Hydrogeochemical study of the Ungaran Geothermal System, Central Java, Indonesia

PHUONG, Nguyen Kim, Dr.Ir. Heru Hendrayana

2005 | Tesis | S2 Teknik Geologi

The Ungaran geothermal prospect is located in the Central Java province, about 30 km southwest of Semarang. The Gedongsongo is the main geothermal resource, associated with the Upper stratovolcanic system of Ungaran volcano. The stratovolcano consists of a series of andesitic to basaltic lava and breccias with occasional interbedded tuff. This formation is overlying the Tertiary marine sediment formation. The pre-caldera volcanic rocks and the Tertiary marine sedimentary rocks are inferred to be the main reservoirs rocks. The permeable features are dominated by faults, but some primary permeable features are the interconnected vugs or gas bubbles, and the contact between two lithologies with different physical characteristic. The main steam zone is capped by a steam-heated layer. The water in the upper part of this layer is steam condensation of acid sulphate composition, and the water in the lower part is of near to neutral pH. Due to water chemistry analysis, type of water in Gedongsongo area is bicarbonate water presented a mixing process between geothermal and shallow groundwater. Only water in Kaliulo has a very high Na and Cl concentration (more than 4000ppm) as well as HCO3. Thus, it can be explained that these water flow through marine sediment at depth. Besides, from Giggenbach diagram (1989), these waters are not suitable for applying geothermometry to estimate the real reservoir temperature in this geothermal system. On the other word, it maybe states that the minimum range of reservoir temperature given by Kaliulo water sample is around 1400C. Isotopic results confirmed that a similar origin of the geothermal fluids and the meteoric water as well as small oxygen shift due to interaction with country rock. The enrichment of d18O and dD (3.95‰ and -20.78‰) of water in Kaliulo is quite as similar as water has low-tempareture exchange in sedimentary basin. On the other word, it is elucidated that water in Kaliulo represents not only mixing between meteoric water and connate water that trapped into Tertiary marine sediment at depth but also rock/water interaction. Soil air gas samples were collected at 50 – 100 m intervals along valleys near fumarole and around geothermal manifestation in Gedongsongo area. Soil air gas data exhibit that a fault zone trending NS along the valley near the fumarole. On one hand, a possible heat source located around Gedongsongo area up to the north (near collapse wall. Moreover, soil air gas also demonstrates a margin that limited geothermal activities in Ungaran in a relatively small region without extending toward the westernmost and easternmost part of this area.

Kata Kunci : Geologi,Sistem Geothermal Ungaran,Hidrologi Kimia