SUBSURFACE GEOLOGY AND HYDROTHERMAL ALTERATION
IN DIENG GEOTHERMAL FIELD, CENTRAL JAVA, INDONESIA
A' A' calibuganr, N. H. wibowotA. Harijokor, D. wintolor, M. R. B. Tarriela2,
T. silitonga3, T. purwantoro3
I
Depafiment of Geological Engineenng,
Facurlv of Engineering, Gadjah Mada University. 552g1, Indonesia
2
National Institute of Geological Sciences,
college of Science, University of the philippines,
euezon city, l l0
tPT
l,
philippines
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 volcanic
complex' Manifestations of thermal activities qre extensive in this
field and consist of solfutara, .fumaroles, hot
springs, mud pools, mofettes, phreatic explosion craters and alterecl grounds.
The Diengf eld is the only geothernral
area in Central Java that has been exploited and it is currently producing 6a MI4/ of
electrical power.
This paper
tries 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
and logged. Sampling was done per change in tithologt and alteration
f eld were examinecl
type.A total of II0 rock samples were
collected and prepared .lbr L'-ray dilfractometer (XRD) onel petrographic analyses. Basecl on ntegascopic
onrl
petrographic 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 and
prospects 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 of
approximately 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
Proceedings of the 3'd International Symposium
Earth Resources and Geological Engineering Education
443
wells (1977-1994) and
l8 wells by Himpurna Califonia
(HCE wells in Sileri area)
a.re
Energy Ltd. (1995-1998)). Of the 45 wells, onty 6 wells
currently utilized to supply steam to the power plant and 3 rvells sen'e as injection
wells. 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
of
tllennal fluids with reservoi.r rocks. From the study, it can be determined whether the silica scaling is due to lvaterrock 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
euaternary
stratovolcanoes and smaller craters and cones which are distributed over an area
of l4km x 6km at an elevation of
2000 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 in
Wintolo, 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 is
characterized by ten lithological units which include, from oldest
to youngest, products of G. prau (lava
and
tuffaceous 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 hydrothermally
altered rocks (Boedihardi, et al.,
l99l). Moreover, major lineaments in the area, from oldest to the youngest, are
oriented E-w, NW-SE and NE-sw or N-S (Boedihardi, et al., 1991).
A simplified geological map of the Dieng
geothennal field is shown in Figure
3.
surface thermal manifestations in Dieng geothermal field are
extensive and occur in both high and low elevation
areas'
At higher elevations such
as volcanic peaks, the thermal features consist of
fumaroles, acid sulphate
boiling
springs' mud pools and inte;nsely altered
8lound whereas, at lower elevations, the therrnl areas are characterized bv
Proceedings of the 3'd International Symposiunt
Earth Resources and Gealogical Engineiring Education
444
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
(CalEnergy, 1998) (Figure 4).
).t{Lr
...^
Figure 1. Location map of the Diorg geothermal field
(Modified from HCE, 1998).
.i5ul
CI vntor,.tccrqi6r ffif
a-reas
<rr
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
mounts). On the other hand,
it is then mounted on foulder (i.e. un-oriented
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.
Proceedings of the -l'd lniernational Symposium
Earth Resources and Geclogicol Engineering Education
445