumulation. Except for site W11, hydrates were recovered below the structure I inferred BSR at sites W17, W18, and W19. The gas-hydrate stability zone calculations reveal that the structure I hydrate stability zone differs from the BSR depth and is generally shallower than the base of the logging anomaly, indicating the coexistence of structure I and II hydrates. The BSR is not indicative of the BGHSZ; it is rather regarded as a transitional indicator of structure I and II gas hydrates in the GHPTR. The appearance of free gas and hydrates below the structure I inferred BSR indicates that the Shenhu area is characterized by a complex hydrate formation and accumulation system resulting from the supply of biogenic and thermogenic gases. Despite fine-grained host sediments predominating the GHPTR, the coupling of favorable conditions including efficient hydrocarbon generation, sufficient gas supply, multiple pathways for gas migration, and relatively high reservoir porosity have led to the development of highly saturated gas-hydrate accumulations within relatively thick sedimentary sections, which demonstrates a significant resource potential.

A preliminary study of the gas hydrate stability zone in a gas hydrate potential region of China

AbstractDue to its significance for gas hydrate accumulation, the gas hydrate stability zone (GHSZ) is an essential condition for successful gas hydrate exploration and is usually controlled by three main factors: gas components, geothermal gradient, and permafrost thickness. Based on the core and conventional log data of gas hydrate potential region, the CSMHYD program was adopted to simulate the influences of above three factors on the thickness of the GHSZ. When the gas components of the gas hydrate were CH4, C2H6, and C3H8, with the CH4 percentage ranging from 50% to 100%, the average GHSZ thickness can reach 1000 m according to the forward simulation results. When the gas hydrate was composed of CH4 and one of C2H6, C3H8, H2S, and CO2, the influence order of the above four gases on the thickness of GHSZ is as follows: H2S ＞ C2H6 ＞ C3H8 ＞ CO2. Under the gas components of 90% CH4, 5% C2H6, and 5% C3H8, the thickness of GHSZ decreased rapidly as the geothermal gradient increased following an exponential relation with a correlation of 99.92% and increased slowly as permafrost thickness increased following a linear relation with a correlation of 99.9%. In addition, the thickness of a gas hydrate favorable reservoir was determined by comprehensive log methods as 500 m. We conclude that a reservoir within 500 m below the permafrost layer is favorable for gas hydrate reservoir formation.

Influence of silica-containing materials surface properties on concrete structuring processes

The paper observes positive the influence of silica containing materials on cement systems. Practical and ecological parameters are shown. The data of a number and types of exchanges centers resulted from milling of industry-related materials are reported. Analysis of exchanges centers influence on cement hydration activity and “cement-mineral surface” contact zone interaction is provided. Theoretical justification of quartzitic sandstone and magnetic separation waste application as raw material for fillers is created. The influence of fillers surface on concrete structuring based on concrete plastic strength research is studied. It is determined that industry-related quartz-containing fillers application in amount 15 to 40% by weight of cement increases the plastic strength of cement paste and the speed of hydration processes. The period of structure formation of the cement stone is shortened, the intensity of hydrate accumulation processes increases. These data confirm the results of x-ray phase analysis of cement stone samples. A large amount of low and high basic capacity hydrosilicates is discovered during magnetic separation waste and quartzitic sandstone application. The use of the researched materials as fine fillers in cement concrete allows obtaining a composite with improved physico-mechanical characteristics and durability.

Studies on temperature characteristics and initial formation interface during cyclopentane-methane hydrate formation in large-scale equipment with bubbling

In this work, temperature characteristics during hydrate formation were investigated in the cyclopentane-methane system using large-scale equipment with bubbling. The volume in each experiment was increased by a factor of 80 compared with previous work. A group of scaled-up experiments, under the optimal condition based on experiments in the smaller crystallizer, was carried out to verify the scale-up potential of hydrate-based thermal fluid production process. Another group of scaled-up experiments were performed with different variables (volume ratio of cyclopentane to water, gas/liquid ratio and pressure) to figure out the initial hydrate formation interface and optimize the temperature of the thermal fluid. Experimental results illustrate that hydrate-based thermal fluid production process can be scaled up by a factor of 80, however, the temperature of the thermal fluid needs to be optimized by adjusting variables accordingly. Moreover, the liquid cyclopentane/water interface more than the gas/liquid interface affects hydrate formation and hydrate accumulation. In particular, the hydrate formation interface initially occurs on the water side near the liquid cyclopentane/water interface. A large amount of hydrate accumulates in the bulk liquid cyclopentane phase, forming new hydrate formation interfaces. Thermal fluid with a specific temperature range can be produced based on the heat released and conducted from the hydrate formation interfaces.

Quantitative determination of pore‐structure change and permeability estimation under hydrate phase transition by NMR

AbstractQuantitatively characterizing the seepage features is critical important for multi‐fluid flow in gas hydrate accumulations; however, limited researches concern water permeability during hydrate phase transition. In this work, nuclear magnetic resonance (NMR) measurement is employed to observe the in situ formation and dissociation of tetrahydrofuran (THF) hydrate in porous media. Results indicate that the relative free water and bound water consumption during hydrate phase transition can affect the seepage features of sediments. In addition, we investigate the growth habits of THF hydrate in quartz glass sand and find the growth pattern of the hydrate transforms from suspension to cementation when its saturation exceeds approximately 35%. The Tokyo model shows that the hydrate are heterogeneous distribution of pore‐filling and likely to evolve in larger pores; The findings clearly show that NMR is an efficient and direct technique for investigating the seepage characteristics during hydrate phase transition as well as pore fluid distribution in sediments.

Origin of the hydrate bound gases in the Juhugeng Sag, Muli Basin, Tibetan Plateau

The Juhugeng Sag, located in northwest of the Muli Basin, Tibetan Plateau, has been investigated for coal and petroleum resources during the past several decades. There have been successful recoveries of gas hydrates during recent years from the Middle Jurassic Yaojie Formation that offer insight into the origin of the hydrocarbon gases from the complex sag feature. This study examines the organic geochemical and stable carbon isotopic characteristics of shale and coal samples from the Middle Jurassic Yaojie Formation of the Juhugeng Sag, as well as compares with carbon isotopes, gas amounts and components of hydrate-bound gas. A total of 19 samples from surface mining, including 12 samples of black shale and 7 samples of coal, were analysed using a micro-photometer, a gas chromatograph, Rock–Eval and isotope methods. All the shale samples contained 100% type I kerogen, and the random vitrinite reflectance values vary from 0.65% to 1.32% and achieve thermal pyrolysis phase. Isotope values of methane (δ13C ranging from − 52.6‰ to − 39.5‰ and δD ranging from − 285‰ to − 227‰) in the hydrate bound gases suggest that the methane originates mainly from thermogenic contributions. It is proposed that ethane from the gas hydrate is thermogenic-produced, and this conjecture is supported by the fact that most of the gas hydrate also contains more than 30% of thermogenic C2+ hydrocarbons and is similar to structure II hydrate. Carbon isotope data from the gas hydrates show a positive carbon isotope series (δ13C1 < δ13C2 < δ13C3), with ethane δ13C values being lighter than − 28.5‰, as high consistency with source rocks from the Jurassic period indicate thermal oil-prone gas. A model of the accumulation of gas hydrate is plotted. However, the gaseous sources of gas hydrates may be a subject for more research.

Effect of multiphase flow on natural gas hydrate production in marine sediment

Natural gas hydrates (NGHs), regarded as an alternative future energy source. Currently, tests for hydrate exploitation from marine sediment have been performed in the Nankai Trough of Japan and the Shenhu area of the South China Sea. Hydrate exploitation is influenced by water-gas flow in the sediment, and considering the huge seawater reserves in hydrate accumulation areas, an experiment of seawater-gas flow was performed to dissociate hydrate. The effects of seawater-gas flow rates and initial hydrate saturation on methane hydrate (MH) production were analyzed. The results showed that seawater-gas flow efficiently promotes hydrate dissociation and inhibits hydrate reformation. Moreover, there was a faster heat and mass transfer with increasing seawater flow rates and decreasing gas flow rates, which enhanced the average MH dissociation rate. In addition, the variation time of the flow channel increased with higher initial hydrate saturation. Additionally, seawater-gas flow promotes MH dissociation stronger than deionized water-gas flow.

Peculiarities of geological and thermobaric conditions for the gas hydrate deposits occurence in the Black Sea and the prospects for their development

The actuality has been revealed of the necessity to attract the gas hydrate depos- its of the Black Sea into industrial development as an alternative to traditional gas fields. This should be preceded by the identification and synthesis of geological and thermobaric peculiarities of their existence. It was noted that the gas hydrates formation occurs under certain thermobaric conditions, with the availability of a gas hydrate-forming agent, which is capable of hydrate formation, as well as a sufficient amount of water necessary to start the crystallization process. The gas hydrate accumulation typically does not occur in free space – in sea water, but in the massif of the sea bed rocks. The important role in the process of natural gas hydrates formation is assigned to thermobaric parameters, as well as to the properties and features of the geological environment, in which, actually, the process of hydrate formation and further hydrate accumulation occurs. It was noted that the source of formation and accumulation of the Black Sea gas hydrates is mainly catagenetic (deep) gas, but diagenetic gas also takes part in the process of gas hydrate deposits formation. The main component of natural gas hydrate deposits is methane and its homologs – ethane, propane, isobutane. The analysis has been made of geological and geophysical data and literature materials devoted to the study of the offshore area and the bottom of the Black Sea, as well as to the identification of gas hydrate deposits. It was established that in the offshore area the gas hydrate deposits with a heterogeneous structure dominate, that is, which comprises a certain proportion of aluminosilicate inclusions. It was noted that theBlack Sea bottom sediments, beginning with the depths of 500 – 600 m, are gassy with methane, and a large sea part is favourable for hydrate formation at temperatures of +8...+9oC and pressures from 7 to 20 MPa at different depths. The characteristics of gas hydrate deposits are provided, as well as requirements and aspects with regard to their industrialization and development. It is recommended to use the method of thermal influence on gas hydrate deposits, since, from an ecological point of view, it is the safest method which does not require additional water resources for its implementation, because water intake is carried out directly from the upper sea layers. A new classification of gas hydrate deposits with a heterogeneous structure has been developed, which is based on the content of rocks inclusions in gas hydrate, the classification feature of which is the amount of heat spent on the dissociation process.

Numerical Modeling of Gas Migration and Hydrate Formation in Heterogeneous Marine Sediments

The formation of marine gas hydrates is controlled by gas migration and accumulation from lower sediments and by the conditions of the hydrate stability zone. Permeability and porosity are important factors to evaluate the gas migration capacity and reservoir sealing capacity, and to determine the distribution of hydrates in the stable region. Based on currently available geological data from field measurements in the Shenhu area of Baiyun Sag in the northern South China Sea, numerical simulations were conducted to estimate the influence of heterogeneities in porosity and permeability on the processes of hydrate formation and accumulation. The simulation results show that: (1) The heterogeneity of the hydrate stability zone will affect the methane migration within it and influence the formation and accumulation of hydrates. This is one of the reasons for the formation of heterogeneous hydrates. (2) When the reservoir is layered heterogeneously, stratified differences in gas lateral migration and hydrate formation will occur in the sediment, and the horizontal distribution range of the hydrate in a high porosity and permeability reservoir is wider. (3) To determine the dominant enrichment area of hydrate in a reservoir, we should consider both vertical and lateral conditions of the sedimentary layer, and the spatial coupling configuration relationships among the hydrate stability region, reservoir space and gas migration and drainage conditions should be considered comprehensively. The results are helpful to further understand the rules of hydrate accumulation in the Shenhu area on the northern slope of the South China Sea, and provide some references for future hydrate exploration and the estimation of reserves.

Clustered BSRs: Evidence for gas hydrate-bearing turbidite complexes in folded regions, example from the Perdido Fold Belt, northern Gulf of Mexico

We describe previously undocumented but extensive gas hydrate accumulations in the mouth of Perdido Canyon in the northern Gulf of Mexico. The accumulations are located within central parts of structural domes (four-way closures) and are characterized by stacked, high-amplitude bottom simulating reflections (BSRs) that we call clustered BSRs. Seismic data from Perdido Canyon show two clustered BSRs associated with turbidite sequences within two dome folds formed from tectonic folding and salt diapir rise. The northwestern (NW) and southeastern (SE) clustered BSRs have aerial extents of ∼25 km2 and 50 km2, respectively. Well log data confirm gas hydrate occurs above the NW clustered BSR, within a 225 m-thick consistently high-resistivity interval that we interpret as gas hydrate in near-vertical fractures and turbidite sands. The SE dome is only drilled at the edge of the BSR; nevertheless, the well log data indicate that a 30 m-thick gas hydrate accumulation is present. Gas chromatographic logs in both domes suggest a gradual transition from predominantly microbial gas below the BSR (500–1000 m below seafloor (mbsf)) to thermogenic gas at 1000–2000 mbsf. Based on the well log data and seismic stratigraphic analysis, we find gas hydrate is concentrated in fractures in marine mud, as well as in the pores of submarine fan turbidities, where saturations reach as high as 75%. An estimate of the total gas hydrate-bound gas volume at standard temperature and pressure is between 0.04 and 0.17 trillion cubic meters (TCM) assuming average hydrate saturation of 5-20% in a ∼45 m thick turbidite sand unit above the Perdido Canyon BSR area. Measured BSR extent and gas volume estimates indicate that the NW and SE reservoirs are among the largest gas hydrate occurrences known in the Gulf of Mexico.

Geological occurrence and accumulation mechanism of natural gas hydrates in the eastern Qiongdongnan Basin of the South China Sea: Insights from site GMGS5-W9-2018

Seismic and logging while drilling (LWD) data, gas hydrate samples, and hydrate gas geochemical testing results acquired during the GMGS5 expedition in 2018 have been used to explore the geological occurrence and accumulation mechanism of gas hydrates in the eastern Qiongdongnan Basin (QDNB) of the northern South China Sea (SCS). A large gas chimney with diameter of 4 km associated with the Songnan Low Uplift and the Central Channel and characterized by acoustic blanking on the seismic profile was identified at site GMGS5-W9-2018 in the deep water QDNB. Seismic indications for hydrocarbon migration and gas hydrate accumulation including low frequency features, enhanced reflections, pull-ups/downs and the bottom simulating reflector (BSR) were recognized within the chimney area. Gas hydrates were confirmed to be deposited at the depth of 7–158 mbsf based on the coring and sampling results and pilot hole LWD anomalies, which showed a high resistivity, low density, high gamma rays, and elevated acoustic velocity. Leakage type gas hydrates with multiple geomorphology occurrence, including massive, layers, nodules, fracture filling and disseminated, were recovered at the top of the gas chimney, suggesting favorable conditions for the accumulation of gas hydrates in the eastern QDNB. Although methane was the dominant hydrocarbon gas in all hydrate-bound gases, heavier hydrocarbons (C2+) were also prevalent. C2–C5 hydrocarbons made up to ~21% of the hydrocarbon gases, indicating their thermogenic origin and close relationship with deep reservoirs. Due to the presence of the C2+ hydrocarbons, structure I and structure II gas hydrates may coexist at site GMGS5-W9-2018. The base of structure II gas hydrate stability zone (BSIIGHSZ) was ~42–44 m deeper than the base of structure I gas hydrate stability zone (BSIGHSZ). The hydrocarbon migration and hydrate accumulation were closely related to the gas chimney in conjunction with the Songnan Low Uplift and the Central Channel, which transported the deep thermogenic gas to the GHSZ, forming hydrates in the fractured clay-dominated fine-grained sediments. The fractures occurred in the strata were likely driven by pneumatic forces. Overpressure in the sediments derived from gas charging and accumulation through the gas chimney may have caused the fractures in the sediments that dissipated the hydrocarbons into the GHSZ. We propose a model of gas hydrate accumulation that may aid in the future exploration of gas hydrates in the QDNB of the SCS.

Geochemical responses and implications for gas hydrate accumulation: Case study from site SHC in Shenhu Area within northern South China Sea

Abstract A knowledge of gas hydrates is critical for understanding complex geological, geophysical, and biogeochemical processes and for explaining the proposed link between gas hydrate destabilization and global sea level changes. With the aim of discussing the geochemical characteristics and implications for gas hydrate accumulation, this study assesses the changes in a series of organic and inorganic geochemical gas hydrate-bearing sediments within a core section obtained at site SHC in the Shenhu Area of the northern South China Sea that range from the Late Miocene to the present. High S/C ratios (0.53–5.6) and low TOC contents (

Fluid migrations at the Krasny Yar methane seep of Lake Baikal according to geochemical data

Sediments of the deep water Krasny Yar methane seep area of Lake Baikal contain channels of 1 cm in diameter and up to a depth of 150 cm that have a high water content and are characterized by higher oxidized and reduced conditions than the surrounding sediment. New data of lithological and geochemical studies, including the full anionic and cationic composition of pore waters, indicate that chemical flow at the sediment surface of the seep is heterogeneous in space with both areas of inflow of fluids and areas with outflow of fluids. We suggest that much of this flow is modulated by more complex hydrological processes that may include aqueous pumping driven by gas expulsion- and changes in the permeability of the conduits due to gas hydrate formation and dissociation. We discuss the seep output fluid chemistry and heat flow in relation to the above hydrological processes and propose a seep model where fluid and gas migrations act at different spatial scales partly favored by hydrate accumulation and the fresh water conditions of Lake Baikal.

Origin of natural gases and associated gas hydrates in the Shenhu area, northern South China Sea: Results from the China gas hydrate drilling expeditions

Geochemical data for hydrate gases acquired from the GMGS3 and GMGS4 gas hydrate drilling expeditions conducted by the Guangzhou Marine Geological Survey (GMGS) are used to explore the origin of hydrate gases and their relationship to deep hydrocarbon reservoirs, and to evaluate the contribution of different genetic types of gases to the formation and accumulation of gas hydrates in the Shenhu area of the northern part of the South China Sea (SCS). Compositionally, methane is the dominant gas (>90%) in the void gas and pressure core gas. In addition, as much as ~3% of the gas is composed of C2+ hydrocarbons, including ethane, propane, iso-butane, butane, iso-pentane, and n-pentane. The δ13C-CH4 and δD-CH4 values indicate a mixed biogenic-thermogenic origin for the hydrate-forming gas. The methane isotope correlation indicates that the source of the hydrate gas is closely related to the deep conventional gas reservoirs discovered in the Baiyun Sag-Panyu Low Uplift area. Both the hydrate gases and the deep reservoir gases are sourced from the hydrocarbon kitchens in the Baiyun Sag, revealing a paragenetic relationship within the same petroleum system. The composition of the hydrocarbons and the isotopic variation of methane with depth suggest that the thermogenic gas was likely affected by compositional and isotopic fractionation during the long-distance migration from the deep source rocks to the shallow gas hydrate stability zone (GHSZ). The impact of biodegradation on a solely thermogenic gas could also affect the final composition of the hydrate-forming gas. Analysis of the GHSZ based on gas hydrate compositions suggests that the occurrence of thermogenic gas could also indicate the coexistence of structure I (SI) and structure II (SII) gas hydrates in the Shenhu area, with the SII hydrates accumulating in or below the lower part of the SI GHSZ. The confirmed presence of SII hydrates in the Shenhu area relocated the base of the GHSZ deeper than was indicated by the bottom simulating reflector, which warrants further study in future explorations for gas hydrates in the Shenhu area.

Factors Controlling Short‐Range Methane Migration of Gas Hydrate Accumulations in Thin Coarse‐Grained Layers

AbstractNatural gas hydrate is often found in marine sediment in heterogeneous distributions in different sediment types. Diffusion may be a dominant mechanism for methane migration and affect hydrate distribution. We use a 1‐D advection‐diffusion‐reaction model to understand hydrate distribution in and surrounding thin coarse‐grained layers to examine the sensitivity of four controlling factors in a diffusion‐dominant gas hydrate system. These factors are the particulate organic carbon content at seafloor, the microbial reaction rate constant, the sediment grading pattern, and the cementation factor of the coarse‐grained layer. We use available data at Walker Ridge 313 in the northern Gulf of Mexico where two ~3‐m‐thick hydrate‐bearing coarse‐grained layers were observed at different depths. The results show that the hydrate volume and the total amount of methane within thin, coarse‐grained layers are most sensitive to the particulate organic carbon of fine‐grained sediments when deposited at the seafloor. The thickness of fine‐grained hydrate free zones surrounding the coarse‐grained layers is most sensitive to the microbial reaction rate constant. Moreover, it may be possible to estimate microbial reaction rate constants at other locations by studying the thickness of the hydrate free zones using the Damköhler number. In addition, we note that sediment grading patterns have a strong influence on gas hydrate occurrence within coarse‐grained layers.

India National Gas Hydrate Program Expedition 02 Summary of Scientific Results: Gas hydrate systems along the eastern continental margin of India

Abstract The primary objectives of the India National Gas Hydrate Program Expedition 02 (NGHP-02) were to obtain new data on the occurrence of gas hydrate systems and to advance the understanding of the controls on the formation of gas hydrate accumulations in the offshore of India. In accordance with the ultimate overall goal of the NGHP effort to assess the energy resource potential of marine gas hydrates in India, particular focus was placed on the exploration and evaluation of gas hydrate occurrences at high saturations in sand-rich systems. NGHP-02 operations were conducted from 3-March-2015 to 28-July-2015 off the eastern coast of India and included logging while drilling (LWD) operations at 25 locations, and coring and wireline logging operations at 10 locations, in the Krishna-Godavari and Mahanadi Basins. The formation of highly concentrated gas hydrate accumulations, which are more suitable for energy extraction, requires the presence of relatively coarse-grained sediments with porosity needed to support the migration and accumulation of gas, and the nucleation of gas hydrate. The results of downhole logging, coring and formation pressure testing operations during NGHP-02 have confirmed the presence of extensive sand-rich depositional systems throughout the deepwater portions of the Krishna-Godavari and Mahanadi Basins. Two areas of the Krishna-Godavari Basin, referred to as Areas B and C, contain substantial gas hydrate accumulations in sand-rich systems and therefore represent ideal candidate sites for future gas hydrate production testing. This summary and technical report includes a comprehensive synthesis of the geologic, geophysical, geochemical, and physical property data acquired during NGHP-02 as it relates to the controls on gas hydrate occurrence, particularly with regards to sand-hosted accumulations. In the Mahanadi Basin, despite the confirmation of extensive reservoir capacity, gas supply at the NGHP-02 sites was insufficient to charge the reservoirs with gas hydrates. In the Krishna-Godavari Basin, extensive reservoir systems were confirmed with sediment grain-sizes ranging from coarse-silts to gravels. These reservoirs range from fully- to partially-filled with gas hydrate. The gas is determined to be from only microbial sources, and in part migrated into the reservoirs from deeper systems. The controls on gas hydrate occurrence are complex and varied; and include substantial reservoir heterogeneity and sufficient permeability throughout the reservoirs and seals that allowed pervasive fluid flow into and through the hydrate-bearing systems. These discoveries are the most significant confirmation of the exploration approach that focuses on direct detection of hydrate reservoirs supported by comprehensive petroleum systems analyses.