Methane Hydrate Exploration Research Articles

Numerical investigation on gas production from Shenhu (China): Influence of layer inclination and horizontal inhomogeneities

Worldwide methane hydrate exploration activities have entered a new stage after the real extraction tests in Japan (2013, 2017) and China (2017). However, from the current technological development stage to the next commercial production stage, there still exist a wide gap: complex underground reservoir conditions and flow parameters. Therefore, the target of the current study is to investigate the effects of layer inclination and horizontal reservoir parameter inhomogeneities on the gas production. It is found that for 10° inclination of model, the equivalent permeability show 50% decrease to achieve the same production rate. The horizontally-set “inner column” and “outer column” (with changes in porosity from 0.20, 0.38, 0.50; and 0.3, 0.4, 0.5 for hydrate saturation) have different trends: for cases with horizontal porosity changes, the production rate difference happens in the mid-term (>300 d) observation; for cases with horizontal hydrate saturation changes, the production differs in each case from the production initiation. For real production trial, such inhomogeneity effects have to be considered in order to estimate the initial production and to maintain stable production in the long-term.

Governing the transboundary risks of offshore methane hydrate exploration in the seabed and ocean floor—an analysis on international provisions and Chinese law†

Abstract Considering the fact that its existence is abundant while maintaining the ability to generate freshwater while burning, methane hydrates have been classified as sources of sustainable energy. China currently maintains an international role in developing technology meant to explore offshore methane hydrates buried under the mud of the seabed, their primary laboratory being the South China Sea. However, such a process does not come without its hazards and fatal consequences, ranging from the destruction of the flora and fauna, the general environment, and—the greatest hazard of all—the cost of human life. The United Nations Convention on the Law of the Sea (hereinafter ‘UNCLOS’), being an important international legal regime and instrument, has assigned damage control during the exploration of methane hydrates, as being the responsibilities and liability of individual sovereign states and corporations. China adopted the Deep Seabed Mining Law (hereinafter the DSM Law) on 26 February 2016, which came into force on the 1 of May 2016; a regulation providing the legal framework also for the Chinese government’s role in methane hydrate exploratory activities. This article examines the role of the DSM Law and its provisions, as well as several international documents intended to prevent transboundary environmental harm from arising, as a result of offshore methane hydrate extraction. Despite the obvious risk of harm to the environment, the DSM Law has made great strides in regulating exploratory activities so as to meet the criteria of the UNCLOS. However, this article argues that neither the UNCLOS nor the DSM Law are adequately prepared to address transboundary harm triggered by the exploitation of offshore methane hydrates. In particular, the technology of such extraction is still at an experimental stage, and potential risks remain uncertain—and even untraceable—for cross-jurisdictional claims. The article intends to seek available legal instruments or models, to overhaul the incapacity within the current governing framework, and offers suggestions supporting national and international legislative efforts towards protecting the environment during methane hydrate extraction.

The Exploitation of Oceanic Methane Hydrate: Legal Issues and Implications for China

Abstract With the growth of global energy demand, States are actively considering the exploration for new energy. Methane hydrate is one of the world’s new energy sources with high energy density and abundant reserves, which have great strategic significance. This article focuses on three aspects, namely, project preparation, risk prevention and accident management, and addresses the risk issues arising from the exploration of methane hydrate. It is important to apply the United Nations Convention on the Law of the Sea and other treaties, as well as customary international law, while examining the rules applicable to the exploration of methane hydrate. State practice such as those of the United States, Russia, Japan, the European Union and China, are also discussed. The article puts forward some suggestions on the development of China’s methane hydrate resources. The core objective is to achieve a balanced approach to the development of environmental protection and energy development.

Offshore Methane Hydrates in Japan: Prospects, Challenges and the Law

On 19 March 2013, Japan became the first-ever country to announce successful continuous-flow production of methane gas from a solid methane hydrate layer located under the seabed in ultra-deep waters, establishing the technical feasibility of this novel natural resource and of its novel extraction technology. Methane hydrates are a unique natural resource that could provide both a tremendous amount of natural gas and freshwater. Given this abundance of methane hydrate in both permafrost and in sub-seabed sediments, scientists in Japan have begun exploring how to combust the produced natural gas from methane hydrates to produce electricity and how the greenhouse gas emissions can be re-injected back within the hydrate reservoir system. Such a system could enable the production of carbon-neutral electrical power for dryland nations. Additionally, in a hydrogen fuel era, the freshwater can be converted to steam to use in generating hydrogen from the natural gas co-produced with the water. Thus, freshwater can be produced from offshore methane hydrates with co-production of natural gas; electrical power, hydrogen fuel, and carbon capture and storage can be implemented alongside the production activities. The extraction of methane hydrates from the seabed raises questions about various risks and hazards to the environment, including the potential of seafloor destabilization, submarine mudslides, and tsunami. Many of such risks and hazards are not present with the production of traditional natural gas. The regulatory scheme that would eventually govern extraction of natural gas from methane hydrate within Japan's 200nm continental shelf and EEZ may not be robust enough to deter these risks. This study examines the potential prospects and benefits from the commercial development of offshore methane hydrates in Japan and ascertains whether existing legal frameworks are sufficiently robust to address the foreseeable environmental risks posed by methane hydrate exploration and exploitation operations in Japan and beyond national jurisdictions.

Oceanic methane hydrate utilization system design and reservoir scale numerical modeling

In recent years, the exploration and utilization of methane hydrate from permafrost and off-coast regions has become a very hot topic. Aiming at its potential as a vast energy resources in future use, the current study is focused on the low emission extraction and utilization system for oceanic methane hydrate and its economic and technological feasibility. In this study, recent real extraction tests in both China and Japan in 2017 are firstly discussed and compared, where the challenges in production period and production rate are explained. In those tests, depressurization method is used to dissociate the methane hydrate for gas production form the unconsolidated reservoir layers below the seabed. Based on the conditions of Japan, a systematic utilization system is proposed for oceanic methane hydratewhich is consisted of extraction system, power generation system, Carbon Capture and Sequestration (CCS) system and hot fluid injection system. Vertical well systems are adopted in this design and the strategic warming up process is used before depressurization extraction, so as to obtain optimal production rate of methane gas. The inclusion of CCS makes it possible to utilize methane hydrate with very low carbon emission. It is estimated that the electricity price obtained by this system can be comparable with the current civil electric price in Japan if the production rate is around 1.0×105 m3/d. Also in this study, the real extraction scenarios and possible strategies of Nankai Trough in Japan and Shenhu Area in China are re-visited and discussed by numerical models. It is found that the only depressurization or thermal stimulation method have the problems of wellbore deformation, pressure problem and energy efficiency problems, while combined strategic modes show the possibility of long-term continuous production. For the basic dissociation parameter analysis, the conditions near the equilibrium curve show higher production rate, which is due to higher initial reservoir temperature. Such results indicate the importance of heat supply inside the reservoir layer as the methane hydrate dissociation is endothermic. It is shown that with the increase of reservoir pressure, such temperature effects become more critical as the optimal initial temperature increases according to equilibrium curve. This results support the system design with strategic warming-up process in a sister well during methane hydrate extraction. In addition, the effects of reservoir layer thermal management, hot reservoir fluid flow effects on the dissociation behaviors are analyzed in this study. For vertical well analysis, the production rates under different permeability parameters show the same trend with real production, which in turn help confirm the in-lab permeability tests. Horizontal well simulation show the importance of overburden and under burden. If the confining layers are less permeable, the production rate becomes higher due to the limited intrusive water flow from the confining layers. In sum, this study summarized the advantages and disadvantages of oceanic methane hydrate extraction/utilization methods and put forward a low-carbon emission system. It is hoped that this study will contribute to the future development of both production prediction and real extraction tests.

Ion Exclusion at the Ice-Water Interface Differs from That at the Hydrate-Water Interface: Consequences for Methane Hydrate Exploration

When water-ice grows into salt solutions ion species are excluded by the ice differentially due to non-identical solubility in the ice lattice. This causes an electrical potential across the interface during the ice growth process, initially named the Workman Reynolds Freezing Potential, and may be one of the causes for lightning. However, by measuring the voltage between the ice and water, we have found that when tetrahydrofuran hydrate crystals are grown into salt solutions all ion species are excluded equally and the potential does not manifest. When considered together, this marked difference in ion exclusion scenarios may have ramifications for hydrate exploration because of the chlorine anomaly, which is often used as an indicator of the presence of hydrate reserves.

Reservoir properties and heterogeneity of turbidite sediment revealed from the exploration of methane hydrate in the eastern Nankai Trough, Japan

Reservoir properties and heterogeneity of turbidite sediment revealed from the exploration of methane hydrate in the eastern Nankai Trough, Japan

DEM Analysis of Geomechanical Properties of Cemented Methane Hydrate–Bearing Soils at Different Temperatures and Pressures

AbstractMechanical properties of methane hydrate–bearing soils are influenced by the surrounding temperature and pore pressure. Studies of such influences are of great significance for the safe exploration of methane hydrate. First, a thermo-hydro-mechanical bond contact model is introduced to elucidate the microscopic contact scale behavior of grains with methane hydrate bonds. Second, such a model is incorporated into the distinct-element method (DEM), and a dimensionless temperature-pressure distance parameter is introduced. The influences of temperature and pore pressure along heating and depressurization paths are analyzed by conducting DEM biaxial compression tests, which are also compared with the results of laboratory triaxial compression tests conducted by others. The variation rules of the macromechanical properties (i.e., strength, elastic modulus, peak friction angle, cohesion, and dilatancy angle) with changing temperatures and pore pressures are then analyzed for different effective confinin...

Gas outburst with sediments because of tetrahydrofuran hydrate dissociation

SummaryMethane hydrate (MH) is a new energy resource in the 21st century. But the dissociation of MH from sediments during the MH exploration or oil/gas exploration under a hydrate layer accompanied by the softening of soils and formation of excess pore gas pressure may lead to ground failures and environmental disasters. In this study, experiments on modeling the weakening and failure of the sediment by heat‐induced dissociation of tetrahydrofuran (THF) hydrate were presented. The failure mode of gas outburst was observed. Gas outbursts is a process where gas and soils in hydrate‐dissociation zone burst out after the continuous skeleton of over‐layer is fractured during the expansion of the dissociation zone and the formation of gas zone and excess pore gas pressure. An analytical method is presented by decoupling heat transfer and soil deformation. The geometrical and mechanical similarities for gas outburst are obtained. An empirical criterion for the occurrence of outburst is proposed using the theory of thermal conduction, rigid plastic mechanics, and the experimental data. Copyright © 2015 John Wiley & Sons, Ltd.

RETRACTED: A global survey of gas hydrate development and reserves: Specifically in the marine field

Gas hydrates, also known as methane hydrates, are formed due to the high hydraulic pressures present under the cold seabed over long periods of time. Gas hydrates are mainly composed of methane produced in the seabed by bacteria in the use of the remains of animals and plants as food. Often appearing as translucent or opaque ice, gas hydrates can be separated into water and methane gas, which can be burned at normal temperatures and pressures, giving this substance the nickname “combustible ice.” As global oil reserves continue to be depleted, scientists are regarding methane hydrates as a new energy source that is very likely to replace oil in the 21st century. According to reports by the United States Geological Survey, the potential natural gas energy that can be recovered from global methane hydrate formations is two times the amount of fossil fuel energy available to the world. Therefore, many countries that are deeply engaged in the development of gas hydrates, such as the United States, Japan, Canada, China, India, and Taiwan, hope that this new energy source can become a substitute for more conventional petroleum sources. Japan—the first country to develop methane hydrates—will be ready for commercial mass production in the eastern Japanese Nankai Trough prior to 2018, according to Japan׳s Methane Hydrate R&D Program-MH 21. However, the exploitation of methane hydrates in terrestrial permafrost requires less technical risk and costs. Joint explorations in areas of Alaska by the United States, Japan, and Canada will enter the preparation phase for commercial output as early as 2015. In Taiwan, cooperation with Germany and the United States has led to methane hydrate exploration and the initiation of drilling sampling in the South China Sea that is expected to be completed in 2016, with commercial production ready as soon as 2026.

Granular experiments of thrust wedges: Insights relevant to methane hydrate exploration at the Nankai accretionary prism

The accumulation mechanism of methane hydrates has been a central issue in previous hydrate research regarding the Nankai accretionary prism, southwest of Japan. Expulsion of formation fluids is significant during the prism accretion process, and the migration of these methane-bearing fluids exerts a strong control on the accumulation of hydrates. Two types of fluid pathways, inter-granular porosity and faults, need to be evaluated to understand hydrate accumulation. Fluid migration along faults can be partly modeled by examining faulting activity. Our study modeled the accretion process by using two granular methods that approximated the geologic body as an assemblage of particles: (1) analog experiments using granular materials, and (2) a numerical simulation based on the distinct element method. The analog experiments closely reproduced the prism geometry observed in seismic profiles across the Nankai accretionary prism. Digital image correlation analysis indicated that the frontal thrust is generally active but older structures are also frequently reactivated. The numerical simulations produced prism geometries similar to those of the analog experiments. The velocity distributions of the particles showed evidence of episodic faulting and reactivation, but the internal stress field exhibited little change in the deeper part of the prism during deformation. The frequent and substantial changes in fault activity displayed by the models indicate episodic fluid flow along fault surfaces. Active frontal thrusting suggests that formation fluids generally migrate from deep within the prism to the deformation front, but may move along reactivated older faults. Inter-granular permeability also fluctuates, as it is controlled by temporal and spatial variations in the internal stress field. However, fluid flow is likely to be relatively stable in the deeper segment of the prism.

Sensitivity of Deep-Towed Marine Electrical Resistivity Imaging Using Two-Dimensional Inversion: A Case Study on Methane Hydrate

Uncertain physical properties of methane hydrate (MH) above a bottom simulating reflector should be estimated for detecting MH-bearing formations. In contrast to general marine sediments, MH-bearing formations have a relatively high electrical resistivity. Therefore, marine electrical resistivity imaging (MERI) is a well-suited method for MH exploration. The authors conducted sensitivity testing of sub-seafloor MH exploration using a two-dimensional (2D) inversion algorithm with the Wenner, Pole-Dipole (PD) and Dipole-Dipole (DD) arrays. The results of the Wenner electrode array show the poorest resolution in comparison to the PD and DD arrays. The results of the study indicate that MERI is an effective geophysical method for exploring the sub-seafloor electrical structure and specifically for delineating resistive anomalies that may be present because of MH-bearing formations at a shallow depth beneath the seafloor.

Molecular dynamics simulations of the mechanisms of thermal conduction in methane hydrates

The thermal conductivity of methane hydrate is an important physical parameter affecting the processes of methane hydrate exploration, mining, gas hydrate storage and transportation as well as other applications. Equilibrium molecular dynamics simulations and the Green-Kubo method have been employed for systems from fully occupied to vacant occupied sI methane hydrate in order to estimate their thermal conductivity. The estimations were carried out at temperatures from 203.15 to 263.15 K and at pressures from 3 to 100 MPa. Potential models selected for water were TIP4P, TIP4P-Ew, TIP4P/2005, TIP4P-FQ and TIP4P/Ice. The effects of varying the ratio of the host and guest molecules and the external thermobaric conditions on the thermal conductivity of methane hydrate were studied. The results indicated that the thermal conductivity of methane hydrate is essentially determined by the cage framework which constitutes the hydrate lattice and the cage framework has only slightly higher thermal conductivity in the presence of the guest molecules. Inclusion of more guest molecules in the cage improves the thermal conductivity of methane hydrate. It is also revealed that the thermal conductivity of the sI hydrate shows a similar variation with temperature. Pressure also has an effect on the thermal conductivity, particularly at higher pressures. As the pressure increases, slightly higher thermal conductivities result. Changes in density have little impact on the thermal conductivity of methane hydrate.

メタンハイドレート探査手法とその基本コンセプト

Methane hydrates, which are ice like materials and constructed with methane and water molecules, are spread widely under permafrost and deep water areas. It is thought that methane hydrates are ones of occurrence type of methane and can be future energy resources.BSR (bottom simulating reflector) occurs at the basement of GHSZ (gas hydrate stability zone) and therefore it is used to detect occurrences of methane hydrates in deep waters using reflection seismic data. At the same time, BSR is only the first step for methane hydrate exploration because BSR does not provide information about thickness, saturation and type of methane hydrate bearing zone. From the viewpoint of resource exploration, practical methods to analyze details of methane hydrate resource fields have been required.We constructed an effective strategy to delineate methane hydrate concentrated zones, of which reservoir are turbidite sand bodies, utilizing four following indicators in 3D seismic data: (A) BSR, (B) turbidite sequence (above BSR), (C) strong seismic reflectors and (D) relatively higher interval velocity, suggesting methane hydrate concentrated sand layers. It enabled us to delineate methane hydrate concentrated zones and evaluate their rock volume successfully in the eastern Nankai Trough as similar as prospects and leads in conventional oil and gas fields are delineated by geological and geophysical approaches.

東部南海トラフ海域のメタンハイドレート探査における堆積学的手法の適用

An integrated sedimentological workflow was applied to the methane-hydrate exploration project in the eastern Nankai Trough area, central Japan, as a useful tool to investigate the distributions and actual volume of methane hydrates. Since previous research revealed that most of the methane hydrates in the eastern Nankai Trough area occur in matrix pores of submarine-fan turbidite sandstones, the facies distribution of turbidite sandstones may be one of the important keys to evaluate the occurrence of methane hydrates. Accordingly, this study conducted a) core facies analysis, b) log facies analysis mainly using FMI, c) depositional sequence division, d) seismic facies analysis on the 2D/3D seismic sections, and e) seismic geomorphological analysis on the 3D seismic survey data to map submarine-fan turbidite facies distributions for 17 depositional sequence horizons in the targeted Pleistocene interval. The obtained facies maps reveal that submarine-fan depositional styles changed throughout Pleistocene from a braided channel type, through small radial fan, trough-fill fan, and muddy sheet fan types, to a channel-levee system type. As the next step, the facies maps of each depositional sequence were overlaid with bottom simulating reflector (BSR) distributions as a proxy of methane hydrates. The overlaid maps indicate that the BSRs occur on feeder channels, distributary channels, and proximal lobes of submarine fans, suggesting that methane hydrates selectively occur in coarser grained portions of a submarine fan. The facies maps were also used for calculation of average net-to-gross ratio of the methane hydrate occurrence zones, as the maps provide information on the spatial distributions of seismic facies class, which has individual value of net-to-gross ratio. Finally, this study analyzed reservoir-scale geologic bodies, such as depositional lobes and braided channels of a submarine fan, using 3D seismic survey data, and constructed a detailed geologic model for simulations of methane-hydrate generation and production.

計量魚群探知機によるメタンプルームの観測とメタン運搬量の見積もり

During methane hydrate exploration and research, remote and on-board acoustic surveying and monitoring of methane hydrate can be easily and economically conducted using a quantitative echo sounder. Simultaneously, the structure and the floating-up speed of methane plumes can be obtained from an analysis of acoustic data. We conducted a survey of methane plumes from 2004 through 2008 at a spur situated southwest off the coast of Sado Island (tentatively called Umitaka Spur) and at the Joetsu Knoll. In 2007 and 2008, we performed experiments by releasing methane hydrate bubbles and methane hydrate, and letting them float upward. Consequently, we demonstrated that acoustical reflection from the methane plumes correlates with water temperature and depth, that the floating-up speed is constant but depends on the conditions of methane hydrate, that the discharge of methane hydrate bubbles changes, and that there is a wide scattering of materials below the seafloor where methane plumes are located. Furthermore, the amount of methane hydrate bubbles seeping was estimated by a preliminary calculation. The method will be applied not only to basic research on methane hydrate but also to assessments of the environmental impact of methane hydrate exploitation.

海底電磁探査の近年の進歩とメタンハイドレート検出への適用

Marine electromagnetic (EM) sounding methods were developed originally for imaging the deep mantle, partial melt below mid-oceanic ridges, active faults in the crust around subduction zones, etc. Recently, marine EM sounding has been applied extensively for oil, gas, and methane hydrate exploration. In this paper, we introduce various marine EM sounding methods and case studies. Natural fluctuations of EM fields from the ionosphere can be observed on the seafloor using ocean-bottom electromagnetometers (OBEMs), and the marine magnetotelluric sounding method can be applied for imaging the deep structure (> several km below the seafloor). However, the natural signal is less powerful in the higher frequency band and provides less resolution for near-seafloor structures. A high-frequency controlled electromagnetic source on/near/below the seafloor is necessary for evaluating the shallow structure. Marine controlled-source EM (CSEM) sounding using a horizontal electric dipole, towed near the seafloor and sending an electric signal, and OBEMs as receivers settled on the seafloor is widely used for oil and gas exploration, and occasionally for methane hydrate explorations. A vertical electric dipole is also used for magnetometric resistivity (MMR) sounding when exploring hydrothermal systems around ridges. On the other hand, both sources and receivers attached to a cable can be towed near the seafloor for a continuous survey. Towed-CSEM sounding with a time-domain analysis is used especially for methane hydrate exploration. Another type of towed-CSEM with magnetic signal obtains porosity information on near-seafloor sediments. A deep-towed marine DC resistivity survey has been applied near the seafloor, and successfully imaged shallow methane hydrate distributions in sedimentary layers. Thus, various marine EM soundings can illuminate sub-seafloor methane hydrate, and are useful for discussing the accumulation/dissolution process of methane hydrate and the potential of methane hydrate as a new energy resource.

東部南海トラフメタンハイドレート資源の生産性評価

The Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium), which was organized to attain the exploration and exploitation of methane hydrate (MH) resources, has been evaluating MH reservoirs located in the Eastern Nankai Trough from the viewpoints of geology, geophysics, petrophysics and reservoir/production engineering. As one of these studies, we have been predicting gas/water production performances from these MH reservoirs showing diverse characteristics. This paper presents the results of our examinations on the producibility of gas from MH reservoirs located in the Eastern Nankai Trough by a variety of MH dissociation/production methods and on the feasibility of the future development of MH in terms of gas production and economics.Eastern Nankai Trough MH reservoirs, which are composed of alternating beds of sand, silt and clay in turbidite sediments, have various conditions of clay distribution as well as of initial pressure, temperature, permeability and MH saturation. Some of these reservoirs contain MH of high saturation at a certain interval (MH concentrated reservoirs), while in the others MH is deposited sparsely (MH non-concentrated reservoirs). First, the numerical near well model for one of the typical MH reservoirs was constructed mimicking this alteration of sand, silt and clay layers by simplified grid layer system, which was used for the simulation to clarify the mechanisms of MH dissociation and production by the application of the depressurization method.The detailed numerical reservoir models were then constructed for the vicinity of the wells located both in MH concentrated and in non-concentrated reservoirs, consulting the well log and core interpretation results. MH dissociation/production performances were predicted through numerical simulation assuming the application of various MH dissociation methods such as depressurization, wellbore heating, hot water huff'n'puff and hot water flooding. The simulation studies revealed the difference in the gas production between MH concentrated and non-concentrated reservoirs. These studies also suggested that the methane recovery from MH concentrated reservoirs by depressurization methods ranged from 30 to 60% and that the energy efficiency by thermal methods was very small despite high methane recovery. Furthermore, simple economic analyses on the basis of these simulation results exhibited the promise that some MH reservoirs in the Eastern Nankai Trough could be economically developed if the well spacing and MH dissociation/production methods were appropriately designed.

東部南海トラフのメタンハイドレート層とメタン湧出を伴う海底面現象の関係

It is important to understand the relations between the formation of methane hydrate in shallow sediments and seafloor manifestations accompanied by methane discharges to delineate the following issues concerning the exploitation of methane hydrate: (1) establishing methane hydrate exploration method through geological and geochemical surveys of the seafloor; (2) understanding methane hydrate system; (3) clarifying relations between methane hydrate-bearing formations and global warming; and, (4) understanding production problems during methane hydrate development. As a preliminary study to solve the above-mentioned issues, we attempted to clarify the relations between methane hydrate-bearing formations and various seafloor manifestations accompanied by methane releases from the seafloor, such as pockmarks and carbonate precipitations, using 3D seismic data in the three survey areas of the eastern Nankai Trough. Bathymetric and seafloor amplitude maps constructed from high-resolution 3D data provided extensive information on the seafloor. We also constructed BSR depth anomaly maps to interpret geothermal gradient and structure of the P-wave velocity in shallow formations because methane hydrate is sensitive to temperature change. It is likely that methane hydrate-bearing formations and seafloor manifestations have a strong relationship with geological migration conduits of methane-bearing fluid in the eastern Nankai Trough, e.g., permeable sandy sediments, and large and shallow faults. New geological and geochemical surveys of the seafloor are required to clarify the relationship.

Fusion of 3D seismic exploration and seafloor geochemical survey for methane hydrate exploration

The MH21 Research Consortium has conducted a high-resolution 3D seismic survey and a seafloor geochemical survey, to explore methane hydrate reservoirs in the eastern Nankai Trough, offshore Japan. Excellent geological information about shallow formations was obtained from the high-resolution 3D seismic survey, which was designed to image the shallow formations where methane hydrates exist. The information is useful in constructing a geological and geochemical model, and especially to understand the complex geology of seafloor, including geochemical manifestations and the structure of migration conduits for methane gas or methane-bearing fluid.By comparing methane seep sites observed by submersibles with seismic sections, some significant relationships between methane hydrate reservoirs, free gas accumulations below the seafloor, and seafloor manifestations are recognised. Bathymetric charts and seafloor reflection amplitude maps, constructed from seismic reflections from the seafloor, are also useful in understanding the relationships over a vast area. A new geochemical seafloor survey targeted by these maps is required.The relationships between methane hydrate reservoirs and seafloor manifestations are becoming clearer from interpretation of high-resolution 3D seismic data. The MH21 Research Consortium will continue to conduct seafloor geochemical surveys based on the geological and geochemical model constructed from high-resolution 3D seismic data analysis.In this paper, we introduce a basis for exploration of methane hydrate reservoirs in Japan by fusion of 3D seismic exploration and seafloor geochemical surveys.