Borehole geology of well ThG-16 in Theistareykir georhermal field, Iceland

Abstract

Well ThG-16 in Theistareykir geothermal field was drilled in 2017 and provides geological information from drill cuttings and logging data. The data is analysed in an integrated borehole geology study using multiple methods, such as binocular, petrography, X-ray diffractometry, the methylene blue dye adsorption test, and well logging interpretation. The objective of this study is to describe and interpret geological conditions in well ThG-16. The stratigraphy of the well is mainly composed of various hyaloclastite formations in the uppermost 1478 m. Between 1478 and 2290 m, lava flows are dominant and from 2290 m to the bottom of the well at 2702 m a coarse-grained intrusion zone is observed with dolerite and intermediate intrusions. The alteration zone is divided into five zones. From the surface to 80 m is an unaltered zone, followed by a smectite zone (80 – 170 m), a MLC zone (170-476 m), an epidote chlorite zone (476-1394 m), and an epidote chlorite actinolite zone (1394-2700 m). Three main feed points are observed in the well that are located at 2550-2590 m, 1980-2000 m, and 1450-1480 m. During drilling of well ThG-16 total loss of circulation was not experienced. It is suggested that the reservoir rock in the well has low permeability which is confirmed by the small size of fractures found in the televiewer log, the decreasing of good permeability mineral indicators as pyrite and calcite in the deeper part, and the long heating up period of the well after drilling