All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
59.
Water contents and hydrogen isotopic ratios of rocks and minerals from the 1991 eruption of Unzen volcano, Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
231 – 242
1999
ISSN: 03770273Keywords:▾
Japan; degassing; hydrogen isotope; igneous geochemistry; volcanic eruption; water content
Abstract: ▾ Water contents and hydrogen isotopic ratios were determined for blocks from pyroclastic flow deposits, and bread-crust bombs and blocks from the 1991 Vulcanian eruptions of Unzen volcano, Japan. Groundmass water contents and δD values of samples were calculated by subtracting the contribution of major hydrous minerals (hornblende and biotite) from the bulk rock analyses, and range from 0.1 to 0.5 wt.% and -83 to -49‰, respectively. The samples do not show a systematic H2O-δD relationship, although the block samples tend to have lower δD values than the bomb samples. The non-systematic H2O-δD relationship is likely a result of near surface, kinetically-controlled gas loss. High viscosity of this magma would hinder attainment of hydrogen isotopic equilibrium between exsolved vapor and melt in the final degassing stage. The near surface degassing, however, was accompanied by kinetic fractionation resulting in enrichment of deuterium in the final products as exemplified by bread-crust bombs with high H2O-low δD margins and low H2O-high δD cores. Relatively high δD values of the blocks and bombs as well as high temperature volcanic gas (-30 to -35‰) suggest a closed system degassing of an initial water-rich magma (H2O = 6 wt.%) until its water content was reduced to 0.5 wt.%. The pre-eruptive δD value (-46‰) was estimated from the volcanic gas data and D/H analysis of hornblende phenocrysts coupled with assumed isotopic equilibration in the initial hydrous magma.
58.
Well-log interpretation of gas-hydrate-bearing formations in the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
281-293
1999
ISSN: 00687626Abstract: ▾ Techniques for evaluating subsurface natural gas hydrate were part of the JNOC/GSC/J APEX joint research project. The physical properties of pure methane hydrate, related to well-log responses, were directly measured and/or calculated based on its physico-chemical properties. A petrophysical model of the pore-filling gas hydrate was built considering the existence of thermally dissociated free gas in the pores of the formation. Tool sensitivity to gas hydrate content was analyzed, and formation resistivity and acoustic transit time were found to show distinct sensitivity. Three practical methods for evaluating gas hydrate content were proposed and were tested to confirm their applicability: 1) the resistivity method, 2) the acoustic-velocity method, and 3) the statistical-inversion-analysis method. The porosity and gas hydrate saturation results calculated from these methods agreed quite well. Thus, reasonable interpretations can be achieved using these methods if the drilling and log measurements are carefully designed, and the zoning and parameter settings are made properly in pore-filling-type gas hydrate occurrences similar to those found in the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well.
57.
[English]
Vertical seismic profile in the mallik 2L-38 gas hydrate research well in the Canadian arctic
Publisher
Society of Exploration Geophysicists
1999
CREWES Project, University of Calgary, Canada; SEOS, University of Victoria, CGG Geophysics Canada, Canada; Pacific Geoscience Centre, Canada; Japan Petroleum Exploration Ltd., Japan56.
Occurrences and X-ray computerized tomography (CT) observations of natural gas hydrate, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
197-204
1999
ISSN: 00687626Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was drilled through a thick interbedded sequence of gas-hydrate-bearing sediments between 896 and 1106 m. In total, 37.3 m of core were collected between 886 and 952.6 m, using a variety of coring systems. Visual observations at the drill site identified a predominance of pore-space gas hydrate in varying concentrations within framework-supported sands and pebbly sands. Gas hydrate was mainly fine grained (<2 mm), filling the intergranular pores and/or coating mineral grains. Although rare, thin veins (1-2 mm) and clasts or nodules of gas hydrate (up to 0.5 mm) were also observed. The largest gas hydrate occurrence (2 cm in diameter) formed the matrix of a granular sand at 913 m. X-ray CT imagery, carried out in Japan, has identified eight well constrained signatures of the constituent components of the gas-hydrate-bearing sands and granular sands. These images have been used to establish textural characteristics and the relationship between sediment grains and gas hydrate.
55.
Micropaleontology of the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
69-75
1999
ISSN: 00687626Abstract: ▾ Core and cuttings samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well in the Mackenzie Delta have yielded sparse terrestrial microfossils and abundant reworked Cretaceous foraminifers (silicified) and plant microfossils. No definitely in situ marine microfossils were recovered in the borehole (total depth 1150m; gas hydrate at 896-1110m). Cores from 110-118 m and 173-175 m contained terrestrial microfossils including fungi, seeds, insect fragments, and abundant macerated plant fragments typical of the Pliocene-Pleistocene Iperk Sequence in the Mackenzie-Beaufort Basin. Core from 886-951 m and cuttings samples from 670-870 and 960-1140 m yielded reworked algal cysts, seed casings, and megaspores. In addition, cuttings contained reworked Cretaceous agglutinated foraminifers. Core and cuttings samples were also characterized by quartz, chert, brownish-black lignite, coaly fragments, and rare amber. The lithology of the section below 670 m is characteristic of the Oligocene Kugmallit Sequence in the Mackenzie-Beaufort Basin.
54.
Mount Dutton volcano, Alaska: Aleutian arc analog to Unzen volcano, Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
275 – 301
1999
ISSN: 03770273Keywords:▾
Japan; United States; Holocene; lava dome; magma chemistry; volcanic eruption
Abstract: ▾ Holocene eruptions from Mount Dutton, a small Late Quaternary volcano near the tip of the Alaska Peninsula, bear strong physical and petrologic similarities to the 1990-1995 Unzen Fugendake eruption in Japan. The volcano had a protracted phase of effusive calcalkaline andesitic (54-59 wt.% SiO2) cone-building in the late Pleistocene followed by an abrupt switch to more silicic (~ 65 wt.% SiO2) lavas, emplaced as a central summit cluster of steep-sided domes beginning in the early Holocene. The flanks of the volcano are mantled by pyroclastic flows, debris flows, and talus formed as a result of gravitational dome collapse. Disequilibrium mineral assemblages, including coexisting quartz and olivine in eruptive episodes ranging from the initial cone-building basaltic andesite lavas to the latest Holocene dacite domes, suggest extensive magma mixing. In addition, up to meter-sized, pillow-like cognate mafic enclaves of hornblende + plagioclase + glass are common in the latest of the summit dacite domes. Mineralogical evidence and bulk chemical data indicate the enclaves represent a high-alumina basalt parent with variable and subordinate reservoir contaminant, and the host lava is reservoir magma with variable and subordinate basaltic contaminant. Mount Dutton's history and petrology can be interpreted as reflecting the monotonous repetitive intrusion of mantle-derived mafic magma into a silicic crystal-rich crustal reservoir. During the Holocene, these injections resulted in the extrusion of partially crystallized, viscous, 'sticky' central domes which typically failed by collapse resulting in small volume Merapi-type flowage deposits. We speculate that slow introduction of mafic magma into the silicic chamber leads both to enclave formation and to the effusive eruption style. Mount Dutton volcano experienced severe shallow earthquake swarms in 1984, 1988, and to a lesser extent in 1991; although none of these swarms resulted in an eruption, their epicenter distribution and volcanic-tectonic character indicate they recorded the movement of magma at shallow depth beneath the volcano. The lessons of Unzen and Montserrat suggest that, despite limited tephra production, these small volcanic centers with central dome complexes present a serious hazard to life and property.
53.
Generation of block and ash flows during the 1990-1995 eruption of Unzen Volcano, Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
123 – 137
1999
ISSN: 03770273Keywords:▾
Japan; ash flow; lava dome; volcanic ash; volcanic eruption
Abstract: ▾ Processes generating block and ash flows by gravitational dome collapse (Merapi-type pyroclastic flow) were observed in detail during the 1990-1995 eruption of Unzen volcano, Japan. Two different types were identified by analysis of video records and observations during helicopter flights. Most of the block and ash flows erupted during the 1991-1993 exogenous dome growth stage initially involved crack propagation due to cooling and flowage of the dome lava lobes. The mass around the crack became unstable, locally decreasing in tensile strength. Finally, a slab separated from the lobe front, fragmented progressively from the base to the top within a few seconds, and became a block and ash flow. Rock falls immediately followed, in response to local instability of the lobe front. Clasts in these rock falls fragmented and merged with the preceding flow. In contrast, block and ash flows during the endogenous dome growth stage in 1994 were generated due to local bulge of the dome. Unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows.
52.
A new regime of volcanic eruption due to the relative motion between liquid and gas
Journal of Volcanology and Geothermal Research,
89
(1-4)
303 – 315
1999
ISSN: 03770273Keywords:▾
explosive volcanism; magmatism; two phase flow; volcanic eruption
Abstract: ▾ In explosive magma eruptions, magma ascends through a conduit as a Poiseuille flow at depth, and gas exsolves gradually and expands as the pressure decreases (bubbly flow regime). When the volume fraction of gas becomes sufficiently large, liquid or solid parts of magma fragment into droplets or ashes, and the flow dynamics becomes governed by the gas phase (gas-ash flow regime). We propose a new flow regime, which we call fractured-turbulent flow regime, between the bubbly flow regime and the gas-ash flow regime. In the new regime, both liquid magma and gas are continuous phases. The high connectivity of the two phases allows the relative velocity between them to increase significantly. We present one sample calculation, which displays basically explosive characteristics, but has three features distinct from previous models. The explosive characteristics are manifested as the fragmentation of the magma and the high speed jet that issues from the vent. The first distinct feature is a nearly lithostatic pressure distribution, which results from the increase of the height of the fragmentation surface. The second one is the atmospheric pressure at the vent; the flow is not choked. The third one is that the relative velocity between the gas and the ash is large at the vent despite the large interaction force between the two phases. The large relative velocity is established in the fractured-turbulent regime, and is maintained in the subsequent gas-ash flow regime.
51.
Microbial communities from core intervals, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
189-195
1999
ISSN: 00687626Abstract: ▾ Microbial cell distribution in sediment core samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well on the Mackenzie Delta (Canada) was studied using acridine orange direct counts of stained cells for total cell estimation, and by most probable number statistical enumeration for culturable methanogens. The purpose was to characterize the microbial communities in gas-hydrate-bearing sediments. Results indicated that the total cell count values were in the range of 1.1 × 105 cells/g to 2.8× 106 cells/g with culturable methanogens present at 1×10-4% to 1.0% of those values. These results also indicated that culturable methanogens may be more numerous in the porous sandy strata of the Mackenzie Bay Sequence than in clay and silt units of the Kugmallit Sequence. These data expand the known distribution of methanogens in deep sediments and establish the presence of microbial communities in subpermafrost environments.
50.
Chemical and physical analysis of natural gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
251-262
1999
ISSN: 00687626Abstract: ▾ Gas hydrate samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well were analyzed on both macroscopic and molecular scales using several complementary experimental techniques. These included gas volume analysis, thermogravimetric analysis, precision gas analysis, powder X-ray diffraction, differential scanning calorimetry, Fourier transform infrared spectroscopy, and Raman Spectroscopy. Powder X-ray diffraction indicated that the samples were Structure I gas hydrate. Enclathrated gas species were identified to be mostly methane (98-100%); however, some samples contained significant amounts of heterogeneously dispersed propane and carbon dioxide (at least 1.5-2.0%). These samples were found to be significantly more stable than samples containing methane only. In addition, Raman spectra indicate subtle variations in the cage occupancies of the mixed gas hydrate as compared to those in pure methane hydrate.
49.
Clay minerals in the sediments of Lake Baikal: A useful climate proxy
Journal of Sedimentary Research,
69
(3)
588 – 596
1999
ISSN: 15271404Keywords:▾
Russian Federation; clay mineral; proxy climate record; stratigraphic correlation
Abstract: ▾ Lake Baikal, in south-central Siberia, has been the focus of an international effort (the Baikal Drilling Project; BDP) to obtain continuous long cores (upwards of 100 m) from this unique rift-valley lake and to interpret the paleoclimatic history from various proxy data. As part of this effort, the clay minerals were examined by two research teams. A consistent clay-mineral assemblage, containing illite, inter-stratified illite-smectite, chlorite, and kaolinite as the major minerals, characterizes much of the modern sediments. The relative abundance of these minerals changes with depth in both short piston cores from various parts of the lake and in 100-m-long cores taken from the distal toe of the Selenga Delta (BDP-93). Independent analyses of the abundance changes and correlation with other data from the cores show that the clays are responding to two influences: (1) climatic fluctuations, particularly in the upper 40 m of the sedimentary record, which show the relative amount of illite-smectite (and sometimes kaolinite) increasing during warmer climate episodes, and (2) source-area changes, which are most evident below 40 m in the BDP-93 cores and mark a shift from an eastern (Buguldeika River) to a western (Selenga River) source. The clay-mineral-based climate fluctuations are correlative with the marine oxygen-isotope record through stage 7, and provide a relatively simple and cost-effective tool for gaining insight into the paleoclimate of this interior continental site.
48.
Co-ignimbrite ash-fall deposits of the 1991 eruptions of Fugen-dake, Unzen Volcano, Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
95 – 112
1999
ISSN: 03770273Keywords:▾
Japan; ash flow; ignimbrite; pyroclastic flow; volcanic ash; volcanic eruption
Abstract: ▾ Fugen-dake, the main peak of Unzen Volcano, began a new eruption sequence on November 17, 1990. On May 20, 1991, a new lava dome appeared near the eastern edge of the Fugen-dake summit. Small-scale, 104-106 m3 in volume, Merapi-type block and ash flows were frequently generated from the growing lava dome during May-June, 1991. These pyroclastic flows were accompanied by co-ignimbrite ash plumes that deposited ash-fall deposits downwind of the volcano. Three examples of co-ignimbrite ash-fall deposits from Unzen pyroclastic flows are described. The volume of fall deposits was estimated to be about 30% by volume of the collapsed portions of the dome that formed pyroclastic flows. This proportion is smaller than that described for other larger co-ignimbrite ash-fall deposits from other volcanoes. Grain size distributions of the Unzen co-ignimbrite ash-fall deposits are bi-modal or tri-modal. Most ashes are finer than 4 phi and two modes were observed at around 4-7 phi and 9 phi. They are composed mainly of groundmass fragments. Fractions of another mode at around 2 phi are rich in crystals derived from dome lava. Some of the fine ash component fell as accretionary lapilli from the co-ignimbrite ash cloud indicating either moisture or electrostatic aggregation. We believe that the co-ignimbrite ash of Unzen block and ash flows were formed by the mechanical fracturing of the cooling lava blocks as they collapsed and moved down the slope. These ashes were entrained into the convective plumes generated off the tops of the moving flows.
47.
Crustal deformation in the mid-May 1991 crisis preceding the extrusion of a dacite lava dome at Unzen volcano, Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
43 – 55
1999
ISSN: 03770273Keywords:▾
Japan; crustal deformation; dacite; lava dome; tilt; volcanic eruption
Abstract: ▾ Tilt observations at FG1 station located about 680 m west of the central crater of Unzen volcano, southwestern Japan, revealed details about the process of magma ascent during the mid-May 1991 crisis, just before the appearance of a lava dome. Crustal deformations measured during this period can be interpreted by a combination of upward growth of a buried magma column and lateral intrusion of a dike, modeled by a vertical line source and a planer tensile dislocation in an elastic half space. Marked change in tilt was observed at first in the EW direction at FG1 during May 11-14, suggesting the ascent of a magma column with a diameter of about 40 m from about 300 m to 160 m in depth beneath the central crater. After the interruption of the rise of the magma column, magma began to intrude laterally as a dike on May 15 toward approximately the N80°W direction. This resulted in a large tilt change essentially in the NS component at FG1. The horizontal extent of the dike might have exceeded 400 m in a few days with a thickness of about 6 m. On May 17, the magma column started to rise again, probably because the impediment to magma ascent was fractured by the growth of the dike. The top of the magma column finally reached to the crater bottom probably on May 19, and a new lava dome was actually observed on May 20. After which, the remarkable upward tilt to the south at FG1 due to the dike intrusion decreased rapidly. The deformation after May 20 can be interpreted by a gradual increase in the thickness of the dike up to 13 m by the end of May 1991. Although large errors are inevitable in estimated values because of a lack of sufficient data, the present results confirm the importance of making continuous observations of crustal deformation near the summit areas of active volcanoes in order to clarify the underground processes of magma movement leading to eruptions.
46.
Detailed evaluation of gas hydrate reservoir properties using JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well downhole well-log displays
Bulletin of the Geological Survey of Canada
(544)
295-311
1999
ISSN: 00687626Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well project was designed to investigate the occurrence of in situ natural gas hydrate in the Mallik area of the Mackenzie Delta of Canada. Because gas hydrate is unstable at surface pressure and temperature conditions, a major emphasis was placed on the downhole logging program to determine the in situ physical properties of the gas-hydrate-bearing sediments. Downhole logging tool strings deployed in the Mallik 2L-38 well included the Schlumberger Platform Express with a high resolution laterolog, Array Induction Imager Tool, Dipole Shear Sonic Imager, and a Fullbore Formation Microlmager. The downhole log data obtained from the log- and core-inferred gas-hydrate-bearing sedimentary interval (897.25-1109.5 m log depth) in the Mallik 2L-38 well is depicted in a series of well displays. Also shown are numerous reservoir parameters, including gas hydrate saturation and sediment porosity log traces, calculated from available downhole well-log and core data. The gas hydrate accumulation delineated by the Mallik 2L-38 well has been determined to contain as much as 4.15109 m3 of gas in the 1 km2 area surrounding the drill site.
45.
Dinoflagellate cysts from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
77-80
1999
ISSN: 00687626Abstract: ▾ Sparse occurrences of organic-walled dinoflagellate cysts were recorded from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, Mackenzie Delta area, Northwest Territories, Canada. Some of the recorded taxa are indicative of a Paleocene-Eocene age, while others are considered to be of Cretaceous origin. Because the deepest parts of the section are correlated to the Oligocene Kugmallit Sequence, all the dinoflagellate cysts are interpreted to be reworked. According to this interpretation, the total absence of in situ marine dinoflagellate cysts suggests that the studied samples were deposited under nonmarine conditions, most likely within a fluvial system.
44.
Dissociation properties of natural gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well by X-ray computerized tomography (CT) experiments
Bulletin of the Geological Survey of Canada
(544)
269-279
1999
ISSN: 00687626Abstract: ▾ Core samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well contained a variety of forms of gas hydrate within sands and granular sands in the interval from 896 to 926 m. A number of these samples were placed inside pressure vessels charged with nitrogen gas and subsequently transported to Japan for specialized dissociation experiments. X-Ray CT images were acquired, at constant intervals, from a granular sand (collected from 913.76 m) containing pore-space gas hydrate during warming from -35 to 4°C, and subsequently during stepped decreases in cell pressures (0.1 MPa) below assumed threshold stability conditions. Dissociated gas flow and sample temperatures were monitored continuously. Changes in CT values indicated that gas hydrate dissociated simultaneously, both on the exposed surfaces and within the pore spaces of the sample, in response to pressure changes. This suggested that pressure reductions were effectively transmitted through the sample, most likely because the samples were not fully saturated with gas hydrate. Gas-flow measurements indicated that a larger pressure drawdown caused a higher dissociation rate.
43.
Drilling operations, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
19-30
1999
ISSN: 00687626Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, located in the Mackenzie Delta, Northwest Territories, Canada, was completed to 1150 m on March 30, 1998, after 39 days. Operations were undertaken through a collaborative agreement between the Japan National Oil Corporation and the Geological Survey of Canada. Research goals included evaluation of engineering technologies used to drill and core gas-hydrate-bearing strata. Eight coring runs were conducted within the permafrost interval (0-640 m) in a surface hole drilled to 687 m. Subsequently, a 340 mm surface casing was installed and the main hole was advanced to a depth of 1150 m with 16 coring runs. A cooled (∼2°C) KCl/polymer drilling mud and Drilltreat, a chemical mud additive, successfully stabilized gas hydrate within cores and formation sediments. No serious hole problems, accidents, or mishaps occurred; however, delays caused by adverse weather and mechanical problems caused cancellation of planned production testing. Coring in the main hole was successful, allowing the evaluation of four different core barrels. Gas-hydrate-bearing cores were collected in a variety of sediments between 896 and 952 m.
42.
Estimating in situ gas hydrate saturation from core temperature observations, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
101-108
1999
ISSN: 00687626Abstract: ▾ During drilling of the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, core temperatures were measured immediately upon recovery in the core-logging trailer. Gas-hydrate-bearing cores were typically frozen, with temperatures as much as 6°C lower than cores containing no gas hydrate. This temperature depression is attributed to the endothermic dissociation of gas hydrate during uphole tripping, and can be used to estimate minimum in situ gas hydrate saturation. Numerical modelling of heat exchange between core and circulating mud during tripping demonstrates that cores cool to mud temperature before leaving the methane hydrate P-T stability field. Simple arguments support the hypothesis that the endothermic heat of gas hydrate dissociation is supplied largely by the release of latent heat during coincident freezing of pore waters. Assuming minimal heat exchange with circulating mud, energy-balance calculations yield estimates of the quantity of gas hydrate lost to dissociation during recovery. These estimates are comparable to the in situ gas hydrate concentrations inferred from downhole geophysical logs.
41.
Gas composition and isotopic geochemistry of cuttings, core, and gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
143-163
1999
ISSN: 00687626Abstract: ▾ Molecular and isotopic composition of gases from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well demonstrate that the in situ gases can be divided into three zones composed of mixtures of microbial and thermogenic gases. Sediments penetrated by the well are thermally immature; thus the sediments are probably not a source of thermogenic gas. Thermogenic gas likely migrated from depths below 5000 m. Higher concentrations of gas within and beneath the gas hydrate zone suggest that gas hydrate is a partial barrier to gas migration. Gas hydrate accumulations occur wholly within zone 3, below the base of permafrost. The gas in gas hydrate resembles, in part, the thermogenic gas in surrounding sediments and gas desorbed from lignite. Gas hydrate composition implies that the primary gas hydrate form is Structure I. However, Structure II stabilizing gases are more concentrated and isotopically partitioned in gas hydrate relative to the sediment hosting the gas hydrate, implying that Structure II gas hydrate may be present in small quantities.
40.
Amount of gas hydrate estimated from compressional- and shear-wave velocities at the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
313-322
1999
ISSN: 00687626Abstract: ▾ The amount of in situ gas hydrate concentrated in the sediment pore space at the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was estimated by using compressional-wave (P-wave) and shear-wave (S-wave) downhole log measurements. A weighted equation developed for relating the amount of gas hydrate concentrated in the pore space of unconsolidated sediments to the increase of seismic velocities was applied to the acoustic logs with porosities derived from the formation density log. A weight of 1.56 (W=1.56) and the exponent of 1 (n=1) provided consistent estimates of gas hydrate concentration from the S-wave and the P-wave logs. Gas hydrate concentration is as much as 80% in the pore spaces, and the average gas hydrate concentration within the gas-hydrate-bearing section from 897 m to 1110 m (excluding zones where there is no gas hydrate) was calculated at 39.0% when using P-wave data and 37.8% when using S-wave data.
39.
Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
81 – 94
1999
ISSN: 03770273Keywords:▾
Japan; historical perspective; lava dome; pyroclastic flow; volcanic eruption
Abstract: ▾ During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200-300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400-500 ka. The volcanic rocks of the Older Unzen exceed 120 km3 in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100-70 ka, 30-20 ka, and <20 ka, respectively. Mayuyama volcano formed huge lava domes on the eastern flank of the Unzen composite volcano about 4000 years ago. Total eruptive volume of the Younger Unzen volcano is about 8 km3, and the eruptive production rate is one order of magnitude smaller than that of the Older Unzen volcano.
38.
Isotope constraints on the origin of pore waters and salinity in the permafrost and gas hydrate core intervals of the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
177-188
1999
ISSN: 00687626Abstract: ▾ Pore waters have been extracted from sediments in the permafrost interval (110-176 m) and the gas hydrate interval (886-952 m) of the JAPEX/JNOC/GSC Mallik 2L-38 drill core and analyzed for δ18O,δ2H, and geochemistry. Pore waters from the permafrost interval have δ18O values of -19.5 ± 0.5‰ (upper permafrost) and -23.1 ± 1.0‰ (lower permafrost) indicating the likely origin to be local, contemporary meteoric waters infiltrating these sediments during a period of subaerial exposure. Pore waters in the subpermafrost gas hydrate zone are isotopically depleted from seawater values, with δ18O ranging between -14‰ and -8‰c. A weak correlation between δ18O and Cl- exists in the gas-hydrate-bearing sands, consistent with the combined effect of isotopic depletion during gas hydrate formation, and enrichment associated with gas hydrate decomposition. The upper silt and deeper clayey silt sections also retain a minor correlation between isotopes and Cl-, and show strong variability in both δ18O and Cl- with depth, suggesting a history of gas hydrate formation, decomposition, and fluid migration. The Cl--δ18O relationships demonstrate that the original pore waters are a mixture of seawater with greater than 50% meteoric water.
37.
Methane and carbon dioxide gas-generation kinetics, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
125-141
1999
ISSN: 00687626Abstract: ▾ Optimized, first-order, discrete Arrhenius kinetic parameters have been determined for the thermogenic generation of methane and carbon dioxide for several low-maturity, organic-rich core samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. Pyrolysis was carried out using a thermogravimetric analyzer heated at 10, 25, and 50°C/min. The specific products were detected using a directly coupled Fourier Transform Infrared spectrometer. Results indicated that at typical geological heating rates of 3°C/Ma, significant (about 10% of the total) thermogenic carbon dioxide was released at very low temperatures (<60°C) and would be coproduced with microbiologically mediated, diagenetic carbon dioxide. At the same geological heating rate, the first 10% of thermogenic methane was determined to have been released between about 110 and 140°C while significant methane generation from the kerogen continued beyond 250°C. The absolute kinetic parameters for methane indicate that below about 60°C essentially no thermogenic methane should be expected. Thus no in situ thermogenic methane should be expected in the Mallik 2L-38 well.
36.
Magma ascent beneath Unzen Volcano, SW Japan, deduced from the electrical resistivity structure
Journal of Volcanology and Geothermal Research,
89
(1-4)
35 – 42
1999
ISSN: 03770273Keywords:▾
Japan; magmatism; phreatomagmatism; resistivity; volcanic eruption
Abstract: ▾ The resistivity structure of Unzen Volcano has been revealed by extensive magnetotelluric surveys since the first eruption on November 17, 1990. This structure comprises a highly resistive surface layer, a low-resistive second layer at several hundred meters depth, interpreted as a water-saturated layer, a resistive third layer, and a low-resistive fourth layer at 10 km depth, possibly related to the deep magmatic activity. The structure has influenced the volcanic activity of Unzen. This activity was characterized by a series of dramatic changes in eruption type: a minor phreatic eruption on November 17, 1990; phreatic eruptions after February 12, 1991, preceded by several weeks of volcanic tremor; phreatomagmatic eruptions after April 9, and dome effusion beginning May 19, 1991. This paper presents a hypothesis in which the top of the magma column rose about 20 m/day, reached the base of the water-saturated layer at the end of January, 1991, and approached the upper boundary of this layer on April 9. Thus, the temporal change of eruption type and associated phenomena are systematically explained by an interaction between magma and groundwater contained in the saturated layer.
35.
Long Valley Coring Project, Inyo County, California, 1998, preliminary stratigraphy and images of recovered core
US Geological Survey,
99-158378
1999
DOI:10.3133/ofr99158