Travertine Deposits Indicated of a Medium Enthalpy for a Geothermal Reservoir Beneath Dolok Marawa Area, Simalungun, North Sumatera Province - Indonesia

Abstract

The Dolok Marawa geothermal area is located on the northern part of Sumatera Island, North Sumatera Province. The geothermal manifestation here is reflected by hot springs in an elevation between + 330-370 m asl, along the northwest-southeast trending faults (N 320-330º E). Geothermal manifestations here are covered by volcanic, and sedimentary rocks, and also underlain by Miosen limestone basement. The surface temperature varies between 36.4-66.5º C and has neutral pH (6.57-7, 63). The surface deposit includes travertine deposits of carbonate sinter in an area nearby hot springs.
According to the geochemical fluids analyzed, the Partulatula, Panggaruan and Lakparan hot waters have a high concentration of chloride, the Tinggi Raja, Balakbak, and Bahoan hot waters are situated in the border between chloride and bicarbonate waters, and the Bahbotala hot water is a bicarbonate waters. All hot waters are immature waters.
The estimation of water temperature in the fluid reservoir beneath the area is 180º C (medium enthalpy). An abundant volume of travertine deposits in a nearby surface manifestation characterizes this reservoir. The travertine is also indicative of lime stone basement rocks beneath the area. Than lime stone is strongly disolved by hot fluids at temperature below 180 C
The hydrogen and oxygen isotope ratios of thermal spring waters (18O Vs Deuterium) for the Tinggi Raja hot spring show that fluids have depleted 18O content and lie on the meteoric water line (MWL), but for the Bahoan, Balakbak and Lakparan hot springs, waters are enriched in 18O content. This means tthat the hot fluids are presumably derived directly from a medium enthalpy geothermal reservoir beneath the area with a temperature of about 180º C.