Hydrate Stability Zone Research Articles

HOW THE LOWERING OF PRESSURE OR THE FORMATION OF ICE AFFECT THE FORMATION AND DISSOCIATION OF METHANE HYDRATES: VARIATIONS IN TERMS OF METASTABLE PHASE AND ANOMALOUS SELF-PRESERVATION

This research proposes the multiple and consecutive formation of methane hydrates with the same gas-water mixture and in a small-scale reactor. The scope consisted of better exploring how the variation of the initial pressure of the system modified the formation and dissociation processes. One of the most promising solutions for gas hydrates exploitation consists in the replacement of methane with carbon dioxide. This process inevitably requires the formation of CO<sub>2</sub> hydrates; thus, more details and analyses on the driving forces of the formation process are required in order to improve the overall replacement efficiency. Also, the competition with ice was analyzed. Nine tests were carried out in order to consider all these parameters. Each test was characterized thermodynamically and the time required to complete each specific phase of the process was measured. The tests were carried out at two different initial pressures: 60 and 50 bar, respectively. It was found that the main effect, associated with a lower initial pressure and/or with the presence of ice, is the stretching of the so-called metastable phase; since the local conditions became suitable for the formation of hydrates, the system required more time to begin in these latter cases. In the tests made at 60 bar, the delay in hydrate formation related to the metastable phase was equal to 0.73-1.04 h, while it ranged from 7.29 to 15.16 h in the tests made at 50 bar. Moreover, the results proved that the presence of ice hindered the formation process by reducing the heat transfer rate and by lowering the volume available for the process. Conversely, the begin of the dissociation phase was similar in all the experiments: the anomalous self-preservation, or the capability of the system to preserve itself even outside the hydrate stability zone was not observed, proving again that these limiting factors (lower initial pressure and ice formation) mainly affected the process during the formation phase.

Flat Spots Within Cenozoic Sediments of the Nansen Basin, Arctic Ocean: Indicators for Serpentinization, Gas Generation and Accumulation Processes

Flat spot anomalies in the Quaternary part of the section of the Nansen Basin are imaged in seismic records and are interpreted to be related to gas-rich fluid accumulations. The flat spots are mainly located above basement highs between magnetic spreading anomalies C20 (~43 Ma) and C12 (~33 Ma). The complex morphometric analysis of flat spots show that serpentinization processes identified from modelling of gravity anomalies could be original gas source. This process also makes smoothing of the basement highs amplitudes. The depth of the top of the flat spots below the seafloor has an almost constant value of ~390 m indicating the ascent of gases from variable basement depths to a common subsurface fluid trap. The depth of the anomalies below the seafloor corresponds to a theoretical thickness of gas hydrate stability zone in the studied region. Gravity modeling along the Arktika-2011-03 section showed the position of the upper mantle blocks with lower (to 2.95 g/cm3) density within the highs of the acoustic basement. The flat spot anomalies occur above basement highs, below which blocks with lower density typical of serpentinized rocks are modeled. Thus, the serpentinization of the upper mantle ultramafic rocks is considered a main geochemical process, which can explain generation and accumulation of gas in oceanic abyss at a 1–3 km thick sedimentary cover, as well as small vertical movements of the basement blocks due to density reduction and expansion of serpentinized rock.

Can kinetic hydrate inhibitors inhibit the growth of pre-formed gas hydrates?

Low dosage kinetic hydrate inhibitors (KHIs) are one of the most promising techniques for inhibition at the earliest stages of hydrate formation. Although KHI polymers are generally considered to inhibit nucleation, they also show powerful properties in terms of crystal growth inhibition. Some KHIs may even induce anomalous gas hydrate dissociation inside the hydrate stability zone depending on the conditions. While KHIs have been proven for both inhibition of hydrate nucleation and initial crystal growth, their performance when it comes to pre-formed hydrates – i.e. where significant hydrate has already formed prior to KHI injection/contact with the KHI – has yet to be investigated. In this work, the ability of PVCap polymer to inhibit pre-formed hydrates has been evaluated for a methane-water system. Results are consistent with KHIs being able to inhibit further hydrate growth where a considerable volume of hydrate is already present in the system prior to chemical injection. Not only that, but that in the case of the PVCap system studied here, injection can, under certain conditions, directly result in the partial to complete anomalous dissociation of all pre-formed hydrates, even though conditions are inside the hydrate region. It is speculated this behaviour is related to the transition from metastable to stable hydrate structures/phases; PVCap not preventing the dissociation (of unstable phases) step of this process but inhibiting subsequent regrowth of stable equilibrium structures.

Yam Seep at Four-Way Closure Ridge: a prominent active gas seep system at the accretionary wedge SW offshore Taiwan

Hydroacoustic surveys at the accretionary wedge southwest of Taiwan reveal a confined active hydrocarbon seepage area of ~ 49,000 m2 in ~ 1350 m water depth on the northern crest of the Four-Way Closure Ridge, which we call Yam Seep. In this study, multibeam and side-scan sonar data acquired during surveys with an autonomous underwater vehicle during an expedition with R/V Ocean Researcher I in 2017 showed that the area is characterized by rough topography and high seafloor backscatter. Seafloor observations with a video sled and sediment sampling with gravity corers and the MeBo seafloor drill rig during an expedition with R/V SONNE in 2018 revealed that the area is almost entirely covered by intensely fractured methane-derived carbonates, which indicate that seepage has been ongoing for thousands of years. Hydroacoustic anomalies (‘flares’) in the water column indicated the presence of several gas bubble emission sites mostly at the center and eastern flank of the area in 2019. Drilling through massive carbonates in the northwestern part of Yam Seep induced free gas escape from a depth of ~ 5.1 m. This suggests the presence of gas hydrates in the subsurface as the seep area is located well within the gas hydrate stability zone. The inter-disciplinary investigations of the Yam Seep demonstrate that upward migration of light hydrocarbons and seafloor discharge has a considerable influence on the seabed properties.

Quantitative Simulation of Gas Hydrate Formation and Accumulation with 3D Petroleum System Modeling in the Shenhu Area, Northern South China Sea

Gas hydrates have been considered as a new energy that could replace conventional fossil resources in the future because of their high energy density, environmental friendliness, and enormous reserves. To further analyze the potential distribution of gas hydrate stability zones (GHSZ) and the formation of a gas hydrate system in the Shenhu area of the South China Sea (SCS), a 3D petroleum simulation model (PSM) was built from 3D seismic interpretations and all available geological data. Based on the thermal calibration of the 3D model, the evolution of the GHSZ, hydrocarbon generation and migration, and the formation and accumulation of gas hydrates were simulated for the first time in the area. Thermal simulation shows that the methane source of gas hydrate originated from shallow biogenic gas and deep thermogenic gas. Most areas are dominated by shallow biogenic gas, while, only about 3% of the deep thermogenic gas derived from Enping Formation source rock and contributed to the gas hydrate formation within a few areas in the southeast. The thermogenic gas migrated vertically into the GHSZ through connecting faults, mud diapir, and/or gas chimney to form gas hydrate. The source rocks of the Wenchang Formation, a deep thermogenic gas source, began to enter the main hydrocarbon generation window at 28.4 Ma. The Enping source rock began to generate oil from 25 Ma on and gas from 16 Ma on. Since 5.3 Ma, most areas of the source rocks have generated a gas window, and only the shallower parts in the east still in the oil window, which had lasted until now. The shallow biogenic gas source rocks from the Hanjiang, Yuehai, and Wanshan formations generated gas in different periods, respectively. The Qionghai Formation began to generate hydrocarbon from 0.3 Ma and until now. Other results show that the GHSZ developed mainly during the Quaternary and Neogene (Wanshan Formation) and the GHSZ is thicker in the southern area and thinner in the northern part with a positive correlation with water depth. Starting at 11.6 Ma, the GHSZ developed in the Hanjiang Formation in the south of the Shenhu area and gradually expanded to the north to cover most of the study area at 5.3 Ma during the Yuehai Formation. From 1.8 Ma on, the GHSZ covered the entire study area. At the same time, the GHSZ in the Hanjiang Formation disappeared because of the change in temperature and pressure. At present, the GHSZ in the Yuehai Formation has disappeared, while the Quaternary and Wanshan are the two main formations for GHSZ development. The formation and distribution of gas hydrates are fundamentally controlled by the space-time coupling between the hydrocarbon generation and expulsion time and distribution of the GHSZ. The simulation results of gas hydrate accumulation and distribution were verified by drilling results and the matching rate is 84%. This is the first time that 3D simulation was successfully conducted with PSM technology in the Shenhu area and it provides important guidance for gas hydrate study in other areas of the SCS.

Subsurface heat and salts cause exceptionally limited methane hydrate stability in the Mediterranean Basin

Abstract Knowledge of the global reservoir of submarine gas hydrates is of great relevance for understanding global climate dynamics, submarine geohazards, and unconventional hydrocarbon energy resources. Despite the expected presence of gas hydrates from modeling studies, the land-locked Mediterranean Basin displays a lack of evidence of extensive gas hydrate presence from samples and seismic data. We modeled the theoretical Mediterranean distribution of methane hydrate below the seafloor and in the water column using available geological information provided by 44 Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) boreholes, measured geothermal gradients, and thermohaline characteristics of the water masses. We find that the pervasive presence of high-salinity waters in sediments, coupled with the unique warm and salty water column, limit the thickness of the theoretical methane hydrate stability zone in the subsurface and deepen its top surface to 1163–1391 m water depth. The theoretical distribution of methane hydrates coincides well with the distribution of shallow, low-permeability Messinian salt deposits, further limiting the formation of pervasive gas hydrate fronts and controlling their or distribution due to the prevention of upward hydrocarbon gas migration. We conclude that the Mediterranean Basin, hosting the youngest salt giant on Earth, is not prone to the widespread formation and preservation of gas hydrates in the subsurface and that the gas hydrate potential of salt-bearing rifted continental margins may be considerably decreased by the presence of subsurface brines.

Thermal structure and sediment bulk density of the MV420 mud volcano on the continental slope of the Canadian Beaufort Sea and its implications for the plumbing system

The shallow migration path of mudflow of the mud volcano MV420 on the continental slope of the Canadian Beaufort Sea is investigated in terms of thermal and geotechnical characteristics. MV420 is a nearly flat topped active mud volcano that emits methane and fluidized mud. Its top is at a depth of water of 420 m, within the gas hydrate stability zone. During the summer 2017 IBRV Araon expedition, several measurements of marine heat flow were conducted in the area of juvenile mud mound/pond morphologies identified by multibeam bathymetry and backscatter intensity images obtained by the Monterey Bay Aquarium Research Institute’s autonomous underwater vehicle. The heat probe (a gravity corer with temperature, pressure, and tilt sensors) appeared to penetrate to a depth of >70 m below the seafloor. The sediment bulk density in the mudflow migration path of MV420, estimated by pressure change with controlled stretches of a winch cable, was 1.56 g/cm3. The subsurface temperature reaches up to 18°C and profiles show that the maximum local temperature occurs from 20 to 40 m below the seafloor, implying substantial vertical variability of geothermal gradients. Our finding of large positive and small negative geothermal gradients above and below the depth of the local temperature maximum may represent stagnant hot mud along the mudflow migration path, indicating a pulsative eruption of the mudflow. Gas hydrate is stable only within a few meter thick layer near the seafloor above the path because of the cold bottom water (0.4°C) covering the top of the mud volcano. Furthermore, the thermal conductivity of 0.939 W/m/K and the marine heat flow of 22.5 mW/m2 at a control site outside MV420 is estimated as the background heat flow in the slope, for which little data exists.

Mechanisms for upward migration of methane in marine sediments

Methane, a non-negligible component of the global carbon budget, could be discharged upward through marine sediments to ocean floor by certain migration mechanisms. Although quite some studies have been conducted, the mechanisms for methane migration have not been well reviewed yet, especially in hydrate-bearing sediments. In this study, methane migration mechanisms are classified into diffusion and advection processes which include water movement, free gas flow, sediment failures, and recently developed gas migration through hydrate channels. The occurrence of natural gas hydrate might affect methane migration in three ways: (1) reducing the permeability of marine sediments and consequently hindering the upward movement of methane either in gas or liquid phase, (2) enhancing the geomechanical strength of marine sediments, which prevents the creation of new pathways for methane escape by sediment failures, and (3) benefiting upward methane migration by constructing hydrate channels at the interface of continuous gas columns. Generally, dissolved methane could hardly break through the gas hydrate stability zone and sulfate-methane transition zone because of the high consumption rate for methane in these two zones. For free methane gas, the capillary force is a strong resistance to free gas flow in porous sediments. However, whether for dissolved methane or free methane gas, discharge along pre-existing fractures or failure surfaces might be considerable. In addition, methane discharge by gas flow through hydrate channels is still hard to constrain. Finally, based on current research uncertainties in constraining the methane flux to the ocean, the research outlook is also addressed. It is suggested that more investigations should be conducted in three aspects: the flow characteristic of high-permeability conduits, the quantitative correlations of geomechanical properties and hydrate distribution, and the occurrence conditions of hydrate channels.

Evaluation of Natural Gas Hydrate Fault System: A Case from a Sag in Deep-Water Slope Area of the Northern South China Sea

AbstractThe fault system is one of the structural carrier systems of gas hydrate accumulation, which plays a vital role in controlling the distribution of natural gas hydrate (NGH) accumulation. The previous studies mainly focus on summarizing the vertical migration mode of high flux fluid along the fault with obvious geophysical response characteristics on the seismic profile, such as “fault with gas chimney,” “fault with mud diapir,” and “fault with submarine collapse”, but lack of evaluation methods for the fault carrier system. We use the X sag in the deep-water continental margin slope area of the northern South China Sea as an example to study the fault systems closely related to NGH. This paper puts to use attribute technologies, such as coherence, curvature, and fusion, to analyze the characteristics and combination of the fault systems. We discussed migration patterns and evaluation methods of dominant fault carrier systems. This research proves that the strike-slip fault system in the platform area can directly connect the gas source bed with high-quality hydrocarbon generation to the gas hydrate stability zone (GHSZ). The activity of this fault system is more conducive to the accumulation of hydrocarbon in the GHSZ. This area has a good site for pore-filling gas hydrate prospecting and a preferential favorable fault carrier system. The composite fault system, consisting of a normal dip-slip fault system and a polygonal fault system, in the slope area can jointly communicate the biogenic gas-rich reservoir. Its activity and well-migration performance are the main reasons for the submarine gas leakage and collapse. It is a secondary favorable fault carrier system in the study area. There may be massive and vein natural gas hydrate formation in fractures in the leakage passage, and pore-filled gas hydrate may exist in the submarine nonleakage area. In this work, a three-factor evaluation method of the fault carrier system is proposed for the first time. This method is of great significance for the evaluation and exploration of NGH reservoirs in the continental margin slope area of the northern South China Sea.

Gas Hydrate Resource Potential Of Deepwater Sabah, Malaysia: A Preliminary Assessment

Offshore NW Sabah is one of the localities identified in the United States Geological Survey (USGS) global hydrates database but not much work has been done on this potential source of energy for Malaysia and the surrounding region. The presence of gas hydrates in this area is mainly inferred from bottom-simulating reflectors (BSR) identified in seismic reflection profiles across the margin. BSRs have been mapped across almost the entire length of the deepwater fold-thrust belt in the Sabah Trough where they are commonly observed within the crests of fold-thrust anticlines. Based on an average geothermal gradient of 62.5 °C/km, the thickness of the gas hydrate stability zone is predicted to vary with water depth from zero at 640 m water depth to 300 m at 2900 m water depth. The total in-place methane resource from the Sabah gas hydrates is estimated to range from 72 to 852 trillion cu. ft. (TCF) (2.06 – 24.1 x 1012 m3) with a mean of 364 TCF (10.3 x 1012 m3 ) and a most likely (P50) value of 252 TCF (7.1 x 1012 m3). These preliminary estimates may seem large but they are comparable with those from other gas hydrate deposits in the region. More work is required to refine them in order to determine how much of the in situ volume is technically and economically recoverable.

Numerical Modeling of Gas Hydrate Recycling in Complex Media: Implications for Gas Migration Through Strongly Anisotropic Layers

AbstractBurial driven recycling is an important process in the natural gas hydrate (GH) systems worldwide, characterized by complex multiphysics interactions like gas migration through an evolving gas hydrate stability zone (GHSZ), competing gas‐water‐hydrate (i.e., fluid‐fluid‐solid) phase transitions, locally appearing and disappearing phases, and evolving sediment properties (e.g., permeability, reaction surface area, and capillary entry pressure). Such a recycling process is typically studied in homogeneous or layered sediments. However, there is mounting evidence that structural heterogeneity and anisotropy linked to normal and inclined fault systems or anomalous sediment layers have a strong impact on the GH dynamics. Here, we consider the impacts of such a structurally complex media on the recycling process. To capture the properties of the anomalous layers accurately, we introduce a fully mass conservative, high‐order, discontinuous Galerkin (DG) finite element based numerical scheme. Moreover, to handle the rapidly switching thermodynamic phase states robustly, we cast the problem of phase transitions as a set of variational inequalities, and combine our DG discretization scheme with a semi‐smooth Newton solver. Here, we present our new simulator, and demonstrate using synthetic geological scenarios, (a) how the presence of an anomalous high‐permeability layer, like a fracture or brecciated sediment, can alter the recycling process through flow‐localization, and more importantly, (b) how an incorrect or incomplete approximation of the properties of such a layer can lead to large errors in the overall prediction of the recycling process.

Spatial-temporal variations of the gas hydrate stability zone and hydrate accumulation models in the Dongsha region, China

It is of great significance to study the spatial-temporal variations of the thickness of the gas hydrate stability zone (GHSZ) to understand the decomposition, migration, accumulation and dissipation of gas hydrate, the corresponding relationship between bottom-simulating reflectors (BSRs) and gas hydrate, and the distribution of heterogeneous gas hydrate. We selected the Dongsha region in the South China Sea (SCS) as the research object to calculate the spatial-temporal variation of the GHSZ since 10 Ma, analyzed the main factors affecting the thickness of the GHSZ, discussed the dynamic accumulation processes of gas hydrate, and proposed an accumulation model of gas hydrate in the Dongsha region. The results show that the thicknesses of the GHSZ in the study area were between 0 m and 100 m from 10 to 5.11 Ma, and the relatively higher bottom water temperature (BWT) was the key factor leading to the thinner thickness of the GHSZ during this period. From 5.11-0 Ma, the thickness of the GHSZ gradually increased but showed several fluctuations in thickness due to changes in the geothermal gradient, seawater depth, BWT, and other factors. The decrease in the BWT was the main factor leading to GHSZ thickening from 5.11 to 0 Ma. The thicknesses of the GHSZ are between 110 m and 415 m at present. The present spatial distribution features show the following characteristics. The GHSZ in the deep canyon area is relatively thick, with thicknesses generally between 225 m and 415 m, while the GHSZ in other areas is relatively thin, with thicknesses between 110 m and 225 m. Based on the characteristics of the GHSZ, two hydrate accumulation models are proposed: a double-BSRs model due to thinning of the GHSZ and a multilayer hydrate model due to thickness changes of the GHSZ, with single or multiple BSRs.

Shallow gas and gas hydrate accumulations influenced by magmatic complexes in the Pearl River Mouth Basin, South China Sea

We analyse occurrences of gas hydrates and free gases above a 1200 km2 complex of igneous volcanoes and sills using newly acquired three-dimensional (3D) seismic data in the Pearl River Mouth Basin of the South China Sea. In strata above volcano summits, we recognize three enhanced reflection layers at different stratigraphic levels, including discontinuous Bottom Simulating Reflectors (BSRs) and pluming BSRs. We also show that the lateral tips of eight sills, which at times appear to terminate against supra-sill faults, underlie gas chimneys linked to gas hydrate and free gas in shallower strata. These supra-sill faults and gas chimneys, as well as supra-volcano faults, provided pathways for upward fluid (gas) migration from deep sources. Given the spatial relationship between volcanoes, sills, free gas, and gas hydrates, we suggest that the igneous structures focused fluid migration into overlying faults and gas chimneys. We also explore whether higher geothermal gradient at the summits of the volcanoes caused the base of gas hydrate stability zone to be shallower, forming a pluming BSR. Our study highlights how volcanic systems can influence the formation and distribution of later gas hydrate and free gas.

Monitoring Offshore CO2 Sequestration Using Marine CSEM Methods; Constraints Inferred from Field- and Laboratory-Based Gas Hydrate Studies

Offshore geological sequestration of CO2 offers a viable approach for reducing greenhouse gas emissions into the atmosphere. Strategies include injection of CO2 into the deep-ocean or ocean-floor sediments, whereby depending on pressure–temperature conditions, CO2 can be trapped physically, gravitationally, or converted to CO2 hydrate. Energy-driven research continues to also advance CO2-for-CH4 replacement strategies in the gas hydrate stability zone (GHSZ), producing methane for natural gas needs while sequestering CO2. In all cases, safe storage of CO2 requires reliable monitoring of the targeted CO2 injection sites and the integrity of the repository over time, including possible leakage. Electromagnetic technologies used for oil and gas exploration, sensitive to electrical conductivity, have long been considered an optimal monitoring method, as CO2, similar to hydrocarbons, typically exhibits lower conductivity than the surrounding medium. We apply 3D controlled-source electromagnetic (CSEM) forward modeling code to simulate an evolving CO2 reservoir in deep-ocean sediments, demonstrating sufficient sensitivity and resolution of CSEM data to detect reservoir changes even before sophisticated inversion of data. Laboratory measurements place further constraints on evaluating certain systems within the GHSZ; notably, CO2 hydrate is measurably weaker than methane hydrate, and >1 order of magnitude more conductive, properties that may affect site selection, stability, and modeling considerations.

Neural net detection of seismic features related to gas hydrates and free gas accumulations on the northern U.S. Atlantic margin

Bottom-simulating reflections (BSRs) that sometimes mark the base of the gas hydrate stability zone in marine sediments are often identified based on the reverse polarity reflections that cut across stratigraphic layering in seismic amplitude data. On the northern U.S. Atlantic margin (USAM) between Cape Hatteras and Hudson Canyon, legacy seismic data have revealed pronounced BSRs south of the deepwater extension of Hudson Canyon and more subtle ones from offshore Delaware south to Cape Hatteras, where the reflections sometimes follow stratigraphic layering. Using high-resolution seismic data acquired during the 2018 Mid-Atlantic Resource Imaging Experiment and a supervised neural net, we identify seismic features associated with gas hydrates and/or the top of gas between Hudson Canyon and Cape Hatteras. Using seismic attributes especially sensitive to the presence of gas, we train a neural network algorithm on seismic data from an area with strong BSRs and then apply the model to the rest of the data set. The results indicate that gas hydrate and/or shallow free gas are significantly more widespread on the northern part of the USAM than previously known. Seismic indicators of gas extend landward from the 2000 m isobath to the upper continental slope in sectors with (offshore Virginia) and, to a lesser extent, without (offshore New Jersey) pervasive upper slope methane seeps. Higher sand content and intermediate sediment thickness, factors related to the container size and gas charge in a petroleum systems framework, are associated with more robust gas indicators.

Gas Sources, Migration, and Accumulation Systems: The Shallow Subsurface and Near-Seafloor Gas Hydrate Deposits

Compared with the deeply buried marine gas hydrate deposits, gas hydrates in the shallow subsurface, close to and at the seafloor, have attracted more attention owing to their concentrated distribution, high saturation, and easy access. They accumulate at relatively shallow depths <100–120 m and occur as gas hydrate-bearing mounds (also known as hydrate outcrops, pingoes) at the seafloor derived from the growth of hydrates in the shallow subsurface or as pure hydrate chunks formed by gas leakage. This paper reviews and summarizes such gas hydrate systems globally from the perspective of gas sources, migration pathways, and accumulation processes. Here, we divided them into four categories: fault-chimney-controlled, diapir-fault-controlled, fault-controlled, and submarine mud volcano-controlled deposits. Gas chimneys originate immediately above the restricted regions, mostly affected by faults where high gas concentrations trigger elevated pore fluid pressures. Diapirism derives a dendritic network of growth faults facilitating focused gas discharge and hydrate formation near the seafloor. Furthermore, pre-existing faults or fractures created by overpressured gas from greater depths in accretionary tectonics at convergent margins act as preferential pathways channeling free gas upwards to the seafloor. Gas flux rates decrease from the submarine mud volcano center to its margins, creating a concentric pattern of distributing temperature, gas concentrations, and hydrate contents in shallow sediments around the mud volcano. Hydrate-bound hydrocarbons are commonly of thermogenic origin and correspond to high-background geothermal conditions, whereas microbial gas is dominant in a few cases. The presence of heavier hydrocarbons mitigates the inhibition of hydrate formation by salt or heat. Fluid migration and pathways could be compared to the “blood” and “bones” in an organic system, respectively. The root of a pathway serves as the “heart” that gathers and provides considerable free gas concentrations in a restricted area, thereby triggering pore fluid pressures as one important drive force for focused fluid flow in impermeable sediments (the organic system). Besides the suitable temperature and pressure conditions, a prerequisite for the formation and stability of hydrate deposits in the shallow subsurface and at the seafloor is the sufficient supply of gas-rich fluids through the hydrate stability zone. Thus, the proportion of gas migrating from deep sources is significantly larger than that trapped in hydrates. As such, such marine hydrate deposits seem more like temporary carbon storage rather than the main culprit for climate warming at least in a short period.

Seismic evidence of gas hydrate and seafloor fluid escape on the upper Amazon deep-sea fan, Brazilian equatorial margin

The Amazon fan contains a gas hydrate province known from a bottom-simulating reflection (BSR) that lies within an upper slope compressional belt. In this study, the extent and character of the BSR and its relation to thrust-fold structures is examined using a grid of 2D and 3D seismic data. We show the BSR to comprise a series of elongate patches up to 16 km wide that are present along 300 km of the slope in water depths of 750-2250 m and extend over a total area of 6800 km2. The elongate BSR patches show a strong spatial correspondence with the arcuate crests of thrust-fold anticlines. In profile, the BSR patches exhibit convex forms and/or locally irregular relief that rises toward the seafloor. In plan, 3D seismic horizon maps reveal columnar BSR elevations up to 1 km wide, which rise beneath seafloor mounds and depressions, up to 0.5 km wide and 30 m in relief, interpreted as small mud volcanoes and possible pockmarks. The elongate BSR patches are interpreted to record the structurally-controlled rise of warm, gas-rich fluids into the crests of thrust-folds and their leakage into the gas hydrate stability zone, and in places to seafloor, through a near-surface system of faults, hydrofractures and vents.

Investigating geological storage of carbon dioxide in a gas hydrate reservoir of the Shenhu area, South China Sea

Deep-water shallow-buried sand layers above the base of methane hydrate stability zone (BMHSZ) have been recognized as favorable reservoirs for gas hydrate accumulation. Carbon dioxide (CO2) injection into the sand layers provides significant advantages over onshore geological storage options because of CO2 hydrate formation and high CO2 density under conditions of pressure and temperature. However, reservoir characterization and geological conditions for storing CO2 in gas-charged sand layers below the BMHSZ have not been well studied.We investigate deep-water shallow-buried strata in the Shenhu area by analyzing and interpreting 3D seismic and well logs data. We present the W07–W23 gas hydrate reservoir that consists of two Lower Pleistocene sand zones: gas hydrate and free gas accumulation within levee facies in the upper zone and free gas accumulation within channel facies in the lower zone. Analysis of well data shows a 51 m thick total gas section interbedded with gas hydrate and inferred gas hydrate saturation as high as 20% of sediment pore space. Average porosity in the reservoir is estimated from log data at approximately 50%. Permeability in the hydrate-free sediments, as inferred from empirical equations retrieved from the offset wells, ranges 3–5 mD. We estimate that a total of 13–40 × 109 m3 of gas could be contained within the two zones and they may store roughly 5.7–11.4 × 107 ton of CO2. These results provide an improved understanding of gas hydrate reservoirs for occurrence of gas hydrate, free gas accumulations and CO2 storage capability below the traditional BMHSZ.

Saturation sensitivity and influencing factors of marine DC resistivity inversion to submarine gas hydrate

The submarine gas hydrate usually exists in the sediment on the continental slope. The bottom simulating reflector on the reflected seismic was identified as the bottom of the hydrate stability zone. However, many BSRs may not find the hydrate’s effective storage and its underlying free gas in many places. It is essential to identify the saturation of the hydrate. The resistivity can be used to evaluate the hydrate’s porosity and saturation. The hydrate boasts a high resistance to the surrounding sediments. The sensitivity of the marine Direct Current resistivity method (DCR) to the high resistance of the sediment can be used to evaluate the saturation of the hydrate. We have assessed the sensitivity of various DCR array arrangements, towed depths, hydrate thicknesses, and saturation. These influencing factors for improving recognition ability were also systematically analyzed. We have compared the inversion results of various DCR array arrangements, as well as different depths, thicknesses, and hydrate saturation, and calculated the saturation. We suggest using the corrected saturation equation to analyze the DCR results, which can improve the ability of hydrate identification. Evaluating these parameters will help develop or select DCR instruments for detecting the submarine gas hydrate.

Global Estimates of Biogenic Methane Production in Marine Sediments Using Machine Learning and Deterministic Modeling

AbstractWe have developed a model of geospatially estimating carbon accumulation and methanogenesis in seabed sediments that uses more accurate and sophisticated inputs to models than used in previous estimates. Using this hybrid stochastic and deterministic model, we estimate the maximum carbon available for methanogenesis in the global seabed, and subsequent microbial methane generated as a function of location and depth (including the gas hydrate stability zone). Global integration over present and previously microbially reactive sediments column yields total carbon and methane to be ∼0.8–2.2 × 106 and 1.1–3.0 × 106 Pg C and CH4, respectively. Our improvements to accuracy include using geospatially machine learned estimates of seafloor inputs to which the methanogenesis modeling is most sensitive (e.g., total organic carbon, heat flux, porosity). Our improvements to model sophistication include geospatially dependent modeling (on a 5 × 5 arc‐minute grid), a new model of sediment compaction (allowing for non‐linear geothermal gradients), and variable age versus depth at each grid cell. A carbon reservoir of the magnitude we estimate here is consistent with the recent IPCC suggestion that long‐term carbon sinks could explain imbalances in reduction of atmospheric CO2 over the last 50 million years. Our technique provides a foundation of using globally updateable machine learning parameters as the input to geologic and geochemical models, allowing for new observations to update global budgets of carbon available for methane, and subsequent total estimates of seabed methane.