Geostatistical Analysis of Geothermal Temperatures in Yanaidzu-Nishiyama Geothermal Field, Northeastern Japan

Abstract

Spatial continuity of geothermal temperatures has been analyzed by variograms using temperature data of 35 wells drilled in Yanaidzu-Nishiyama geothermal field, northeastern Japan. The trend that temperature increases with increasing depth is approximated by T(z)=T₀+b·In(z-z₀), where T(z) is temperature at elevation z, and b, z₀, T₀ are constants. In order to obtain temperature deviations from the trend, the function is applied to each well in the individually regressed case (IRC), or to all wells in the totally regressed case (TRC). The analysis gives the following conclusions : 1) 2-D variograms of measured temperatures, and both temperature deviations show patterns caused by the data point distribution ; 2) a vertical variogram of measured temperatures diverges, and do not show a sill ; 3) temperature deviations in IRC seem to be completely free from the trend, and their 3-D variogram is almost isotropic, though ranges varies between 200 and 70 m (the longest and shortest directions are vertical and horizontally W-E, respectively) ; 4) variograms of temperature deviations in TRC show the intermediate character between measured temperatures and temperature deviations in IRC, and hence it is not clear that temperature deviations in TRC are free from the general trend ; 5) a horizontal 2-D variogram in TRC was almost the same as that of measured temperatures (γ=0.9999), and ranges toward N40∼50°W and N10∼40°E directions are extremely long ; 6) N40∼50°W which is one of the directions having long ranges coincides with the strike of faults playing a pathway of hydrothermal fluids ; 7) wells are hoped to be distributed randomly, and be inclined toward many directions.