Hydrate Stability Zone Research Articles

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.

Drilling Cores and Geophysical Characteristics of Gas Hydrate-Bearing Sediments in the Production Test Region in the Shenhu sea, South China sea

Gas hydrate production testing was conducted in 2017 in the Shenhu Area in the northern part of the South China Sea, and unprecedented success was achieved. In order to obtain gas production and physical properties of gas hydrate reservoirs in the study area and determine the location of test production wells, the seismic and logging data and drilling cores were analyzed in detail, the physical characteristics of the sediments, faults, gas components, and reservoir were studied. The results show that 1) the gas hydrates are diffusion type, with reservoirs dominated by clayey silt sediments, and the gas hydrate-bearing layers are characterized by soup-like, porridge-like, cavity, and vein structures; 2) the resistivity and acoustic velocity of gas hydrate formation are significantly higher than those of the surrounding sediments, while the neutron porosity, density, and natural gamma are slightly lower; the Bottom Simulating Reflectors (BSRs) in seismic profiles exhibit the exist of gas hydrates; 3) gas chimneys and faults are well-developed beneath the BSRs, and hydrocarbon gases can easily migrate into the gas hydrate reservoirs in areas with stable temperature and pressure conditions; 4) the gas hydrate saturation is high, the highest saturation in site W17 was up to 76%, with an average of 33%; while the highest saturation in site W19 was up to 68%, with an average of 31%. The gas source is considered as mixed gas of thermogenic gas and microbial gas. By comparing the core samples and geophysical characteristics of sites W17 and W19 in the study area and calculating the thickness, distribution area, and saturation of the hydrate deposition layer, it was found that site W17 is characterized by a thick layer, large area, high saturation, and good sealing, and thus, site W17 was established as the test production site. The development of gas chimney and faults provides pathways for the upward migration of deep gas, and the gas migrates to gas hydrate stable zone in forms of diffusion, water soluble and free state, forming high saturation of diffusion gas hydrates.

Numerical modeling of methane hydrates dissociation in the submarine permafrost

In this study, we evaluate the sensitivity of gas hydrate reservoirs associated with submarine permafrost conditions to changes in global climate. We apply numerical simulations to assess the timings of methane hydrate dissociation on the East Siberian Arctic Shelf after flooding the shelf with the seawater. The modeling combines a model of submarine permafrost evolution with a model of methane hydrate dissociation that accounts for the consumption of latent heat during hydrate dissociation. Based on the analysis of the performed experiments, we found that the endothermic reaction is a significant mechanism for slowing hydrate dissociation in frozen sediments. As a result, it additionally increases the lag of the subsea permafrost and hydrates stability zone response to glacial cycles.

Microbial Methane Generation and Implications for Stability of Shallow Sediments on the Upper Slope, U.S. Atlantic Margin

Dissociation of methane hydrates in shallow marine sediments due to increasing global temperatures can lead to the venting of methane gas or seafloor destabilization. Along the U.S. Atlantic margin there is a well-documented history of slope failure and numerous gas seeps have been recorded. However, it is not fully understood whether the observed gas seepages can lead to slope failure as seafloor data is often sparse. We used machine learning algorithms to predict total organic carbon (TOC) and porosity at the seafloor on the U.S. Atlantic margin. Within this region, an area of high TOC predictions (1.5—2.2% dry weight) occurred along the continental slope from (35.4°N, 75.0°W) to (39.0°N, 72.0°W), aligning with documented gas seeps in the region. Elsewhere, predicted values of TOC were near or below 1% dry weight. In the area of high TOC, we modeled hydrate and gas formation over a 120,000 years glacial cycle. Along the feather edge, average hydrate saturations at the base of the hydrate stability zone (BHZ) were between 0.2% and 0.7% with some models predicting hydrate saturation above 3% and average peak gas saturations ranged from 4% to 6.5%. At these locations we modeled the pore pressure response of sediments at the BHZ to hydrate dissociation due to an increase in temperature. We focused on purely drained and undrained loading environments and used a non-linear Hoek-Brown failure envelope to assess whether failure criteria were met. In a drained loading environment, where excess pore pressure is instantly dissipated, we found that the change in effective stress due to hydrate dissociation is small and no failure is expected to occur. In an undrained loading environment, where excess pore pressure does not dissipate, the change in effective stress due to hydrate dissociation is larger and shear failure is expected to occur even at low hydrate saturations (0.2%—1%) forming final gas saturations below 0.1%. Therefore, we conclude that the dissociation of hydrates along the feather edge can lead to the conditions necessary for sediment failure.

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.

Methane migration mechanisms for the Green Canyon Block 955 gas hydrate reservoir, northern Gulf of Mexico

In marine continental margins, high saturations of natural gas hydrate are observed within the gas hydrate stability zone, but the gas migration pathways to the hydrate reservoir are not often clear. We focus on a site in Green Canyon Block 955 (GC 955) in the Gulf of Mexico, where gas hydrate occurs in a 35 m-thick reservoir with saturations of 79-93% in numerous centimeter- to decimeter-thick sandy silt layers and little to no gas hydrate in the interbedded cm to decimeter-thick clayey silt layers. Different datasets (well logging, coring, and seismic data) suggest different types of potential methane migration mechanisms. For example, relatively high particulate organic carbon content in the reservoir and the microbial methane source indicated by gas chromatography and methane isotopes suggest gas might be made locally and migration could occur through dissolved methane sources or water advection. In contrast, the visible deeply rooted faults intersecting the gas hydrate reservoir and the gas pockets adjacent to the faults on seismic data suggest that free gas may migrate directly into the reservoir. We tested three different mechanisms to form gas hydrate at GC 955: short-range methane diffusion, short-range methane diffusion coupled with upward water advection, and free gas flow. The 1-D and 2-D numerical models show that both the short-range migration and the upward water advection are not enough to form high saturations gas hydrate at GC 955, and that free gas flow is required to form the high saturations of gas hydrate observed at GC 955.

Spatial-Temporal Evolution of the Gas Hydrate Stability Zone and Accumulation Patterns of Double BSRs Formation in the Shenhu Area

The current study examines the methane gas hydrate stability zone (GHSZ) in the Shenhu area in the northern South China Sea (SCS) as an example to calculate the thickness of the GHSZ and reconstruct its evolution since 8.2 Ma. Two mechanisms for typical double BSRs in the Shenhu area are shown, and the relationship between the evolving thickness of the GHSZ and the dynamic accumulation of NGHs at typical stations in the Shenhu area is clarified. The results show that the thickness of the GHSZ varies over time with overall thickening in the Shenhu area. The current thickness of the GHSZ is between 160.98 and 267.94 m. Two mechanisms of double BSRs in the Shenhu area are summarized: the double BSRs pattern based on changes in formation temperature, pressure and other conditions and the double BSRs pattern based on differences in gas source and composition. The formation process and occurrence characteristics of double BSRs and hydrate at site SH-W07-2016 in the Shenhu area are also closely related to the changes in thickness of the GHSZ. In addition, the age when gas source first enters the GHSZ has a considerable influence on the dynamic accumulation process of hydrate. Since the formation of hydrate above the BSR at site SH-W07-2016, the GHSZ has experienced up to two periods of thickening and two periods of thinning at this site. With the changes in the thickness of the GHSZ, up to two stages of hydrate formation and at most two stages of hydrate decomposition have occurred. This paper is of great value for understanding the formation of multiple bottom-simulating reflectors (BSRs) as well as the migration, accumulation and dissipation of natural gas hydrate (NGH) during the dynamic accumulation process.

Impact of Gas Saturation and Gas Column Height at the Base of the Gas Hydrate Stability Zone on Fracturing and Seepage at Vestnesa Ridge, West-Svalbard Margin

The Vestnesa Ridge, located off the west Svalbard margin, is a >60 km long ridge consisting of fine-grained sediments that host a deep-marine gas hydrate and associated seepage system. Geological and geophysical observations indicate the predominance of vertical fluid expulsion through fractures with pockmarks expressed on the seafloor along the entire ridge. However, despite the apparent evidence for an extended free gas zone (FGZ) below the base of the gas hydrate stability zone (BGHSZ), present-day seafloor seepage has been confirmed only on the eastern half of the sedimentary ridge. In this study, we combine the relationships between aqueous phase pressure, capillary pressure, sediment clay fraction, porosity, and total stress to simulate how much gas is required to open preexisting fractures from the BGHSZ towards the seafloor. Data from four specific sites with different lithology and pressure regime along the ridge are used to constrain the simulations. Results demonstrate that fracturing is favored from the FGZ (with gas saturations < 0.1 and gas column heights < 15 m) towards the seafloor. Neglecting the capillary pressure overpredicts the size of the gas column by up to 10 times, leading to erroneous maximum gas vent volume predictions and associated ocean biosphere consequences. Further parametric analyses indicate that variations in the regional stress regime have the potential to modify the fracture criterion, thus driving the differences in venting across the ridge. Our results are in line with independent geophysical observations and petroleum system modeling in the study area, adding confidence to the proposed approach and highlighting the importance of the capillary pressure influence on gas pressure.

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.

The variation of free gas distribution within the seeping seafloor hydrate stability zone and its link to hydrate formations in the Qiongdongnan Basin

The variation of free gas distribution within the seeping seafloor hydrate stability zone and its link to hydrate formations in the Qiongdongnan Basin

Characterization of coexistence of gas hydrate and free gas using sonic logging data in the Shenhu Area, South China Sea

Recent studies have confirmed the coexistence of gas hydrate (GH) and free gas (FG) in gas hydrate stability zone (GHSZ). However, the acoustic properties of the coexisting sediment, GH-bearing sediment, and FG-bearing sediment are needed to be specified, for discriminating these three coexisting phases using sonic logs. In this study, the compressional- (P-) and shear (S-) wave attenuations of the sediment with coexisting GH and FG are calculated from the sonic logs in the South China Sea (SCS). The obtained results reveal that the P-wave and S-wave velocities of the sediment with coexisting GH and FG are decreased by 14% and 11%, P-wave attenuation enhanced by 104%, and S-wave attenuation decreased by 44%, respectively, as compared with the sediment only containing GH at well SHSC-4J1. A similar result can be obtained for well GMGS6-SH02. The highest P-wave attenuation appears in the sediment with coexisting GH and FG, ∼0.66 at well SHSC-4J1 and ∼0.60 at well GMGS6-SH02, respectively. Moreover, cross plots of the measured P- and S-wave velocities and attenuations suggest that attenuation is more sensitive to the coexistence of GH and FG than velocity. The saturation of coexisting GH and FG in sediment can be calculated from the sonic velocities using rock physics modeling, a maximum value of ∼26% (av. 13%) at well SHSC-4J1 and a maximum value of ∼23% (av. 7%) at well GMGS6-SH02.

Sedimentation-driven cyclic rebuilding of gas hydrates

Gas hydrate recycling is an important process in natural hydrate systems worldwide and frequently leads to the high gas hydrate saturations found close to the base of the gas hydrate stability zone (GHSZ). However, to date it remains enigmatic how, and under which conditions, free gas invades back into the GHSZ. Here we use a 1D compositional multi-phase flow model that accounts for sedimentation to investigate the dominant mechanisms that control free gas flow into the GHSZ using a wide-range of parameters i.e. hydrate formation kinetics, sediment permeability, and capillary pressure. In the first part of this study, we investigate free gas invasion into the GHSZ without any sedimentation, and analyse the dynamics of hydrate formation in the vicinity of the base of GHSZ. This helps establish plausible initial conditions for the main part of the study, namely, hydrate recycling due to rapid and continuous sedimentation. For the case study, we apply our numerical model to the Green Canyon Site 955 in the Gulf of Mexico, where the reported high hydrate saturations are likely a result of hydrate recycling driven by rapid sedimentation. In the model, an initial hydrate layer forms due to the invasion of a specified volume of rising free gas. This hydrate layer is consistent with the local pressure, temperature and salinity state. This hydrate layer is then thermally de-stabilised by sedimentation resulting in free gas formation and hydrate recycling. A key finding of our study is that gas hydrate recycling is a cyclic process which can be divided into three phases of 1) gas hydrate melting and free gas nozzling through the hydrate layer, 2) formation of a new gas hydrate layer as the old layer vanishes, and 3) fast uninhibited grow of a new hydrate layer. High hydrate saturations of about 80% can be attained purely through physical, burial-driven recycling of gas hydrates, without any additional gas input from other sources. Hydrate recycling is, therefore, highly dynamic with its own inherent cyclicity rather than a gradual process paced by the rate of sediment deposition.

Modeling the Effects of Sedimentation on Natural Occurrences of CH4 Hydrates in Marine Sediments

This study explores the relationships that sedimentation rate and transport properties have with the formation and evolution of hydrates in fine-grained marine sediments and their corresponding bottom simulating reflector (BSR) responses. Using a series of one-dimensional simulations of multiphase, multicomponent flow and transport of mass and heat through porous media, a slab of sediments through sedimentation is modeled. The boundary conditions are set to emulate the pressure and geothermal gradients and its resulting gas hydrate stability zone (GHSZ). Hydrates are formed by injecting methane gas through the bottom of the grid and letting it migrate and reach the boundary of stability. The resulting hydrate accumulation is subjected to different sedimentation rates and replicated with different intrinsic permeability. With sedimentation, the geothermal gradient is displaced upward and the boundary of stability shoals. Through methane recycling, the distribution of phases changes through cycles of slow melting and rapid reformation. This results in a dynamic flow barrier that relocates the base of the GHSZ over geological time, in response to the variations of both pressure and salinity. The characteristics of a BSR response will be tied to the stage of the melting cycle.

Deep ocean bubble transport model coupled with multiple hydrate behavior characteristics

AbstractSubmarine gas seepage is a widely observed process. In this study, a unified mechanistic model of bubble transport both inside and outside the gas hydrate stability zone (GHSZ) was developed. Multiple hydrate‐related behaviors were considered, including hydrate nucleation, hydrate film lateral spread, hydrate shell dynamic growth, hydrate dissolution and decomposition, and collapse and fracture deformation of hydrate‐coated bubbles. Using the proposed model, a series of simulation studies about bubble dissolution and rising fate were conducted. The results indicate that the formation of solid hydrates in the deep‐sea environment can provide a fairly effective barrier for the dissolution and shrinkage of bubbles, and the deeper the initial release water depth, the smaller the critical size of the bubble required for arriving at the water surface. Furthermore, the majority of gases released from the seafloor would be absorbed by the shallow oceanic layer, but larger bubbles could still pass through the water column to the atmosphere.

Improved Detectivity for Detecting Gas Hydrates Using the Weighted Differential Fields of the Marine Controlled-Source Electromagnetic Data

Gas hydrate is seen as a kind of new energy resources, yet it may also be one of the main greenhouse gases as its dissociation may release methane into the atmosphere. Furthermore, a severe hazard to offshore infrastructures may also be introduced by extensive gas hydrate dissociation associated with the stability of the geological structures after gas production. Therefore, it is essential to investigate the gas hydrate as well as its environmental impacts before drilling and extracting it. The geophysical seismic reflection data is usually used for exploring the gas hydrate. The gas hydrate can be effectively identified by the bottom simulating reflectors (BSRs) on seismic reflection data. However, the BSR is only for identifying the bottom boundary and it is difficult to estimate its space distribution and saturation within the hydrate stability zone. The marine controlled-source electromagnetic (CSEM) data is suitable for detecting the gas hydrate as the resistivity of the seafloor increases significantly in the presence of gas hydrate or free gas. In this study, a weighted differential-field method is applied to improve the detectivity for identifying the gas hydrate. Numerical tests show that the difference of the EM fields can effectively suppress the airwaves in shallow waters. Therefore, the detectivity given by the field ratio between the models with and without the gas hydrate target is enhanced.

The Influence of Confining Stresses on Formation Kinetics of Methane Gas Hydrates

Unconventional natural gas extraction has undergone rapid progress in recent years as an emerging energy source, and the amount of methane gas stored in gas hydrates is estimated to be larger than that of all other natural gas resources combined. In the process of finding suitable hydrate reservoirs for possible exploitation and other applications, it is vital to understand the effect of reservoir conditions on the formation kinetics of methane gas hydrates. The aim of this study is to investigate hydrate formation characteristics in a porous medium under different confining conditions, focusing on the effect of confining pressure on hydrate nucleation, growth, and saturation. A series of experiments was performed to simulate in-situ hydrate reservoir conditions in a custom-designed reactor to synthesise methane hydrates in a porous quartz sand medium under different confining and formation pressures. Temporal changes in gas pressure and temperature were incorporated to quantify the conversion of methane and water into gas hydrates. The results substantiate the extremely random nature of hydrate formation. Formation behaviour in the intermediate stages was observed to be highly individualistic due to the pore-scale configuration of hydrates. The pressure decrease observed during hydrate formation has a direct relationship with the timely progression of hydrate saturation. Moreover, the resultant hydrate saturation at the completion of the formation increases with confining pressure, while the initial formation pressure of methane gas also has a positive correlation with final hydrate saturation. Higher effective stress conditions on the porous medium contribute to greater hydrate saturation within the pores. 40% and 70% increments of effective stress cause the final hydrate saturation to be raised by 20 % and 30 %, respectively, complementing field observations where higher hydrate saturations were detected at the lowermost layers of the hydrate stability zone.

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.

Determination of the lower boundaries of the natural gas hydrates stability zone in the subpermafrost horizons of the Yakut arch of the Vilyui syneclise, saturated with bicarbonate-sodium type waters

This paper considers the possibility of the underground gas storage facilities creating in a hydrate state on the north-western slope of the Yakut arch of the Vilyui syneclise. For this, the boundaries of the hydrate stability zone were determined for 6 promising areas of the considered geological structure. Equilibrium conditions of the natural gas hydrates formation in the model porous media containing bicarbonate-sodium type water (mineralization 20 g/l), characteristic for the subpermafrost horizons of the Yakut arch, have been studied by the method of differential thermal analysis. On the basis of the obtained results, the boundaries of the natural gas hydrates stability zone were determined. It was shown that the upper boundaries of the hydrate stability zone are located in the thickness of permafrost rocks. It was found that the lower boundaries of the natural gas hydrates stability zone in moist unsalted porous medium lie in the range from 930 to 1120 m. When the samples are saturated with mineralized water, the boundaries are located 80-360 m higher. The obtained experimental results allow us to conclude that in subpermafrost aquifers of the Yakut arch has favorable conditions for the formation of natural gas hydrates. Keywords: natural gas hydrates; aquifers; underground gas storage; hydrate stability zone; geothermal gradient; equilibrium conditions of the hydrate formation; bicarbonate-sodium type water.

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.

The evolution of seafloor venting from hydrate-sealed gas reservoirs

We use 3D seismic data to show that three rows of seafloor gas mounds can be traced downward to leak points that lie at the hydrate-gas contact within three individual dipping coarse-grained sand bodies in the Terrebonne Basin, Walker Ridge block 313, northern Gulf of Mexico. We predict the overpressure within the sand bodies by assuming that the gas pressure at the vent points equal the least principal stress. We interpret that free gas accumulates at the base of the hydrate stability zone, trapped by the overlying sand which has a high methane hydrate saturation. The free gas accumulates until the gas pressure at the base of the hydrate stability zone reaches the least principal stress in the overlying mudrock, whereupon hydraulic fractures form and fluids are vented to the surface. The warm rising fluids and perhaps localized exothermic formation of hydrate raise the local salinity and temperature. This process progressively shifts the base of the hydrate stability zone to shallower depths and dissociates the hydrate seal within the sand, which creates new leak points and results in the observed migration of the seafloor vents. Within the southwest corner of the Terrebonne Basin, this process has repeated multiple times within the Blue, Orange, and Green sands. This study shows how the hydrate stability zone can influence the location of fluid expulsion and in turn be affected by the warm, rising, saline fluids sealed by hydrate.