Study of reservoir characteristics: pH, dissolved and precipitate minerals in Atadei geothermal field, Lembata Regency, East Nusa Tenggara

Abstract

Based on consultant X’s reports (2020), there are 24 surface manifestations found in the Atadei geothermal field including fumaroles/steam vents with temperatures ranging from 80°C - 98°C, hot springs with temperatures around 34 - 39.6°C, cold springs and altered rocks. Exploration wells AT-1 and AT-2 were drilled by DISDM in 2004, but no steam or water was found at depth. To determine the reservoir characteristics, geochemical modeling with PHREEQC was conducted. From the saturation index modeling, it is known that the hot and cold springs in the study area are entirely oversaturated with silica minerals (quartz and chalcedony). This is presumably because the host rock is andesitic lava rock with a silica content of 55 - 65%. In addition, most of the springs are also oversaturated with carbonate minerals (aragonite and calcite) but have not yet formed deposits around the manifestations due to the condition of the springs which also predominantly contain silica minerals. Based on geochemical modeling with forward modeling, it is known that the reservoir characteristics in the study area in the temperature range of 180 - 240°C have an acidic pH ranging from 4.64 - 5.18. This is supported by the composite log data of the AT-1 well where in the depth interval of 608 - 830.5 m, the reservoir is suspected to be a chloride sulfate type with a pH that tends to be low (acidic) characterized by the presence of acidic alteration minerals, namely pyrophyllite and dickite (Reyes, 1990).