Borehole geology of well Fiale 3, Asal-Fiale geothermal field, Djibouti

Abstract

The geothermal well Fiale-3 is in the Asal-Fiale geothermal field in the northwestern part of the Fiale caldera. The well is directionally drilled to a total measured depth of 2660 m in a N224° direction. To provide information about the stratigraphy, thermal conditions, and a detailed description of the geothermal reservoir characteristics of the well, binocular microscopy, thin section petrography, XRD analyses and methylene blue analyses have been carried out. The lithostratigraphy of Fiale-3 is formed by four main stratigraphic units, which are the Asal series, the Afar stratoid series, the Dalha basalt series and the Mabla rhyolite. Several alteration minerals haven been observed with progressive depth and the occurrence of hydrothermal alteration minerals is related to their formation temperatures such as smectite (≥40°C), quartz (≥180°C), chlorite (≥220°C), epidote (≥230°C), and actinolite (≥280°C). Further, anhydrite has also been observed from 700 m onwards. Anhydrite as a sulphate mineral is a good indicator for seawater infiltration. In total, five alteration zones can be classified. These are an unaltered zone (0-145 m), a smectite zone (145-550 m), a chlorite zone (550-792 m), an epidote zone (792-1283 m), and an epidote-actinolite zone (1283-2660 m). However, the observed alteration pattern does not present the thermal conditions today.

Measured temperatures in the well from downhole logging indicate a temperature reversal between 733 and 1700 m measured depth (MD) due to a large inflow of cold seawater through the NW-SE major faults of the rift. The absence of low-temperature minerals in the reversal zone may indicate that the geothermal system has not been equilibrated. This may indicate that the cooling is relatively recent. Permeable zones are identified from circulation losses and temperature logs. The largest feed zone at 733 m (MD) is mainly controlled by the rift faults which led to the inflow of cold seawater into the system. In general, the well shows a relative low permeability in the shallow and deep reservoir. The alteration and temperature models of the Asal-Fiale geothermal field provide no evidence for the existence of an upflow.