The First Magnetotelluric Investigation of the Tawau Geothermal Prospect, Sabah, Malaysia

Abstract

The Tawau geothermal prospect is situated about 20 km northeast of the Tawau Town, Sabah, Malaysia. The geological condition of the area with active geothermal indications consists of Quaternary volcanic rocks (i.e. dacitic, andesitic to basaltic lava and tuff) of the Mt Maria and Mt Andrassy covering Tertiary sedimentary formations. The volcanic activity is caused by the extension of the Sulu Archipelago Arc-Trench System into the Semporna Peninsula, which was most active during the Pleistocene. The surface manifestations are steam-heated hot springs and young volcanic craters found in the vicinity of Mt Maria summit as an indication of up-flow zone and chloride hot springs in the lower level indicating direct connection to the reservoir. Subsurface temperature assessed from fluid geothermometry indicates the geothermal reservoir temperature beneath Mt Maria – Mt Andrassy region of about 190oC to 236oC.
The boundary of the prospect area, geometry of the reservoir and geothermal conceptual model was investigated using modern geophysical technique – i.e. Magnetotelluric (MT) and Time Domain Electromagnetic (TDEM) method. The MT and TDEM survey was focused in the central area of thermal manifestations. The total MT and TDEM soundings were 42 measurement points respectively, distributed on the gridding survey design with interval distance of 1200 meter. Remote reference MT sounding was also conducted to overcome the possible noise recorded in the MT data. Most of the MT data quality is excellent while the others are good. MT processing data was conducted following the standard procedure with emphasis on the noise removal and static shift correction using TDEM data.
Two-dimensional resistivity inversion was carried out along 15 lines crossing the survey area using the WinGLink Software. We developed interactive 3-D viewing software for further interpretation of the MT data. Results of this study show a conductive layer (< 15 ohm-m) down to about 1,000 meters distributed in the survey area. The conductive layer is interpreted as the clay cap of the geothermal system. The slightly resistive substratum (20 - 200 ohm-m) was also found underlining the clay cap which is interpreted as the reservoir zone. A more resistive dome shaped basement, distributed in the central area with continuation to the northern part of the survey area, is interpreted as hot rock. Based on this study, a conceptual geothermal model has also been developed for guiding the exploration drilling activities.