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Correlation of micro fractures with P-wave velocities and strain in cored rock samples from the Yutsubo geothermal well deformed under confinig pressures
Ziqiu Xue, Osamu Nishizawa, Yasuto Kuwahara, and Shigeyuki Suzuki
1996
vol. 47 (12), p. 599-617, 25 figs.
Keywords: Yutsubo, P-wave velocity, strain, antisotropy, geothermal well
Abstract: P-wave velocity and strain were measured under confinig pressures up to 100 MPa in four cored rock samples recovered from the two geothermal wells (YT-1 and YT-2), in the Yutsubo area of Oita Prefecture. Samples are octagonal prisms with long axes parallel to the vertical directions of the wells. Lengths of diagonals of octagons are 70-80mm and the heights are 90-100mm, large enough for evaluating effects of macro-cracks. P-wave velocity and strain were measured in the vertical and three horizontal planes (45 degrees apart ), with strain gauges attached at each surface of the prisms.
@A sample YT-1-4 from YT-1(898m) exhibited inhomogeneous deformation, which suggests strong effects of macro-cracks. Anisotropies in velocity and strain between vertical and horizontal planes were observed in two samples (YT-2-4 and YT-1-8) from YT-2 (1212m) and YT-1(1696m), respectively. For investigating changes of microstructure in rock samples after measurements, blue dye epoxy-impregnated thin sections were prepared. New fractures appear along precipitated minerals in pre-existing fractures or pre-existing cracks. High-pressure clastation and new crevice formation around alternated grains may have been produced by the large unrecoverable strains observed in the sample YT-1-4.
Geochemical study on the origin of natural gas hydrocarbons accumulated in volcanic reservoir
Susumu Sakata, Makoto Takahashi, Shun-ichiro Igari, Yukihiro Matsuhisa and Kazuo Hoshino
1996
vol.47 (12), p. 619-642, 18 figs., 7 tables.
Keywords : geochemistry, natural gas, volcanic reservoir, sedimentary reservoir, biogenic origin, abiogenic origin, chemical composition , isotopic composotion, hydrocarbon, helium, northeast Japan, Neogene
Abastract : Natural gases accumulated in volcanic (VR) and sedimentary reservoirs (SR) within a Neogene sedimentary basin in northeast Japan were measured for chemical(C1 to C4 hydrocarbons, carbon dioxide, nitrogen, and helium) and isotopic compositions (3He/4He and 13 C of methane, ethane, propane, and n-butane) to investigate their origin, with special emphasis on a possible biogenic origin for the hydrocarbons in the VR gases.
@In all of the samples irrespective of their reservoir type, the major components are hydrocarbons, accounting for more than 90%. The concentrations of methane to butane show similar range in both the VR and SR gases. VR gases are not necessarily enriched in carbon dioxide relative to the SR gases, which may be used to argue against an abiogenic hypothesis that methane in the VR gases was formed via the reduction of magmatic carbon dioxide.
@The 3He/4He ratios of the VR gases are distinctly higher than those of the SR gases. The maximum value of 7.8 RA is equivalent to that of MORB-type helium, suggesting that the helium in the VR gase is mostly mantle-derived. However, the concentration and isotopic data of the helium in the VR and SR gases can be consistently explained by a mixing model between two end-members : biogenic methane (CH4/3He=2~1013 , 3He/4He=0.02 RA) and mantle helium (CH4/3He=0, 3He/4He=8 RA), which does not assume the presence of magmatic methane.
@The carbon isotopic data of methane (13 C1)in the VR gases scatter wihtin the range of those of the SR gases. In a plot of C1/(C2+C3)[=methane /(ethane+propane)] vs.
13 C1 for the genetic characterization of natural gas, VR gases fall into the field of either thermogenic origin or a mixture of thermogenic and microbial origin. There was another trend observed in the VR and SR gases in that 13 C1 rises and C1/(C2+C3) lowers with increasing depth of the gas horizon, which is consistent with a model of mixing between microbial gas of shallow origin and thermogenic gas of a deep origin.
@The carbon isotopic relationship in methane,ethane, propane, and n-butane can be represented by 13 C1 <13C2 <13C3 <13C4, which may be used to argue against a scenario that C2+hydrocarbons in the VRgaseswere formed via the polymerization of abiogenic methane (Gold and Soter, 1982). A remarkable correlation between 13C413C3 and 13C3 -13C2 was observed in the samples, which reflects an isotope effect in the reaction of thermal cracking of high molecular weight hydrocarbons like petroleum. The 13C213C1 values measured for the gases are similar to, or lower than those expected from the same isotope effect. This can be explained again by assuming an addition of variable amounts of microbial methane to thermogenic hydrocarbons. The abiogenic methanes known so far are enriched in 13C relative to regular thermogenic methanes, thus our data supports the argument against a significant contribution of such methane to the VR gases.
Abstract for Geological Survey Seminar, no. 242. -Methane hydrate, -A natural gas resource in future-
1996
vol. 47, (12), p. 643-646.
