Fracture systematics in and around well WD-1, Kakkonda geothermal field, Japan

Interpretation of FMI (Schlumberger's Fullbore Formation MicroImager) logs from well WD-1 and observation of spot cores from well WD-1 in the Kakkonda geothermal field have provided important clues about the nature and config uration of the fracture networks controlling circulation of high-temperatur e hydrothermal fluid. The fractures comprise four groups: (1) fractures str iking NW, (2) high-angle fractures striking NE to E, (3) low-angle fracture s striking N, and (4) low-angle fractures striking E. Most permeable fractu res in the shallow (< 1500 m depth) and deep reservoirs are of group 3, and likely reflect reverse faulting in the modern stress field. Low-angle, gro up 3 fractures are also permeable, and may have formed in the stress field that existed prior to 0.2 +/- 0.1 Ma. Distributions and densities of fractu res are comparable in the shallow and deep reservoirs; the temperature diff erence between the reservoirs is independent of these variables. (C) 1998 C NR. Published by Elsevier Science Ltd. All rights reserved.