All ICDP Publications with Abstracts
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84.
Occurrences of natural gas hydrates beneath the permafrost zone in Mackenzie Delta. Visual and X-ray CT imagery
Annals of the New York Academy of Sciences,
9121021-1033
2000
ISSN: 00778923
Publisher: New York Academy of Sciences
Keywords:▾
methane; natural gas, Canada; computer assisted tomography; conference paper; gas; gas analysis; oil industry; porosity; sand; sediment; X ray analysis
Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 research well was drilled to a depth of 1150 m beneath the permafrost zone in the Mackenzie Delta, N.W.T., Canada, early in 1998. A large amount of natural gas hydrates were successfully retrieved from a variety of sandy and gravel sediments. Over 110 m of gas hydrate-bearing sediments were found to be distributed between 897 m and 1100 m deep. Approximately 37 meters of core were recovered in this interval with most of the recovered gas hydrates being less than 2 mm in size occurring mainly in intergranular porosity of silty to clean massive sand and conglomerate (granule to pebble). Typically, hydrate-bearing strata were between 10 cm and more than one meter thick with an estimated porosity of 25 to 35%. The largest form of hydrate was about 2 cm in diameter, occurring as clasts and intergranular porosity within granular sands. Occurrences of natural gas hydrate have been observed visually at the drill site and in core samples preserved in pressurized storage vessels utilizing an X-ray CT scanner technique. Quantitative assessments of gas hydrate concentrations in core samples have been made based on pressure response of dissociation vessels and direct volumetric measurements. Six types of gas hydrate have been recognized: (1) pore-space hydrate, (2) platy hydrate, (3) layered/massive hydrate, (4) disseminated hydrate, (5) nodule hydrate, and (6) vein/dyke hydrate. The X-ray CT images proved useful for characterizing macroscopic forms of gas hydrate. Finer grained occurrences were more difficult to study, however the distribution of gas hydrates and granular grains ran be recognized. The occurrences of natural gas hydrates in the Mallik well are compared to the previous natural gas hydrate core samples obtained from ODP/DSDP programs and other field studies.
83.
Petrophysical studies on rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt, Central China: implications for the composition and delamination of the lower crust
Tectonophysics,
301
(3)
191-215
1999
ISSN: 0040-1951Keywords:▾
UHP rocks, seismic properties (V and V), densities, crustal lithology, delamination
Abstract: ▾ The Dabie–Sulu ultrahigh-pressure (UHP) metamorphic belt of Central China represents a zone in which upper and lower continental crust has been subducted and then rapidly exhumed from the mantle back into the crust. About 30 UHP rock samples representing major lithologies were collected from surface exposures and their physical properties were measured in the laboratory at pressures up to 600 MPa (room temperature) and temperatures up to 600°C (600 MPa confining pressure). The experimentally determined data include compressional (Vp) and shear wave velocities (Vs), velocity anisotropy (shear wave splitting), density, and intrinsic pressure and temperature derivatives of Vp and Vs. Using a regional geotherm, velocity–depth profiles were calculated for the different lithologies spanning compositions from felsic through intermediate to mafic, and metamorphic grades up to granulite and eclogite facies. The velocity data, in particular the Poisson ratio (Vp/Vs ratio), were used to interpret lithologically the seismic models evaluated from seismic refraction data. The seismic velocity profiles reveal a four-layer structure (upper, middle, upper-lower and lowermost crust) with an average thickness of 34 km. From our results, we infer that a mixture of about 90% felsic gneiss with variable amounts of high Vp amphibolite/gabbro constitute the middle crust. Intermediate granulite and mafic granulite fit both the Vp and Poisson ratio of the upper-lower and lowermost crust, respectively. From the combined Vp and Poisson ratio (Vp/Vs) data, we conclude that eclogite is not a major constituent of the present Dabie deep crust. Combining the laboratory-derived in-situ seismic data (Vp and Poisson ratio) with refraction seismic data, along with evidence from geological and geochemical investigations, suggests that lower crustal delamination played an important role in the modification of the East China crust.
82.
Production potential of the Mallik field reservoir
Bulletin of the Geological Survey of Canada
(544)
377-390
1999
ISSN: 00687626Abstract: ▾ Immense volumes of naturally occurring gas hydrate in different parts of the world, onshore and offshore, have encouraged the belief that gas hydrate in the next century may become a viable energy resource. Various issues need to be resolved to convert gas hydrate from an energy resource to an energy reserve of real commercial value. The production capability of a gas hydrate reservoir and the gas production technique that could be utilized should be addressed through geological and petrophysical studies, well-production tests and reservoir simulation. To make the simulation of practical value, the controlling mechanisms of fluid flow, kinetics, and heat transfer should be incorporated in the model. The Mallik gas hydrate accumulation in the Mackenzie Delta has exhibited promising potential to be considered a gas reserve through the assessments made of the Mallik L-38 and 2L-38 wells. The data available from both wells and the results of production tests in JAPEX/JNOC/GSC Mallik L-38 gas hydrate research well accommodate basic requirements for comprehensive modelling of the reservoir and production of gas from the in situ gas hydrate through various methods.
81.
Post-eruptive volcanic dome evolution as revealed by deformation and microgravity observations at Usu volcano (Hokkaido, Japan)
Journal of Volcanology and Geothermal Research,
89
(1-4)
255 – 273
1999
ISSN: 03770273Keywords:▾
Japan; crustal deformation; gravity anomaly; lava dome; volcanic eruption
Abstract: ▾ Usu volcano (Hokkaido, Japan) is a dacitic volcano, known for its high production rate of lava domes and crypto-domes. It is thus a good target to study processes of volcanic dome evolution (upheaval and/or relaxation). We carried out repeated GPS and microgravity surveys on the three most recent domes of Mt. Usu (1910: Meiji Shinzan; 1943-1945: Showa-Shinzan and 1977-1982: Usu-Shinzan). The repeat period was 1 to 2 months and extended from October 1996 to June 1997. We also compare new data with results from former studies. More than 20 years after the start of Usu-Shinzan dome growth, there is still subsidence at a maximum rate of about 7 to 8 cm/year. The reasons for this subsidence are discussed. Repeated gravity surveys revealed an increase of gravity on the domes (about 60 ± 10 microgal/year for Usu-Shinzan, about 15 microgal at Showa-Shinzan and 10 to 20 microgal for Meiji-shinzan); this gravity increase exceeds that expected due to subsidence. We discuss and interpret the excess gravity change in terms of a density increase in the edifice, caused by a combination of processes (contraction of the edifice, water level change, devesiculisation, cooling and magma intrusion). Quantification of these processes at Usu volcano may help to understand the processes of evolution at domes on other volcanoes such as Merapi (Indonesia), Unzen (Japan) or Montserrat (West Indies).
80.
Pore-water geochemistry, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
165-175
1999
ISSN: 00687626Abstract: ▾ A pore-water research program was designed to measure dissolved components in interstitial water from sediment core samples collected during the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research-well project. Pore waters from the gas-hydrate-bearing samples had an average salinity of 8 ppt compared to 34 ppt for non-gas-hydrate-bearing samples. The difference in salinities suggests that 80-90% of the pore space in the gas-hydrate-bearing sediment was filled with gas hydrate, which dissociated during recovery. Potassium concentration was also measured in pore water, to estimate the amount of drill-mud contamination in pore-water samples, since the drill mud contained brine solution made from potassium chloride. On average, pore-water salinities were estimated to be enhanced by 2 ppt due to drill-mud contamination.
79.
Physical properties of sediments from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
95-100
1999
ISSN: 00687626Abstract: ▾ A 1150 m deep gas hydrate research well was drilled in the Canadian Arctic in February and March 1998 to investigate the interaction between the presence of gas hydrate and the natural conditions presented by the host sediments. Profiles of the following measured and derived properties are presented from that investigation: water content, sediment wet bulk density, grain size, porosity, gas hydrate quantity, and salinity. These data indicate that the greatest concentration of gas hydrate is located within sand and gravel deposits between 897 m and 922 m. American Society for Testing and Materials 1997: Standard test method for specific gravity of soil solids by gas pycnometer D 5550-94; in American Society for Testing and Materials, Annual Book of ASTM Standards, v. 04.09, Soil and Rock, West Conshohocken, Pennsylvania, p. 380-383.
78.
Overview of science program, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
11-17
1999
ISSN: 00687626Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was drilled in February and March, 1998, in the Mackenzie Delta, Northwest Territories, Canada, to a depth of 1150 m. The scientific program was conducted through a collaborative agreement between the Japan National Oil Corporation and the Geological Survey of Canada with key participation by the Japan Petroleum Exploration Company and the United States Geological Survey. A primary objective of the well was to undertake a comprehensive scientific research program to study an arctic gas hydrate accumulation. Field research conducted as part of the Mallik 2L-38 program included collection of permafrost and gas-hydrate-bearing core samples, downhole geophysical logging, and a vertical seismic profile survey. Laboratory and modelling studies undertaken during the field program, and subsequently as part of a post-field research program, documented the sedimentology, biostratigraphy, physical/petrophysical properties, pore-water and gas geochemistry, geophysics, and reservoir characteristics of the Mallik field gas hydrate accumulation.
77.
Petrophysical environment of sediments hosting gas hydrate, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
109-124
1999
ISSN: 00687626Abstract: ▾ A petrophysical study has been performed on mudstone and sandstone samples from depths of 880-950 m to determine the petrophysical controls on gas hydrate distribution in the sedimentary sequence at the J APEX/JNOC/GSC Mallik 2L-38 well site, Northwest Territories, Canada. Within the cored interval of the Mallik 2L-38 well gas hydrate is hosted in two sandstone horizons with overlying and underlying mudstone horizons, with minor gas hydrate concentrations within some mudstone formations. Results indicate that, although the interbedded mudstone units have relatively high porosities (24-30%) and are at relatively shallow depths, they have a well developed framework-supported texture, probably due to high silt and sand content (56-78 weight per cent), and a maximum burial depth greater than present. Regardless of this, the minor matrix content (13-25 weight per cent) controls the fluid transport characteristics, resulting in extremely low mudstone permeability sections (2×10-21 m to 2×10-19 m2). There are indications that these low permeabilities and the storage pore sizes contribute to the gas hydrate distribution.
76.
Petrology of the 1991-1995 eruption at Unzen: Effusion pulsation and groundmass crystallization
Journal of Volcanology and Geothermal Research,
89
(1-4)
173 – 196
1999
ISSN: 03770273Keywords:▾
Japan; dacite; petrology; volcanic eruption
Abstract: ▾ Effusive eruption of dacite magma (2.1 X 108 m3) during 1991-1995 formed a lava dome at the summit of Unzen Volcano, Japan. The effusion rate was highest at the beginning, 4.0 X 105 m3/day (4.6 m3/s), and decreased roughly with time, to almost zero before this pattern was repeated with a second pulse of magma supply. The whole-rock chemistry of lavas shows significant variation attributable to variations in phenocryst abundance; the more mafic, the more abundant the phenocrysts. The pattern of chemical variation with time shows some difference from that of the effusion rate. All phenocrysts in dacite (plagioclase, hornblende, biotite, quartz and magnetite) show evidence of disequilibrium with melt. Although a glomerophyric aggregation of phenocrysts suggests coexistence with each other, phenocrysts are isotopically heterogeneous from species to species. The calculated initial melt composition was rhyodacite, and was nearly constant throughout the activity. In contrast, the bulk phenocryst population is andesite. A model explaining the textures and the isotopic heterogeneity is the capture of diorite fragments (or xenocrysts) by parental rhyodacite magma. It is suggested that, when effusion rate was high, less viscous crystal-poor magma exited from the reservoir. Groundmass glass and plagioclase microlite rims show temporal chemical variations correlating with the effusion rate; the higher the effusion rate, the more evolved the compositions. Groundmass crystallinity increased with decreasing effusion rate; from 33% to 50%. Textures in dome lavas suggest that groundmass crystallization had been mostly completed when magma reached the conduit top. The Fe-Ti oxide temperature (880-780°C) was low when the crystallinity was high. Micropumice erupted before dome growth provided a sample recording magmatic foam in the conduit. Porosity of dome lavas was lower at lower effusion rates. Collapse of foam magma and simultaneous escape of volatiles through the conduit top were probably responsible for the accompanying low-frequency earthquakes. Phenocrysts were broken and the breakdown rims on hornblende phenocrysts were torn off during collapse and successive compaction. When effusion waned, degassing and the resultant crystallization proceeded more completely, so that the magma became too viscous to flow in the conduit top and behaved as a plug, resulting in a temporary halt of effusion. In turn, groundmass crystallization in magma below the plug increased excess pressure in the upper parts of conduit due to slow cooling. The plug was scavenged when rising excess pressure overcame its effective strength. Then, the second pulse of magma supply began. Strong endogenous growth and extrusion of a lava spine in the later stage probably occurred for the same reason.
75.
Petrology and Fe-Ti oxide reequilibration of the 1991 Mount Unzen mixed magma
Journal of Volcanology and Geothermal Research,
89
(1-4)
213 – 230
1999
ISSN: 03770273Keywords:▾
Japan; dacite; geobarometry; magma chemistry; petrology; volcanic eruption
Abstract: ▾ A dacitic magma (64.5 wt.% SiO2), a mixture of phenocryst-rich rhyodacite and an aphyric mafic magma, was erupted during the recent 1991-1995 Mount Unzen eruptive cycle. The experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas, and the nature of the mixing process. The preeruption rhyodacitic magma was at a temperature of 790 ± 20°C according to Fe-Ti oxide phenocryst cores, and at a depth of 6 to 7 km (160 MPa) according to Al-in-hornblende geobarometry. The mafic magma that mixed with the rhyodacite is found as andesitic (54 to 62 wt.% SiO2) enclaves in the erupted magma and was essentially aphyric when intruded. Phase equilibria indicate that an aphyric andesite at 160 MPa is >1030°C (H2O-saturated) and possibly as high as 1130°C (2 wt.% H2O). The composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt.% SiO2. Using these compositions and temperatures, the temperature of the Unzen magma after mixing is estimated to be at least 850°to 870°C. The groundmass Fe-Ti oxide microphenocrysts and those in pargasite-bearing reaction zones around biotite phenocrysts both give 890 ± 20°C temperatures; the oxide-oxide contacts give temperatures of 910 ± 20°C. The 900 ± 30°C postmixing temperatures are consistent with phase-equilibria experiments which show that the magma was not above 930°C at 160 MPa. Our Fe-Ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption, which is a shorter time than is calculated using available diffusion data. There is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma.
74.
Palynology, age, and paleoenvironmental interpretations from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
81-93
1999
ISSN: 00687626Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was drilled in 1998 to investigate the geological, geochemical, geophysical, and engineering properties of a gas hydrate accumulation previously identified in the Mallik L-38 well. Palynological analysis of core and cuttings from 670 m to 1150 m (TD) are reported here. Detailed quantitative analysis has been done on the 886-952 m cored interval that hosts the main gas hydrate accumulation. The pollen and spore evidence suggests the following biostratigraphic subdivisions for the 670-1150 m succession: 670-785 m, Late Miocene or older; 775-897 m, within the range of Early Miocene to Late Eocene; 897-930 m, probably Late Eocene; 930-995 m, Late Eocene; and 995-1151m within the range Early to Middle Eocene. Below 930 m the rocks are best assigned to the Richards and upper Taglu sequences. A dominantly continental succession is indicated, with a marginal marine and/or estuarine episode between about 945 m and 948 m, in the Late Eocene. The dinoflagellates in this interval are considered to be indigenous to the sampled rock. There is evidence of two episodes of edaphic-climatic dryness in the Late Eocene and probable Late Eocene.
73.
Overview of the 1990-1995 eruption at Unzen Volcano
Journal of Volcanology and Geothermal Research,
89
(1-4)
1 – 22
1999
ISSN: 03770273Keywords:▾
Japan; earthquake; lava dome; pyroclastic flow; volcanic eruption
Abstract: ▾ Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1 x 108 m3 (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230-540 m high). The eruption finished with extrusion of a spin endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO2 flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.
72.
Regional gas hydrate occurrences, permafrost conditions, and Cenozoic geology, Mackenzie Delta area
Bulletin of the Geological Survey of Canada
(544)
31-43
1999
ISSN: 00687626Abstract: ▾ The occurrence of natural gas hydrate within Cenozoic sediments of the Mackenzie Delta-Beaufort Sea region has been well documented. In preparation for the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, a detailed evaluation of terrestrial gas hydrate occurrences was undertaken to assess the geological setting, sediment associations, pressure and temperature conditions, and the presence of free gas in the Mackenzie Delta-Beaufort Sea region. After an exhaustive review, it was determined that the Mallik L-38 site, drilled by Imperial Oil in 1972, offered the highest probability of encountering a thick gas hydrate occurrence with high gas hydrate concentrations. On the basis of openhole well-log evaluation, it was estimated that about Him of gas-hydrate-bearing strata occur at this location from 810.1 to 1102.3 m, within the zone of predicted methane hydrate stability and below the base of icebearing permafrost, estimated to be at 640 m.
71.
Properties of samples containing natural gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, determined using Gas Hydrate And Sediment Test Laboratory Instrument (GHASTLI)
Bulletin of the Geological Survey of Canada
(544)
241-250
1999
ISSN: 00687626Abstract: ▾ As part of an ongoing laboratory study, preliminary acoustic, strength, and hydraulic conductivity results are presented from a suite of tests conducted on four natural-gas-hydrate-containing samples from the Mackenzie Delta JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. The gas hydrate samples were preserved in pressure vessels during transport from the Northwest Territories to Woods Hole, Massachusetts, where multistep tests were performed using GHASTLI (Gas Hydrate And Sediment Test Laboratory Instrument), which recreates pressure and temperature conditions that are stable for gas hydrate. Properties and changes in sediment behaviour were measured before, during, and after controlled gas hydrate dissociation. Significant amounts of gas hydrate occupied the sample pores and substantially increased acoustic velocity and shear strength.
70.
Scientific results from JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well.
In Dallimore S.R., Uchida T., Collett T.S., Editor
Publisher
Geological Survey of Canada
1999
ISBN:
0660177773
69.
Sedimentology of gas hydrate host strata from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
57-68
1999
ISSN: 00687626Abstract: ▾ A detailed sedimentological program has been conducted on gas-hydrate-bearing core samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. Three structurally and texturally distinct sedimentary facies are identified. Facies Csc (952.2-926.5 m) is a weakly bioturbated, clayey silt interbedded with fissile coal and silty sand. Facies Sg (926.5-908.5 m) is comprised of interbedded, finingupward successions of matrix-supported gravel to pebbly sand and fine sand. A dolomite-cemented sandstone (926.5-925 m) forms a distinct basal subfacies (Sst). Facies Ss (908.5-886.2 m) is a fine- to mediumgrained sand interbedded with gravel which fines upward to fine-grained sand with a gradational increase in silt content. The Kugmallit-Mackenzie Bay sequence boundary is interpreted to occur at the base of facies Sg. In situ and self-preserved gas hydrate occurred mainly in the sands and gravels of the Sg and Ss facies. The dolomite-cemented sandstone (subfacies Sst) may be related to complementary geochemical environments resulting from the formation of authigenic pyrite and solute exclusion related to gas hydrate growth within facies Sg.
68.
Seismic- and well-log-inferred gas hydrate accumulations on Richards Island
Bulletin of the Geological Survey of Canada
(544)
357-376
1999
ISSN: 00687626Abstract: ▾ The gas hydrate stability zone is areally extensive beneath most of the Mackenzie Delta-Beaufort Sea region, with the base of the gas hydrate stability zone more than 1000 m deep on Richards Island. In this study, gas hydrate has been inferred to occur in nine Richards Island exploratory wells on the basis of well-log responses calibrated to the response of the logs within the cored gas-hydrate-bearing intervals of the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. The integration of the available well-log data with more than 240 km of industry-acquired reflection seismic data have allowed us to map the occurrence of four significant gas hydrate and associated free-gas accumulations in the Ivik-Mallik-Taglu area on Richards Island. The occurrence of gas hydrate on Richards Island is mostly restricted to the crest of large anticlinal features that cut across the base of the gas hydrate stability zone. Combined seismic and well-log data analysis indicate that the known and inferred gas hydrate accumulations on Richards Island may contain as much as 187 178106 m3 of gas.
67.
Summary of physicochemical properties of natural gas hydrate and associated gas-hydrate-bearing sediments, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, by the Japanese research consortium
Bulletin of the Geological Survey of Canada
(544)
205-228
1999
ISSN: 00687626Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was drilled to a depth of 1150 m in the Mackenzie Delta, Northwest Territories, Canada, in February and March, 1998. A highlight of the project was the successful retrieval of natural gas hydrate samples in a variety of sediments. A summary is presented of research conducted by the Japanese research consortium led by the Japan National Oil Corporation with participation by ten Japanese companies and institutes. Fingerprints of the gas hydrate crystal structure and the molar ratio of water to guest-gas molecules occupying lattice sites are described for gas-hydrate-bearing samples as obtained by NMR and Raman spectroscopy. X-Ray CT imagery is used to describe the texture and gas hydrate/sediment characteristics of recovered samples during controlled dissociation testing. In addition, inorganic and organic chemical, thermal geophysical, and physical properties are described for key core horizons. Results are also presented documenting the rate of gas hydrate dissociation in drilling fluids with different chemistry including lecithin.
66.
The 15 September 1991 pyroclastic flows at Unzen Volcano (Japan): A flow model for associated ash-cloud surges
Journal of Volcanology and Geothermal Research,
89
(1-4)
159 – 172
1999
ISSN: 03770273Keywords:▾
Japan; ash flow; pyroclastic flow; volcanic ash; volcanic eruption
Abstract: ▾ Large-scale collapse of a dacite dome in the late afternoon of 15 September 1991 generated a series of pyroclastic-flow events at Unzen Volcano. Pyroclastic flows with a volume of 1 X 106 m3 (as DRE) descended the northeastern slope of the volcano, changing their courses to the southeast due to topographic control. After they exited a narrow gorge, an ash-cloud surge rushed straight ahead, detaching the main body of the flow that turned and followed the topographic lows to the east. The surge swept the Kita-Kamikoba area, which had been devastated by the previous pyroclastic-flow events, and transported a car as far as 120 m. Following detachment, the surge lost its force after it moved several hundred meters, but maintained a high temperature. The deposits consist of a bottom layer of better-sorted ash (unit 1), a thick layer of block and ash (unit 2), and a thin top layer of fall-out ash (unit 3). Unit 2 overlies unit 1 with an erosional contact. The upper part of unit 2 grades into better-sorted ash. At distal block-and-ash flow deposits, the bottom part of unit 2 also consists of better-sorted ash, and the contact with the unit 1 deposits becomes ambiguous. Video footage of cascading pyroclastic flows during the 1991-1995 eruption, traveling over surfaces without any topographic barriers, revealed that lobes of ash cloud protruded intermittently from the moving head and sides, and that these lobes surged ahead on the ground surface. This fact, together with the inspection by helicopter shortly after the events, suggests that the protruded lobes consisted of better-sorted ash, and resulted in the deposits of unit 1. The highest ash-cloud plume at the Oshigadani valley exit, and the thickest deposition of fall-out ash over Kita-Kamikoba and Ohnokoba, indicate that abundant ash was also produced when the flow passed through a narrow gorge. In the model presented here, the ash clouds from the pyroclastic flows were composed of a basal turbulent current of high concentration (main body), an overriding and intermediate fluidization zone, and an overlying dilute cloud. Release of pressurized gas in lava block pores, due to collisions among blocks and the resulting upward current, caused a zone of fluidization just above the main body. The mixture of gas and ash sorted in the fluidization zone moved ahead and to the side of the main body as a gravitational current, where the ash was deposited as surge deposits. The main body, which had high internal friction and shear near its base, then overran the surge deposits, partially eroding them. When the upward current of gas (fluidization) waned, better-sorted ash suspended in the fluidization zone was deposited on block-and-ash deposits. In the distal places of block-and-ash deposits, unit 2 probably was deposited in non-turbulent fashion without any erosion of the underlying layer (unit 1).
65.
The loss from the upper layer of bottom sediments in the cores from boreholes BDP-96-1 and BDP-96-2 (drilled on lake Baikal)
Geologiya i Geofizika,
40
(8)
1269 – 1271
1999
ISSN: 00167886Abstract: ▾ In 1996, boreholes BDP-96-1 and BDP-96 penetrated the bottom sediments of Lake Baikal. From the lithological and geochemical correlation of the sediments, the loss from the upper layer of sediments in the cores is estimated: 630 cm (BDP-96-1) and 19 cm (BDP-96-2). Correlation of these cores with the 120 cm long core from borehole GC-1, taken with a gravity corer during the drilling through the lake ice, permits compilation of a general continuous record of climatic changes in Central Asia for the last 5 Ma and prevention of loss of Holocene sediments.
64.
The Sr, Nd and O isotopic studies of the 1991-1995 eruption at Unzen, Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
243 – 253
1999
ISSN: 03770273Keywords:▾
Japan; isotopic analysis; magma chemistry; phenocryst; volcanic eruption
Abstract: ▾ The magma generation at Unzen volcano may be considered as the product of crustal material mixed with mantle magma accompanied by fractional crystallization (AFC). The magma in the Unzen volcano is estimated to consist of about 50-80% of residual magma (F) and about 30-70% assimilated crustal material (A) relative to the original magma. Concerning the 1991-1995 eruption, it is estimated that the magma formed as the result of mixing of about 50-60% crustal material and about 55-65% of residual magma. An alternative magma eruption model for the 1991-1995 eruption is proposed here. In the early stage, the isotopic characteristics of 1991 eruption are defined by AFC process in the deeper magma chamber. Later, the magma ascended through the conduit and quiescently stayed for a long time in a shallow reservoir before eruption. The minerals continuously crystallized as phenocrysts especially at the chilled top and outer margin in the shallow chamber. The crystallized phenocryst mush was reworked into the central part of the magma chamber by means of magma convection and rapid magma ascent. Therefore, the reaction between phenocrysts and melt occurs only in internal chemical disequilibrium in the magma chamber. In contrast, the isotopic compositions of the original magma shall be little influenced by the above processes throughout its eruptive history. The 1991-1995 eruptive rocks of the Unzen volcano show their characteristics in Sr and Nd isotopic values independent of their two previous eruptions. However, the isotopic values of early eruptive product could represent the original magma value. This result also supports the previous work of Chen et al. (1993) [Chen, C.H., DePaolo, D.J., Nakada, S., Shieh, Y.N., 1993. Relationship between eruption volume and neodymium isotopic composition at Unzen volcano. Nature 362, 831-834], that suggested the εNd of early or precursory eruptive products could be a qualitative indicator of the maximum size of a continuing or impending eruption.
63.
Thermochronologic constraints on deformation and cooling history of high- and ultrahigh-pressure rocks in the Qinling-Dabie orogen, eastern China
Tectonics,
18
(4)
621 – 638
1999
ISSN: 02787407
Publisher: Blackwell Publishing Ltd
Keywords:▾
China; Qinling-Dabie Orogen; Deformation; Geochronology; Mica; Rocks; Silicate minerals; Early Cretaceous; Lateral extrusion; Mountain ranges; Qinling-Dabie orogen; Rate of cooling; Sinistral shear; Ultra-high-pressure metamorphic rocks; Ultrahigh pressure rocks; cooling; deformation; petrology; ultrahigh pressure metamorphism; Metamorphic rocks
Abstract: ▾ The Hong'an block is the best place to study the exhumation of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks in the Qinling-Dabie orogen of eastern China because it lacks the extensive Cretaceous tectonic and thermal overprint observed in the Dabie Shan. We measured timing of deformation and rate of cooling of the HP-UHP rocks by 40Ar/39Ar analyses of synkinematic minerals from tectonites of key structural zones in the Hong'an and Tongbai Shan. Normal-sense shear along the north dipping Huwan detachment at the northern edge of the Hong'an block occurred between 237 and 231 Ma; this detachment facilitated the bulk of the exhumation of the HP-UHP rocks. Our new 40Ar/39Ar ages, combined with U/Pb zircon and Sm/Nd ages of 245-240 Ma, suggest that exhumation of UHP rocks from mantle depths occurred at rates of 5-25 mm/yr from ~245 to 230 Ma. The mountain range is a warped extensional footwall, within which white mica cooled from 225 to 205 Ma. Locally, younger extension is recorded by white mica recrystallization at 198-194 Ma, after which the entire block had cooled to below 300 °C. Early Cretaceous 40Ar/39Ar ages from the Tongbai shear zone indicate that dextral shear along the southwest boundary of the orogen was contemporaneous with normal to sinistral-oblique slip along the Xiaotian-Mozitan fault along the northern boundary. Coeval dextral and sinistral shear zones along the northern and southwestern margins of the Hong'an and Dabie Shan would have caused eastward lateral extrusion of these two blocks, perhaps driven by collision of the Lhasa block with Eurasia.
62.
Velocity analysis of vertical seismic profile (VSP) survey at JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, and related problems for estimating gas hydrate concentration
Bulletin of the Geological Survey of Canada
(544)
323-340
1999
ISSN: 00687626Abstract: ▾ A VSP survey was conducted at the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well to determine elastic-wave velocities that were estimated by traveltime inversion of zero-offset VSP and wavefield inversion of offset VSP data. Shear-wave velocity is estimated to be slower from VSP data than from wireline DSI measurements in the depth interval from 677 m to 889 m. The compressional-wave velocity difference between the VSP- and DSI-derived velocities are comparatively small. Synthetic seismograms from the drift-corrected DSI velocity log correlate well with VSP sections, especially for compressional waves. Azimuthal anisotropy is suggested in VSP shear-source data and the mode of anisotropy appears to change around the base of permafrost. By comparing computed elastic velocities with drift-corrected DSI velocity logs, two opposing gas hydrate saturation models are examined. Shear wave velocity proved to be the key data to select the correct model. The observed elastic velocity fits the computed elastic velocity for the model of gas hydrate disseminated in pore-space with little cementation at the grain boundaries.
61.
Vertical seismic profile (VSP) in the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
341-355
1999
ISSN: 00687626Abstract: ▾ As part of the JAPEX/JNOC/GSC Mallik 2L-38 field program, a vertical seismic profiling(VSP) survey was carried out at zero and offset-source positions with multicomponent receiver tools and multipolarized vibrators. The results will be integrated with downhole logs and regional seismic data to evaluate the effect of gas hydrate on seismic velocity and to estimate gas hydrate concentrations. The excellent data quality allows accurate compressional-and shear-velocity depth profiles. There are down-going and up-going waves from numerous reflectors, and corridor stacks provide comparison with surface multi-channel data. Velocities in the permafrost zone above 600 m are enhanced, to more than 2500 m/s. In the largely unfrozen section from 600 m to 850 m, the velocities are lower, about 2000 m/s. The gas hydrate zone is well defined below about 900 m, with velocities of 2500-2700 m/s. Poisson's Ratio is ∼0.39 in both the permafrost and gas hydrate sections, compared to -0.44 in the unfrozen sections.
60.
Volcanic deformation at Unzen, Japan, visualized by a time-differential stereoscopy
Journal of Volcanology and Geothermal Research,
89
(1-4)
73 – 80
1999
ISSN: 03770273Keywords:▾
Japan; crustal deformation; lava flow; stereo image; volcanic eruption
Abstract: ▾ Using a pair of old and new photographs taken at the same location, a method named as a time-differential stereoscopy is successfully applied to detect visually the deformation at Unzen volcano, Japan. On September 6, 1992, gradual movement of a lava lobe was visible in a stereo image as relief surrounded by immobile areas. In this case, accelerated movement of a lava block was detected immediately before the time of its collapse, which resulted in the occurrence of a pyroclastic flow. In November 1994, growth of a lava spine was also measured with time-differential stereoscopy. A possible fluctuation of the growth rate might have correlated with an abnormal oscillation of tilt, with a period of about 40 h. This convenient photographic method is recommended for various occasions to detect volcanic and any other deformations as a supplement to precise instrumental measurements.