Geology and hydrothermal alteration in wells TD-5 and TD-6, Tendaho geothermal field, Ethiopia

Abstract

Tendaho geothermal field is one of the two geothermal prospect areas in Ethiopia which have undergone deep and shallow exploratory drilling. The prospect area is located in the Tendaho graben in the inner part of the Afar depression. This report deals with the borehole geology of the shallow wells TD-5 and TD-6. The main purpose of the two wells are to obtain further information on the extent and characteristics of the shallow reservoir and to study the possibility of utilization for electric generation. The formation dissected by the drillholes consist of sedimentary sequence (fine-medium grained sandstone, siltstone and clay) interlayered by thin basaltic lavas in the upper part of the wells. The permeability of the system is apparently controlled by the granularity of the sediment and the boundaries of the contact zones as well as the tectonic structures in the area. The hydrothermal mineralogy assemblage consists predominantly of clays calcite, quartz, stilibite, laumontite, wairakite, prehnite and epidote. Based on the distribution of the alteration minerals, the wells showed zones from low to high temperature zeolites and clay minerals concomitant with the increase of temperature down to the bottom of the wells. These appear in most cases to be in equilibrium between the measured temperature and alteration.