Borehole geology and hydrothermal alteration of well NJ-20, Nesjavellir high-temperature area, SW-Iceland

Abstract

Well NJ-20, drilled in 1999, is located in the Nesjavellir high-temperature field in SW-Iceland. It was drilled as a discharge well to utilize high pressure for electric power generation. Directional drilling was performed to minimize environmental impacts and to gain closer access to the main upflow. The lithology consists predominantly of volcanic rocks. The upper part (from top to 730 m) is mostly composed of tuff, breccia, glass and glassy basalt, which are called hyaloclastites. Several basaltic intrusions have been intruded into the hyaloclastites. In the deeper part (730-1180 m) are predominantly basaltic lavas. According to the alteration mineral distribution, four alteration zones were found, smectite-zeolite zone (0-360 m), mixed layer clay zone (360-820 m), chlorite-epidote zone (820-940 and 1010-1180 m) and epidote-actinolite zone (940-1010 m). Their relevant temperature ranges are <200 C, 200-230 C, 230-280 C and >280°C, respectively. Alteration mineral study results indicate an increasing temperature regime from the top to 1010 m, but slightly decreased below there. According to the temperature log, heat increased below 1180 m. In this well, 15 aquifers were identified according to circulation loss or gain, pressure drops on pumps and temperature logs.