Proceedings World Geothermal Congress 2010
Bali, Indonesia, 25-29 April 2010 Horizontal Derivative from Gravity Data as a Tool for Drilling Target Guide in Wayang Windu Geothermal Field, Indonesia
Yudi Indra Kusumah 1, Suryantini2, Hendro H. Wibowo2
1Star Energy Geothermal (Wayang Windu). Ltd 2 Research Division of Applied Geology, Faculty of Earth Sciences and
Technology, Institut Teknologi Bandung
horizontal derivative, gravity, permeability,
The Wayang Windu geothermal system, associated with the
fracture, gradient, intrusion, drilling
Malabar, Wayang and Windu volcanic centers, is interpreted
to be transitional between vapor-dominated and liquid-ABSTRACT
dominated. Deep wells encounter liquid reservoir with the
top, prior to production, from 0 to 400 m asl (above sea
The density of the gravity survey points allows “horizontal
level), which becomes progressively deeper toward the
derivative” filtering to be applied to the complete Bouguer
south. It is overlain by three separate vapor-dominated
anomaly (CBA) data. The horizontal derivative process
reservoirs, which become progressively shallower to the
produces maximum ridges over the contacts between
north, where the top of the reservoir is at 1150 m asl.
different density body blocks. The image produced by the
horizontal derivative algorithm shows some agreement to
The Wayang Windu volcanic setting is identified as an
the known productive area which lies in the vicinity of the
active geothermal system, characterized by the presence of
maxima of the horizontal derivative. The horizontal
volcanic center and series of intrusion as heat sources.
derivative maximum does not correspond well to the known
Indications of geophysical features representative of
fault pattern. It may possibly indicate the location of the
intrusion bodies were derived from the combination of MT
margins of intrusive bodies or major volcanic facies
and gravity, and then validated by well information. This
suggested the occurrence of microdiorite or andesite
porphyry dykes. There is also a mineralogy study, reporting 1. INTRODUCTION
the presence of high temperature mineral, identified as
The Wayang Windu (WW) geothermal field is located in
amphibole zone in deeper part (Abrenica et al.
West Java Indonesia, at a distance of about 150 km SE of
Jakarta, Indonesia (Figure 1). Currently the installed
In general, the vicinity of the margin in between the
capacity is 220 MWe. The field has been intensively
intrusion body and surrounding rock is susceptible to
explored, with a total of 39 wells drilled, including
fracturing. They are radial and concentric with high angle in
production, reinjection and slim hole exploration wells.
the late stage of fracture development. A hydrothermal
Several geophysical surveys have been performed including
system was then developed around intrusive bodies after
magneto telluric (MT) surveys with 1 km spacing interval,
deep penetrated meteoric water reaction with hot intrusion
gravity surveys which in places reaches 1 km station density,
bodies and stimulated a convective flow circulation through
microearthquake (MEQ) survey and formation imaging well
this clustering of permeable zone.
logs. Figure 1: Location of Wayang Windu Geothermal field in West Java, Indonesia.
Kusumah et al.
The horizontal derivative of potential field data is a
localized anomalies (Figure 3). The residual gravity values
technique used to enhance data. By taking the derivative
around the Malabar complex are clearly defined as high
along the x and y axis, this enhancement aims to define the
anomalies, while in the southern part of Wayang Windu and
anomalous body boundary and to separate with other
Bedil regional pattern still show more localized values. Most
anomalies through the relevance of analytical calculation.
trends of anomalies are more clearly defined based on
The calculation is achieved by comparing gravity profiles or
residual features. In the northern area, residuals show two
contours, as the slope or rate of change of gradient with
peak anomalies, those indicate shallow anomalies.
horizontal displacement, since the sharpness of a gravity
profile is an indication of the depth of the anomalous mass.
This paper outlines the application of gravity information to
understand how the fracture zone is related to intrusion
bodies. This study utilizes enhanced horizontal derivative
analysis, integrated with fracture information from wellbore
data, combined with other geophysical surveys to delineate a
prospect area for well targeting. 2. GRAVITY SURVEY AT WW
Gravity surveys have been conducted regularly at WW since
exploration stage. The initial survey was carried out by
Unocal Geothermal Indonesia in 1982 with 76 stations, and
then followed up by Pertamina in 1985-1986 which carried
out 256 stations. Low density spacing of stations and
coverage area caused low resolution results and difficulties
in interpretation. Unocal then carried out a microgravity
survey in 2002 which was continued by Star Energy in 2008
which carried out a re-measurement of microgravity survey
and systematically gridding gravity survey with high density
spacing, for around 250 m - 1 km spacing and larger Figure 2: Complete Bouguer anomaly in mgal of
coverage area. This survey consists of 135 additional Wayang Windu Geothermal field overlie by well
stations and extended the area to the unexplored piece in the trajectory in mgal.
northern part of the Wayang Windu Geothermal field. The
survey was completed in the end of 2008. Data was gained
with very good quality and showing higher resolution
resulst. All gravity data were analyzed to derive the final
structural gravity map.
The complete Bouguer anomaly was calculated for all
the 1980 formula,
terrain corrections to 50km radius,
reduction in densities of 2.00, 2.20, 2.40 and 2.60
The Bouguer anomaly shows a regional high to
the west and southwest, this positive anomaly
persists at all reduction densities, and cannot be
considered an artifact of the topography vs
reduction density alone. The main features are
relative gravity high in the south and west with Figure 3: Residual Bouguer anomaly in mgal of Wayang
decreasing gravity towards the NE. The gravity Windu Geothermal field using polynomial 3
then increases again to a cluster of anomalies overlie by well trajectories.
around the cluster in the middle area which shows
SE-NW/NNW trending area prior to significant 3. STRUCTURAL MODELING
decrease again in north east and east area. This
feature then indicates a significant regional
To examine the gravity models in relation to the 3D, MT
structure which can be interpreted as high density
modeling carried out by Geosystem in 2009, the residual
bodies around Gunung Wayang and Windu. These
gravity anomaly is overlain on the 3D MT resistivity.
gravity features are interpreted as expression of
General positive correlation gravity high is modeled straight
buried intrusive bodies, while more a attractive
thorough the high, showing how the required dense body
anomaly present in the northern area around the
also corresponds to the 3D MT resistivity high. Modeling
Malabar complex (Figure 2).
was carried out using WinGLink’s 2.75-D software,
allowing the use of up to the limit of body strike length (3
To assess significant local anomalies, separation of regional
km in the case of the modeled Malabar complex intrusive
from residual geophysical data was performed to enhance
stockwork) (Figure 4).
Kusumah et al. Figure 4: Gravity modeling of Bouguer anomaly combined with resistivity MT. 3. HORIZONTAL GRADIENT
Interesting results were found when medium high – high
(0,03-0,05 mgal/m) gradient contrast values coincide with
Blakely (1995) stated that the horizontal gradient of gravity
major productive area between the intrusive and the local
anomaly caused by a tabular body tends to overlie the edges
reservoir rocks. These were interpreted from structural
of the body, if the edges are vertical and well separated from
modeling of gravity and MT. Based on this agreement; it
each other. The biggest advantage of the horizontal gradient
indicates that the geothermal areas in Wayang Windu are
method was its low susceptibility to the noise in the data,
mostly located around intrusion boundaries especially for
because it only requires the calculation of the two first-order
the deep sources.
horizontal derivatives of the field (Phillips, 1998). The
method also has robust delineation, either shallow or deep,
in comparison with the vertical gradient, which is useful
only for the shallower structures. The amplitude of the
horizontal gradient (Cordell and Grauch, 1985) is expressed
⎟ + ⎜⎜ ⎟⎟
) are the horizontal derivatives of the x
gravity field in the x and y directions. The horizontal
gradient amplitude of the complete Bouguer anomaly data of
the Wayang Windu area was computed and illustrated in
Contrast in between high gradient values and low gradient
values were observed as scattered, and the pattern of high
gradient anomalies is broad, but overall localized middle
values are defined. Referring to Grauch and Cordell (1985), Figure 5: Horizontal derivative in mgal/m of the regional
the limitation of the horizontal gradient methods for gravity gravity data overlied with well trajectories.
data is that the horizontal gradient magnitude maxima can be Mostly northern part wells as major productive
offset from a position directly over the boundaries, if the well lying on 0.03-0.05 gradient values.
boundaries are not near-vertical and close to each other.
Integration of data with resistivity, MEQ events, and high
For selecting gradient values, the data have to be integrated
temperature minerals show correlation between maximum
with other information as guidance, to define gradient
gradient, which have values of 0,03 and 0,05, with the
magnitude maxima correlated to area which are interpreted
existence of MEQ cluster, and existence of high temperature
as intrusion boundaries data. MT information and well
minerals showing correlation with each other. (Figure 6).
performance can be used as assistive tools as well as
formation evaluation, based on other geoscientific
This combination indicates that maximum gradient can be
assessment, for better interpretation.
utilized to define fracture zones which are possibly favorable
for production within the geothermal system.
Kusumah et al.
Integrated horizontal gradient as a derivative analysis with
analysis. The events observed during the survey time were
other information can lead to better understanding of the
mostly induced events triggered by injection water into
geothermal system itself and especially identification of a
injector wells through hydro-fracturing, where in 2007 the
permeable fracture area.
survey used MBB-1 as injector well.
Permeable zones in the Wayang Windu reservoir have been
The integrated information of permeable zones based on all
identified primarily from field structural mapping including
available data combined with specific values of contrast
remote sensing interpretation. The fractures were then
gradient horizontal derivative profile enhances the
assessed based on wireline log analyses. Away from well
understanding area of interest, which is interpreted as
bores, the recognition and characterization of fluid paths
contacts between the intrusive bodies and the local reservoir
have been investigated using microearthquake (MEQ)
500 1000 1500 2000 2500m
Y (m) Figure 6: Integrated interpretation of resistivity, Bouguer, and residual gravity compared to MeQ event and high temperature mineral (red for amphibole, green for biotite and yellow for pyrophyllite).
Kusumah et al. MEQ Events (S-N)
500 1000 1500 2000 2500m High Temperature Minerals
16000 Figure 7 (continued): Integrated interpretation of resistivity, Bouguer, and residual gravity compared to MeQ event and high temperature mineral (red for amphibole, green for biotite and yellow for pyrophyllite). 4. APPLICATION FOR WELL TARGETING
reacts with hot intrusion bodies and stimulated a circulation
convective flow through this clustering of permeable zones.
The margin between intrusion bodies and surrounding rock
is favorable development as fractured area. They are radial
Correspondence between horizontal derivatives, MEQ
and concentric and high angle in the late stage of fracture
events and intrusion indications from high temperature
development. Intrusion bodies themselves are less
mineral can help locate narrow bodies that can be delineated
permeable areas. Therefore the delineated margin zone is
as an attractive zone, i.e. a fracture zone within the
essential to identify the geothermal system which developed
geothermal system for drilling targeting.
around intrusive bodies after deep penetrated meteoric water
Kusumah et al. 5. CONCLUSION
Cordell, L., and Grauch, V. J. S., 1985, Mapping Basement
• Gravity and MT results identified possible high
Magnetization Zones from Aeromagnetic Data in the
density bodies that were interpreted as intrusion
San Juan Basin, New Mexico, in Hinze, W. J., Ed., the
Utility of Regional Gravity and Magnetic Anomaly
Maps: Sot. Explor. Geophys., 181&197.
• Enhancement with horizontal gradient analysis
Dobrin Milton, Savit C Carl, (1988). Introduction to
shows correspondence between specific values of
Geophysical Prospecting, McGraw Hill, 14 (581-607).
horizontal gradient and productive fractures within
the geothermal system.
Geosystem S.R.L. 2008. Final Report Passive Seismic
Survey, Wayang Windu Geothermal Field, Processing
• Based on this assessment, the next drilling target
and Interpretative Report: High-Precision Locations,
should take into consideration the result of
Moment Tensor and Shear-Wave Splitting Analysis
horizontal derivative analysis for well targeting.
(Unpublished report to Magma Nusantara Ltd.).
Kusumah, Y.I., De Luca, L., and Bogie, I., (2009) A Recent 6. ACKNOWLEDGEMENTS.
Microearthquake Survey at The Wayang Windu
The author would like to express gratitude to the
Geothermal Field, Indonesia, Proceeding, 30 th, Annual
management of Star Energy Geothermal (Wayang Windu)
EDC Geothermal Conference, 2009; 79-86.
Ltd. for permitting the authors to publish this paper. And
show appreciation to Lukman Sutrisno and Wahyuddin
Milsom, J, Field Geophysics, (2003) The Geological Field
Diningrat for discussion and figure preparation, and to
Guide Series, Jhon Willey & Sons, 15-16, 29-49.
Shanti R.A. Sugiono as well for editing.
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