SUBSURFACE GEOLOGY AND HYDROTHERMAL ALTERATIONIN DIENG GEOTHERMAL FIELD, CENTRAL JAVA, INDONESIA

A' A' calibuganr, N. H. wibowotA. Harijokor, D. wintolor, M. R. B. Tarriela2, T. silitonga3, T. purwantoro3I Depafiment of Geological Engineenng,

Facurlv of Engineering, Gadjah Mada University. 552g1, Indonesia2 National Institute of Geological Sciences,

college of Science, University of the philippines, euezon city, l l0 l , philippinestPT

Geo Dipa Energi, Jl. Karawitan No.32 Bandun g, 4(t264,rndonesia

ABSTRACT

Dieng geothermal fetd is a two-phase system which occurs in a caldera on the eastern margin of the Dieng volcaniccomplex' Manifestations of thermal activities qre extensive in this field and consist of solfutara, .fumaroles, hotsprings, mud pools, mofettes, phreatic explosion craters and alterecl grounds. The Diengf eld is the only geothernralarea in Central Java that has been exploited and it is currently producing 6a MI4/ of electrical power. This papertries to describe the subsurface geologt and hydrothermal alteration mineralogt of the Dieng geothermat field.

In this study, cores and cuttings obtained from the four selected wells in the Dieng geothermal f eld were examinecl

and logged. Sampling was done per change in tithologt and alteration type.A total of II0 rock samples were

collected and prepared .lbr L'-ray dilfractometer (XRD) onel petrographic analyses. Basecl on ntegascopic onrlpetrographic studies, the rock units encountered in the 4 wells include andesite, anclesite breccia, tu.ff anct tuf/breccia. The combined data obtained from megascopic study o.f cores and cuttings and preliminary results 6[petrographic and XRD analyses indicate that the hydrothermal alteration in Dieng geothermal .field is charqcterizecl

by calcite, adularia, pyrite, epidote, silica (quartz, cristobalite), cla.vs (smectite, illite, kaolinite, chlorite, hallo.vsite),

sulphates (gtpsum, anhydrite), and zeolites (laumontite, heulanctite). Based on the obseryetl hyclrothermal alteratio,ntineral assemblage, the alteringJluid is most probobly of neutrol pH.

Keyuords: hydrothermal alteration, subsurface geologt, Dieng geothermat feld

INTRODUCTION

Indonesia is endowed with geothermal energy resources owing to its geotectonic setting. Geothermal energy

exploration in Indonesia commenced in the early 1900's and at least 70 high enthalpy geothermal fields andprospects have been determined (Fauzi, et al., 2000). In Central Java, of the hve identified geothermal prospect

areas. onlv the Dieng geothermal field has been developed for electrical power generation (pertamina, 1994).

The Dieng geothennal field is a high-temperature two-phase geothermal system (Pertamina, 1994). which is located

about 80 kilometers northwest of Yogyakarta City. It occupies a mounlainous terrain with an elcvation ofapproximately 2000 meters above sea level. The estimated electrical power capacity of the field is about 355 MW(Boedihardi. et al.. l99l). A totalof 45 deep wells have been drillcd in the Dieng held which include 27 pertamina

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wells (1977-1994) and l8 wells by Himpurna Califonia Energy Ltd. (1995-1998)). Of the 45 wells, onty 6 wells(HCE wells in Sileri area) a.re currently utilized to supply steam to the power plant and 3 rvells sen'e as injectionwells. The Dieng field is operated by PT Geo Dipa Energi which is joint subsrdiary of pertirmina and pr pLN. The

installed generating capacitv of the field is 60 MW.

In Dieng geothermal field, silica scaling poses a serious problem in power plant operation. In this paper, one

approach considered in addressing the scaling problem is by means of studying the nature of the interaction oftllennal fluids with reservoi.r rocks. From the study, it can be determined whether the silica scaling is due to lvater-

rock interaction process occurring at the subsurface of this field or due to other processes The nature of such $,ater-

rock interaction can be deduced from the analysis of the hydrothennal alteration mineralogy. The subsurface

hydrothermal alteration mineralogy, as well as the subsurface geology of Dieng geothermal field, based on the

combined data obtained from megascopic study of cores and cuttings and preliminary results of petrographic and

XRD arnlyses are presented in this report.

REGIONAL GEOLOGY AND THERMAL MANIFESTATIONS

The Dieng volcanic complex (Figure l) is part of the E-w trending mountain range in Central Java that extends

from Gunung Slamet on tlte west to G. Ungaran on the east (CalEnergy, 1998); and this chain of volcanoes has been

identified as geothermal prospect by Pertamina (1994). The Dieng complex is composed of euaternarystratovolcanoes and smaller craters and cones which are distributed over an area of l4km x 6km at an elevation of2000 m above sea level. Based on its morphological characteristics, the complex can be grouped into four regions

(Figure 2) namely, Batur graben, Sidongkal graben, Rataarnba horst and Old Dieng caldera (Delarue, 1975 cited inWintolo, 1979). The geothermal area is centered at the Old Dieng caldera which is found on the eastern edge of the

Dieng complex. The caldera is believed to have resulted from the collapsed of the western flank of Gunung prahu

(Sukhyar, 1994 cited in van Bergen, et al., 2000).

The Dieng geothermal area is bounded by NW-SE oriented volcanic centers of Gunung Sipandu to pakuwaja on the

western side and by older Gunung Prahu to Patakbanteng on the eastern side (CalEnergy, l99g). This field ischaracterized by ten lithological units which include, from oldest to youngest, products of G. prau (lava andtuffaceous breccias' 3.60 ma), G. Nagasari (andesite, 2.99 ma), G. Bisma (basaltic andesite, 2.53 ma), G.

Pagerkandang (andesite, 0.46 ma), G. Merdada and Pangonan (andesite, 0.37 ma), G. Kendil (dacitic andesite lava,0'19 ma), G' Pakuwaja (quartz latite,0.09 ru), G. Seroja (lava dome,0.07 ma), volcanic plain and hydrothermallyaltered rocks (Boedihardi, et al., l99l). Moreover, major lineaments in the area, from oldest to the youngest, areoriented E-w, NW-SE and NE-sw or N-S (Boedihardi, et al., 1991). A simplified geological map of the Dienggeothennal field is shown in Figure 3.

surface thermal manifestations in Dieng geothermal field are extensive and occur in both high and low elevationareas' At higher elevations such as volcanic peaks, the thermal features consist of fumaroles, acid sulphate boilingsprings' mud pools and inte;nsely altered 8lound whereas, at lower elevations, the therrnl areas are characterized bv

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hot and warm springs with temperature of 35-55 "c and neutral pH (Boedihardi, et al., Jggl). In general, there are

tfuee main areas of vigorous surface thermal activities in Dieng and these are Sileri, Sikidang and pakuwaja a-reas

(CalEnergy, 1998) (Figure 4).

Figure 1. Location map of the Diorg geothermal field(Modified from HCE, 1998).

Proceedings of the -l'd lniernational SymposiumEarth Resources and Geclogicol Engineering Education

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Figure 2. General morphology of the Deng complex(Modified from Delarue, 1 975 citerl in Wintolo, 1979).

SAMPLING AND METHODS

Four boreholes in the Dieng geothermal field were initially selected for sampling. The location of these wells is

shown in Figure 3 and the borehole data are tabulated in Table l. The drill cores and cuttings acquired from the

selected four boreholes were examined megascopically to determine the lithology, presence of fractures or veins.

intensity of alteration, and altemtion minerals (Figure 4). Sampling was then undertaken per change in lithologv and

alteration type. A total of I l0 core and cutting samples were collected and prepared for X-ray diffraction (XRD) and

petrographic studies. Petrographic arnlysis is used to identifo the primary and alteration minerals as well as

mineralogical textures and paragenesis. For this method, thirty samples were made into thin-sections, however, upon

writing this report, only the samples taken from well B have been checked under the microscope. Thus, petrographic

interpretations will be based from the analysis of these samples.

Another method employed in this study is the XRD analysis which is used to determine the minerals present in the

rock samples particularly clay minerals. Two procedures are performed in this method including bulk-rock analysis

and clay analysis. Bulk-rock analysis gives the whole-rock mineralogy of the sample and with this analysis. the

sample is crushed to clav size particles using mortar and pestle and it is then mounted on foulder (i.e. un-oriented

mounts). On the other hand, in clay analysis, the rock particles are poured into the beaker partially filled rvith

distilled water and then place in an ultrasonic vibration machine to disaggregare the particles. The suspension is then

decanted and with the use of centrifuge, the fine suspended particles are extracted and mounted on glass slides (i.e.

oriented mounts). Considering that the XRD analysis is still in-progress, the available XRD data which are used in

this report are obtained from the bulk-rock analysis of the well cuttings recovered in well B.

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