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Proceedings World Geothermal Congress 2015
Melbourne, Australia, 19-25 April 2015
Stratigraphy Characteristic of Reservoir Zone in Hululais Geothermal System
Vivi D. M. Nusantara, M. I. Aziz, Imam M. Prasetyo, Hary Koestono, M. H. Thamrin and M. Y. Kamah
PT. Pertamina Geothermal Energy, Skyline Building 15th floor. Jalan MH.Thamrin 9th. Jakarta 10340. Indonesia
[email protected]
Keywords: Hululais, Reservoir, Metasedimentary, Petrography, Stratigraphy
ABSTRACT
Hululais geothermal field is located in Lebong, 180 km north from Bengkulu, Sumatra, Indonesia. It sits in the Sumatra Fault as a
graben system covered by quartenary volcanic products. Based on the geological map, the oldest rock in this field belongs to
Tertiary Hulusimpang Formation which consists of Cuguk andesite and Cogong diorite unit.
Aggressive exploration was conducted by the drilling of deep geothermal wells beneath the Mt. Beriti Besar crater. Core and
cutting was done in the zone of interest to describe the stratigraphic character of the reservoir rock in Hululais by XRD and
petrographic analyses. Aside from the intrusive diorite which was a probable heat source, the presence of a metasedimentary rock
that originated from the Middle to Upper Miocene Fossilliferous calcilutite between the tertiary Hulusimpang quartenary volcanic
products characterized the reservoir zone in Hululais.
1. INTRODUCTION
The Hululais geothermal field is located in Lebong, which is 180 km north from Bengkulu, Sumatra, Indonesia (Figure 1). It sits in
the Sumatra Fault as a graben system covered by quartenary volcanic products. A series of deep exploratory wells have been drilled
in Hululais to characterize its geothermal potential. From the drilling activities, cuttings and cores are taken as primary reliable
subsurface data. Petrography and XRD analyses of these samples represent the subsurface condition of the field.
Figure 1: Location Map of Hululais Geothermal Field. It lies in Barisan Mountain Ranges, Bengkulu, Sumatra, Indonesia.
Furthermore, analyses on the cuttings is likewise used to build a geothermal stratigraphic model of its reservoir system. The
geothermal stratigraphic model describes the types of alteration, lithology unit, and role of each of the facies units in the Hululais
geothermal system itself. This paper will discuss the stratigraphic model of the Hululais geothermal field and its importance in
understanding the reservoir system in a geothermal field.
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2. GEOLOGICAL SETTING
2.1 Tectonic
The Hululais geothermal system lies in the Sumatra Fault System (SFS) which trends in the NW-SE direction and is divided into 19
segments by Sieh and Natawidjaja (2000). Hululais is an area of two overlapping segments, which are called the Musi and Ketahun
segments. Their movement generates an extensional force that produces a depression called a pull a part basin in Hululais. It is
bounded by two major dextral shear fault and normal faults which are relatively perpendicular to each another (Figure 2).
Figure 2: Hululais is a graben system in stepover area of Ketahun and Musi Segment from Sumatra Fault System (SFS).
The permeability in the Hululais geothermal system is controlled by the NE-SW and NW-SE faults. The NE-SW fault is a normal
fault dipping to northwest. On the other hand, the NW-SE fault belongs to SFS as a dextral shear fault dipping vertically to the
northeast. Moreover, the geothermal fluid flows to the N-NE direction in Semelako which is the outflow zone. Geochemical data
shows that the Suban Agung area is the upflow zone of the Hululais geothermal system (Budiardjo et al., 2001).
2.2 Volcanostratigraphy
Gafoer et al. (1992) constructed a stratigraphy of the Hululais area in the Southern Sumatra Map. The oldest rocks come from the
Tertiary Hulusimpang Formation which consists of the Andesite Cuguk and the Diorite Cogong units. In the quartenary period,
high activities of volcanic explosion in this area deposited pyroclastic rocks from Basaltic to Andesitic Lava. In terms of age, from
older to younger, the units are: Andesite Resam unit, Andesite Lekat unit, Andesite Koleng unit, Andesite Tiga unit, Andesite
Hululais unit, Tuff Pabus unit, Andesite Lumut unit, and Obsidian Pabuar unit (Figure 3).
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Figure 3: Geological Map of Hululais Geothermal Field. The younger formation is Quartenary Volcanic Formation and the
oldest is Tertiary Hulusimpang Formation
3. SUBSURFACE
Cutting samples and the core body of rocks, combined with logging data and drilling parameter are the main data used to
characterize the subsurface condition in Hululais. The process of identification, correlation, and zonation of each unit resulted in a
subsurface model that showed the lithology types, alteration zones, and distribution of mineral assemblages.
3.1 Lithology
Based on the depth, from the shallow to the deep zones, the lithology in the Hululais geothermal field can be divided into
two formations. They are the following:
3.1.1 Quartenary Volcanic Formation
The quartenary volcanic formation has been drilled from the surface until ± 974 to 0 masl. It consists of the Andesit Hululais unit
and the Andesite Resam unit. The lithology is dominated by the intercalation of Altered Tuff Breccia and Altered Andesite Breccia.
Altered Andesite is found as an inset with a thin layer of lava. The degree of alteration ranges from low to high (10% to 60%).
Andesite in quartenary volcanic formation has porfiritic and flow texture rich of glass volcanic and plagioclase as ground mass
(Figure 4).
Figure 4: Petrographic photo shows altered tuff, tuff breccia and flowage andesit from cutting of well C in 550 mMD / 424
masl. Altered tuff, tuff breccia and flowage Andesite characterize lithology in quartenary volcanic breccia.
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Tuff breccia and flowage andesite characterize the lithology in the quartenary volcanic formation. They are predicted as members
of the Hululais and Resam andesite units. Both units resemble each other but they come different periods of volcanism based on
absolute dating data. The Hululais unit derives from the Hululais volcanic (0.907 MYA) whereas the Resam unit is a product from
the Resam volcanic in 1.332-1.220 MYA (Budiardjo et al., 1994).
3.1.2 Tertiary Hulusimpang Formation
The Tertiary Hulusimpang formation can be found from ± 250 masl to -1100 masl. The lithology varies from Andesite,
Microdiorite, and Granodiorite. It consists of Andesite Cuguk with a layer of metasedimentary rock originated from Middle to
Upper Miocene Fossilliferous calcilutite in the top of the formation.
From 150 to –250 masl, a thick layer of dark massive metasedimentary rock originated from Fossilliferous calcilutite is found.
Fossilliferous calcilutite presents as a 400 m thickness layer. Physically, the core sample shows Fossilliferous calcilutite as a very
massive rock, black in color and very tight. However, petrography samples show that the composition of this rock is full of
molussca, especially the microplankton species (Figure 5).
A
B
Figure 5: Petrographic photo shows contact of Fossilliferous calcilutite and Andesite at 1412 mMD / -233 masl (A) and
cutting samples compose of mollusca and flowage andesit at -10 masl (B).
The condition of the microplankton test has been replaced by calcite. Physically the fossil is “burned” and becomes black in color.
The Fossilliferous calcilutite unit is deposited in the Middle to Upper Miocene (N14-N18). Fossil index analyzed are Globorotalia
Pseudomiocenica, Globigerina Nephentes, Sphaeroidinella subdehiscens, and Globotalia Plesiotumida (Figure 6).
Figure 6: Microplankton fossil from Fossilliferous calcilutite in top of Tertiary Hulusimpang Formation. The figure shows
test of microplankton fossil become brown to black because it is burned by volcanism activities in this area
Fossilliferous calcilutite is an insitu fossil. It shows a perfect and ideal test and there is no mixing fossils. The Fossilliferous
calcilutite layer is defined as the top part of the tertiary Hulusimpang formation from the Simpangaur unit. The Simpangaur unit
thickens to the east of Hululais area. It is composed of conglomerate, breccia, tuffaceous sandstone, and claystone containing
mollusca (Gafoer et al., 1992).
In the deeper part of the Hulusimpang Formation, the rocks are composed of the Andesite Cuguk unit. It is dominated by Altered
Andesite and Altered Andesite Breccia. Alteration ranges from mid to high levels (20% to 80%). The lithology type in the Andesite
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Cuguk unit has a higher crystallinity rank than the Andesite Resam and Hululais units. It varies from Andesit to Microdiorite, as
shown in Figure 7.
Figure 7: Petrographic photo of Andesit – Microdiorite from core sample of well A at 2263 mMD / -813 masl. Secondary
biotite and epidote indicating high temperature alteration is abundant in this depth.
Diorite is found in the bottom part of the wells as intrusive rocks. Well C has diorite at 1838 mMD / -468 masl (Figure 8). It is
predicted as a heat source in the Hululais geothermal system and it comes later as the youngest active intrusive area in Hululais.
The correlation of the lithology in Hululais is shown in Figure 9.
Figure 8: Petrographic photo (cross nikol) of Diorit from core sample in well C at 1828 mMD / -468 masl.
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Figure 9: Correllation lithology of wells in Hululais. Rock formation divided into Quartenary Volcanic Formation and
Tertiary Hulusimpang Formation
3.2 Alteration
The alteration zone in the Hululais is divided into three types from the surface to the bottom. The different types are: Smectite zone,
Smectite - Illite zone, and the Chlorite – Epidote – Illite zone. Relict Epidote is distributed from ± 500 masl. A correlation between
alteration zones is suitable with the distribution of the temperature beneath the wells.
The smectite zone is indicated by a thick layer of rich smectite clay. It ranges from the surface to ±1000 masl. The lithology in
smectite zone is composed of pyroclastic rocks, andesite breccia, andesite lava, and tuff breccia from the quartenary volcanic
formation. Alteration ranges low to high levels. Mineral assemblages in this zone are smectite, chlorite, Pyrite, Quartz and Iron
Oxide. In addition, the smectite zone is interpreted as clay cap zone in the Hululais geothermal system. The Smectite – Illite zone
ranges from 1000 to 750 masl. It is interpreted as a transition between the clay cap and reservoir zones. Smectite and Chlorite are
still present in this zone. Illite, Illite-Smectite clay, and Epidote are likewise abundant. The Chlorite – Epidote – Illite zone indicates
a reservoir zone at 750 masl below. Mineral assemblages are composed of epidote, Illite, Secondary Biotite, Actinolite, and
Adularia. The distribution of alteration zone is shown in Figure 10.
Figure 10: Alteration distribution from geothermal wells in Hululais. Rock formation divided into Quartenary Volcanic
Formation and Tertiary Hulusimpang Formation
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3.3 Geothermal Stratigraphy
Lithology and alteration zones in Hululais are used to build a geothermal stratigraphy model. It shows how the alteration and
hydrothermal activities can change the existing rocks in a geothermal area (Figure 11).
Figure 11: Geothermal stratigraphy model of Hululais eothermal system. It shows varies of lithology in Hululais geothermal
system and ditribution of alteration. Different type of alteration can change the lithology into different types.
Geothermal stratigraphy of Hululais geothermal field shows three types of alteration in two rocks. The smectite zone ranges from
the surface to ±1000 masl, where there are: altered quartenary volcanic formation into altered tuff, altered tuff breccia, and altered
flowage andesite. This zone represents the clay cap zone of the Hululais geothermal system.
The Smectite – Illite zone is present between ±1000 to ±750 masl. It likewise has altered quartenary volcanic formation into altered
tuff, altered tuff breccia, and altered flowage andesite. Illite is abundant in this region. This zone represents the transition zone
between the clay cap and reservoir zone beneath.
The reservoir zone is indicated by the Chlorite – Epidote – Illite zone from ±750 masl below. Intensive alteration activities in
reservoir zone altered both the quartenary volcanic formation and the tertiary Hulusimpang formation. It produce altered andesite to
altered microdiorite as reservoir rocks in the Hululais geothemal system. Intrusive diorite is the most probable heat source that
reheat the current geothermal system in Hululais.
4. CONCLUSION
The Hululais geothermal system composed of the quartenary volcanic formation and the tertiary Hulusimpang formation. The
quartenary volcanic formation is altered by a low type of alteration. It produces the smectite zone as a clay cap zone and the illite –
smectite zone as transisition zone. The Tertiary Hulusimpang formation is altered by the high temperature alteration which
produces the chlorite – epidote – illite zone as reservoir zone. The top part of the tertiary Hulusimpang formation is bounded by a
metasedimentary rock that originated from fossiliferous calcilutites. The geothermometer mineral shows epidote, secondary biotite,
actinolite, and illite. The temperature in reservoir zone is about 240 oC to 300oC. Intrusive diorite is the heat source of the current
system which is proven by cutting samples from wells and pressure and temperature measurements.
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