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The Hydrate-01 Stratigraphic Test Well was drilled at the Kuparuk 7-11-12 site on the Alaska North Slope in December 2018. Sonic log data provide compressional (P) and shear (S) slowness from which we determine gas hydrate saturation (S gh ) estimates using effective medium theory. The sonic S gh estimates compare favorably with S gh estimated from resistivity and nuclear magnetic resonance (NMR) logs, showing that gas hydrate occupies up to approximately 90% of the pore space in the target reservoir sands. The informally named B1 sand (2294 feet below mean sea level) shows lower V P /V S ratios than the D1 sand (2770 feet below mean sea level), with the lower part of the B1 sand showing lower V P /V S ratios than the upper part of the B1 sand. This corresponds to a stiffer, or more "cemented", behavior for the lower B1 sand and less cemented behavior for the D1 sand. This trend could be due to differences in the reservoirs themselves or in the gas hydrate morphology or to both factors. We observe that the presence of gas hydrate in the upper B1 sand has greater impact on hydraulic permeability (measurements suggest a greater difference between intrinsic and effective permeability) than in the D1 sand, possibly related to gas hydrate morphology but more likely due simply to higher gas hydrate saturations in the upper B1 sand. Analyses of S gh relative to porosity, shale fraction, and intrinsic permeability show that reservoir quality (as represented by these three metrics) exerts control on gas hydrate saturation. Grain size and mineralogy data show somewhat smaller grains and better sorting in the D1 reservoir relative to the upper B1 reservoir and smaller grains and greater clay fraction in the lower B1 reservoir relative to the other two reservoir zones. Together, these data suggest that reservoir characteristics play a role in the observed V P /V S patterns, but gas hydrate morphology (possibly varying with saturation) must also be considered.

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.

[1] The Research Group for Semi-controlled Earthquakegeneration Experiments in South African deep gold mines (SeeSA) has continuously monitored strain changes with a resolution of 24 bit 25 Hz at the Bambanani mine near Welkom. An Ishii borehole strainmeter was installed at a depth of 2.4 km near the potential M $ 3 earthquake source area. Instantaneous strain steps of $10 À4 strains associated with two M2 events were observed within a length of seismic fault. These steps were followed by significant postseismic creep-like drift, but not preceded by forerunners. Analysis of the continuous 25 Hz data reveals many smaller steps with much longer durations (100 ms $ 100 s) than seen in normal earthquakes (À1 < M < 2) with source durations of 1 ms$50 ms. Some of the especially slow steps were preceded by accelerations in strain, the maximum being as large as one-third of the step.

The authors document strain changes up to ~10 -4 associated with two M>2 events 2.4 km deep at distances less than ~100 m from a strainmeter. This corresponds to a ~7 MPa stress change recorded only within the hypocentral area. This change was recorded with a sensitive, wide-dynamic-range, Ishii strainmeter. A 15-m hole was drilled subparallel to the fault strike, in order to continuously monitor slip-driving shear and normal strains with a sampling frequency of 25 Hz. Two M>2 events took place around the fault. For both events, relaxation in the maximum principal stress at a rate of 10 -6 /week was observed for several days prior to the main shock. In one of the two events, foreshocks were concentrated in the last several tens of seconds, accompanied by strain steps and logarithmic post-seismic deformation. However, no acceleration in deformation was observed, even with a resolution of 1/10,000 of a coseismic step for each event.

We have discussed reservoir architecture of methane hydrate (MH) bearing turbidite channels in the eastern Nankai Trough using 3-D seismic data and well log data. MH bearing turbidite channels exhibit complex patterns of strong reflection comprising patchy-like shape of positive and negative seismic reflectors. The groups of these reflectors obtained by picking represent the internal architecture of the channel complex that can be roughly classified into three depositional sequences. According to a seismic sequence stratigraphic analysis, each depositional sequence results in the different depositional system implying that the reservoir architecture of the turbidite channels varies corresponding to the sedimentary conditions as well as the topographic changes in the study area. Compared with well log data, the thickness of the turbidite channel at ß2 well in the southwestern part of the study area is much greater than that of ß1 well in the northeastern part. However, the depositional sequences of the northeastern part represent sand-dominated turbidite sediments ensuring that the reservoir potential is high despite the relatively smaller thickness of the turbidite channels.For constructing a geological frame model, we examined further details of reservoir characteristics of the turbidite channels around ß1 well. The identified bottom frame of the several channels is oriented along north-to-south and northnortheast-to-south-southwest directions, which coincide with the directions of paleo-current flows determined by the seismic sequence stratigraphic analysis. An interval velocity between BSR (bottom simulating reflector) and the top of the MH bearing sediments is obtained from a high-density velocity analysis. The distributions of the higher interval velocity are identified above the bottom frame of channels in the northeastern part of the study area. The turbidite sediments in the northeastern side of channels are derived from the north-northeast direction of paleo-current flows, which is different from the sediment supply system compared with those of the northern side to southwestern side of the channels. Thus, the higher velocity anomalies in the northeastern side of the channels may be related to the different coarse sediments supply system which may lead to the different reservoir architecture of the turbidite channels.

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.

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