From Exploration to Exploitation

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IOP Conference Series: Earth and Environmental Science

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Hululais Field Review from Geoscientific Exploration to Exploitation Stage

To cite this article: Tavip Dwikorianto et al 2023 IOP Conf. Ser.: Earth Environ. Sci. 1159 012008

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Hululais Field Review from Geoscientific Exploration to Exploitation Stage

Tavip Dwikorianto¹, Yunus Daud¹, Agustya Adi Martha², and Mulyanto³

1Department of Physics, Faculty of Mathematics and Natural Sciences (FMIPA), Universitas Indonesia, Depok 16424, Indonesia

2BMKG Office Jakarta, Jl. Angkasa I, No.2 Kemayoran, Jakarta Pusat 10610

3PT. Pertamina Geothermal Energy Kamojang Area PO. BOX 120 Garut, West Java, Indonesia

Email: [email protected], [email protected], [email protected], [email protected]

https://orcid.org/0000-0001-6308-2515

Abstract. Geothermal studies in Hululais carried out by Pertamina was started in 1993 with an exploration survey. The detailed geological mapping as the earliest geothermal resource assessment of Hululais was done internally by Pertamina. The geochemical study was done together with the geological study. The geophysical study was carried out using three methods.

The gravity survey was run in 1993 for 320 points along 12 trace lines, while the magnetotelluric survey was conducted in 2006 and 2012 for 64 locations with 1-2 km spacing. The microearthquake survey was done in 2012 for 115 days monitoring time by six sensors in 10 km spacing and in 2019. The conceptual model as a product of the integrated study of three geoscientific surveys shows that the upflow zone is interpreted in Suban Agung – Suban Gregok in the southern area. Fumarole, mud pool and steam-heated hot spring are found in this zone.

The outflow zone is interpreted towards the north in Semelako area supported by the chloride hot spring. The appearance of these manifestations is possibly controlled by fault structure in Musi Segment zone of Sumatra Fault Zone (SFZ). The heat source comes from granodiorite intrusion below Bukit Beriti Besar – Bukit Gedang Hululais. The cap rock has 500 meters thick in the upflow zone and increases to 800 meters thick towards the outflow zone in the northern part of the study area. The upflow zone was proved by two exploration wells that drilled until 3000 mMD. The first well in Suban Agung has a high temperature reach 300^oC with neutral reservoir fluids and good fault structure permeability. The second exploration well that drilled towards outflow zone shows a temperature of 205^oC with neutral reservoir fluids but with low permeability. In the exploitation stage, 21 development wells have been drilled for making the update conceptual model. Among them, ten wells are production wells and the remains are reinjection ones. The permeability in the reinjection wells is not so good compared to the production wells. It might be due to secondary mineral filling in the fault structure permeability zone, so this situation causes a problem in injection water to the reservoir. Investigation of the subsurface permeability distribution in the Hululais reservoir is challenging, so further research should be done to answer this challenge.

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1. Introduction

Hululais Geothermal Field (HGF) is one of the fifteen geothermal working areas operated by PT Pertamina Geothermal Energy (PT. PGE). It is situated in Rejang Lebong District, Bengkulu Province, Sumatera Island, Indonesia. This field is located about 150 km from Bengkulu City towards the Bukit Barisan Mountain in the north (Figure 1). HGF is targeted to generate 2 x 55 MW electricity.

Figure 1. Hululais geothermal field location (modified from [1]).

HGF started as a project in 2007 which the scope of project included permit, infrastructure, drilling, well testing and EPC (Engineering, Procurement, Construction) of production facility. Three exploration wells were drilled in 2010 – 2013 and 21 wells were drilled for development until 2018.

1.1. Previous Work

Geoscience studies in Hululais and surrounding areas have been carried out by Pertamina and other institutions since 1992. The earliest publication in the study area focuses on the regional geological mapping of Bengkulu with a scale of 1:250,000 [2]. The more detailed geological and geochemical mapping in the study area was done internally by Pertamina [3] which preceded the earliest geothermal resource assessment of Hululais. Since then, various geoscience publications have been made but few that discuss the structural and volcanism aspect in Hululais. In 2018, the latest geological mapping was carried out to update the data of Hululais volcanostratigraphy by PT PGE.

The geophysical study was carried out using three methods. The gravity survey that was done internally was run in 1993 for 320 points along 12 trace lines, while the magnetotelluric survey was conducted in 2006 and 2012 for 64 locations with 1 – 2 km spacing that was done by PT Elnusa as fund from West Jac and under PGE’s supervision. A microearthquake survey was done internally in 2012 for 115 days monitoring time by using six sensors in 10 km spacing.

24 wells have been drilled since 2010. Retrieved cutting and cores, fluids and borehole images from the drilling wells are substantial data for subsurface geology modelling. Thus, identification and characterization of those data are important for making it to the next field development scenario.

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1.2. Objective

There is a lot of data that has already been taken from HGF for knowing the reservoir characteristic, either from surface mapping or wellbore data. This paper will explain about the geothermal system concept of HGF from geoscientific data, even from exploration surveys and wellbore either exploration or exploitation wells for making the update conceptual model. The discussion will be focused on the parameters that control the geothermal system of HGF, i.e., the heat source, the caprock, the hydrology pattern and mainly the permeability that have different distributions in the southern and the northern area.

1.3. Method

The data of this paper are collected from previous publications and reports. The geological, geochemical, and geophysical data will be compiled for making the conceptual model. The drilling target for exploration wells will be based on this model. The result will be used in fixing the model and for planning the development wells. The whole result, even the exploration and exploitation activity will be compiled as a comprehensive evaluation for updating the conceptual model as basic for the development of the field. This simple method is shown in Figure 2.

Figure 2. Field review flow diagram (modified from [4]).

2. Geoscientific Data

Sumatra Island is influenced by the convergence of two plates, i.e., the Indian Plate beneath the Eurasian Plate. The convergence of these two plates is accommodated by oblique subduction. The result of oblique subduction is depicted on the surface by the NW-SE dextral strike-slip of Sumatra Fault Zone (SFZ). The fault zone forms the NW-SE Barisan Range of Sumatra Island that separates the back-arc basin to the east and forearc basin to the west [5].

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Figure 3. Tectonic setting of Hululais (modified from [5] [6]).

Hululais area lies in the southwestern margin of dextral strike-slip fault couples of Ketaun Segment to the north and Musi Segment to the south as a part of the Sumatra Fault Zone (Figure 3). These segment movements form a basin with approximately 15 km length and 5 km wide. Hululais is one of thirteen geothermal systems situated in pull-apart basins in Sumatra Island which has the special tectonic setting where the major strike-slip Sumatran fault and volcanic zone coexist [5]. The movement of SFZ also provides pathways for magma that resulted from partial melting as the product of the oblique subduction.

Therefore, most major volcanoes in Sumatra Island are located parallel to SFZ and near the fault zone, which HGF included. Boundary normal faults of pull-apart basins play an important role as major discharge zones for geothermal fluid, because the extensional stress is concentrated in the boundary normal faults. Some surface manifestations as indicators of the geothermal reservoir are found, i.e., fumarole, mud pool, hot springs, and surface alteration.

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2.1. Exploration stage

2.1.1. Geology. From geological study, there are six Crown units varying from old to young, i.e., Bukit Resam, Suban Agung, Bukit Beriti, Bukit Gedang, Bukit Lumut and Bukit Pabuar which each of unit is comprised of rocks/deposits from one eruption point or more. There are seven Hummock which each of Hummock is the eruptive material of a particular Crown on its main crater or the flank and mostly composed by homogeneous volcanic rock/deposit (e.g., Parasitic cone, lava cone, lava dome, etc). The lithology types of Crown and Hummock in Hululais can be seen in Figure 4.

HGF has three main quarters of volcanism that compose the volcanostratigraphy, from old to young, namely Bukit Suban Agung, Bukit Beriti, and Bukit Gedang. Suban Agung Volcanoes that collapsed and produced debris avalanches characterized by a collapsed amphitheatre rim and Hummock topography on the avalanche deposit (Sadp 1, Sadp 2, Amap and Cmap). Those volcanoes are influenced by the NW-SE major dextral strike-slip fault of two segments from the SFZ, namely Ketaun and Musi Segment. These segments activate the magma chamber in forming these three volcanoes in the southwest side of Musi Segment and a row of mountain in the northeast side of Ketaun Segment.

Between these segments is formed a depression about 15 x 5 km square wide which is known as pull- apart basin.

Figure 4. Geological map of Hululais (modified from [7]).

HGF mostly lies in the southwest side of Musi Segment as NW-SE major dextral strike-slip fault.

There are at least three main trending geological structures direction, specifically North West-South East (NW-SE), North East-South West (NE-SW) and North-South (N-S) patterns. From satellite images some lineament structure. The NW-SE lineaments orientation are stepping Suban Agung Fault faults, Nibung Fault and Semelako Fault which is interpreted as Musi Segment. The N-S lineament orientations are Cemeh Fault, Gregok Fault and Nusuk Fault. The NW-SE lineament orientation, i.e., Manghijau Fault. All of these faults are a minor fault of Musi Segment that formed by N-S compression stress from subduction Australian-Pacific plate movement.

The Suban Agung Faults in the south part are proven by total lost circulation while drilling wells of HLS-A1 and C1 which show good permeability. But one well of HLS-B1 in the north part has no lost circulation while drilling.

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Figure 5. Distribution of surface manifestation in Hululais.

2.1.2. Geochemistry. The NW-SE to N-S structure and NW-SE structures coincide with surface manifestations and may play a role in fluid flow to the surface. There are three main manifestation types, i.e., Sulphate, Chloride and Bicarbonate type, that are seen in Figure 5. The Sulphate type is found in Suban Agung at southern part as fumarole, about 1500 masl elevation. The Chloride type is found in Semelako at the northern part as a natural chloride-bicarbonate hot spring, about 400 masl elevation and the Bicarbonate type is found in Suban Telbei (Bukit Cemeh) and Suban Salok. Chloride-Sulphate water is found in Suban Agung Hilir, close to Suban Agung.

2.1.3. Geophysics. Three maps are produced from the gravity survey that was conducted in 2013 by PT PGE. A first order of polynomials was used to produce a regional map for knowing the deep regional structure. By subtracting the Bouguer anomaly and regional map anomaly, the residual map anomaly can be defined for knowing shallow bodies and structure that are indicated by high and low anomalies.

Figure 6. Gravity anomaly map; a) CBA, b) regional and c) residual.

In Figure 6b, the regional map shows contrast between high and low gravity anomalies in the center part of study area in trending NW-SE direction which coincide with the regional fault structure of SFZ.

By SVD analysis, the value of the absolute maximum SVD (1.7) is much higher than the absolute minimum value (0.9). Thus, it was concluded that the fault type of the model was a normal fault and the angle of dip was about 80^o [8]. The low anomaly in Bukit Gedang is suggested as deep altered formation

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in shallow depth. The low anomaly in the manifestation area (Suban Agung, Suban Gregok, Suban Salok and Semelako) are correlated with alteration formation.

The magnetotelluric study produces resistivity anomaly maps in which the resistivity values are reflected as conductor for low resistivity and resistor matter for high resistivity. In Figure 7, the horizontal section in some depth indicates the gradually changing resistivity anomaly. The 1000 meter depth section shows the NW-SE elongated pattern of conductive zone whose low resistivity to <10 Ohm-meter in the centre part of the study area in Suban Gregok – Suban Nusuk . This conductive zone becomes more resistive in the 750 meter depth section, even becoming resistive completely in the 250 meter depth section.

Figure 7. Resistivity anomaly maps in some horizontal section in 1000 msl (a), 750 msl (b), 500 msl (c), and 250 msl (d) (modified from [9]).

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Figure 8. Resistivity anomaly map of Hululais in vertical section [9].

The resistivity anomaly model in the S-N vertical section shows 800 metres of conductive layer respectively in the southern side in Suban Agung area which thickens gradually to the northern side in graben. The beyond conductive layer is the resistive layer which is the peak of the dome shape below the Suban Agung area (see Figure 8).

Figure 9. Micro-earthquake map of Hululais,

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Figure 9 shows the epicenter and hypocenter of microearthquake that recorded in 2012 and 2019.

Commonly the events are detected in surround of Musi and Ketaun Segment as part of SFZ. There are more many events are found in the southwestern area of Musi Segment which it is the study area compare in the south-eastern of Ketaun Segment. In vertical section shows the distribution of events have dipping pattern.

2.2. Exploitation Stage

2.2.1. Borehole Geological Data. Twenty one production wells have been drilled since 2015 as development of three exploration wells previously. There are ten wells as production wells, eight wells as injection wells and 6 wells as monitoring wells. The well targeting of exploration wells is based on the boundary of resistivity anomaly of MT and supported by other exploration surveys made in the tentative conceptual model. Then, the development wells are drilled based on the result of previous drilling and the updated conceptual model. The target production wells are directed toward the upflow zone in the Suban Agung area and the injection wells are toward the outflow zone in Suban Gregok – Bukit Cemeh (see Figure 10).

Figure 10. The well location and direction map of Hululais.

Integrated borehole logs as made by compiling the data from cutting, coring, wireline logging and drilling parameters. Characterization, categorization, and zonation process were performed by grouping the rock facies based on its physical properties [12]. There are five types of facies. From shallow to deeper, i.e., Upper Suban Agung Volcanic (USAV), Lower Suban Agung Volcanic (LSAV), Hululais Volcanic (HV), Hululais Granodiorite Intrusions (HGI), and Seblat Metasediment (SM).

In the geothermal system of Hululais, USAV formation represents medial facies of a stratovolcano

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its low permeability rate in its physical rock features. Mineralogy composition in this formation is dominated by smectite, kaolinite, zeolite, chalcedony, and sulphur.

LSAV is situated in the proximal part which associates with a transition zone of Hululais system since smectite content significantly decreases, while the first discontinue epidote appears indicating the top of the reservoir zone. This formation consists of intercalation of andesite lava, andesite breccia, and lithic tuff. Typical secondary minerals in this formation are epidote, albite, adularia, illite, and chlorite.

Mixed layer clay (MLC) in this depth was identified as smectite – illite type.

HV is the reservoir rock of the geothermal system in the upflow area, beneath Mt. Suban Agung.

HV is identified as andesitic lava with trachytic texture, and ignimbrite. It has high-temperature indicator minerals that consist of euhedral epidote, anhydrite, actinolite, and secondary biotite. Chlorite and illite clay are formed in this zone. The fragments of Seblat Metasediment (SM) are found in HV formation as thin lenses. Hululais volcanic has an excellent permeability rate which is related to some trending NW- SE and NNW-SSE faults, i.e., Suban Agung Fault, combined with primary permeability from its rock type that encountered by wellbore in cluster A, C, E and G (see Figure 10).

HGI is a massive batholite body beneath Mt Suban Agung crater, Mt Gedang and Mt Beriti that role play as the reservoir rock of the geothermal system in the upflow area as well. HGI is intensively fractured by the Suban Agung Fault that is encountered by wells in cluster A, C, E, G and H, so total loss circulation (TLC) zones are generally found in this Granite-Granodiorite intrusive rock body. Core, borehole image, and wireline logging data substitute the absence of cutting. The contrast of resistivity value and static image colour in borehole image can be used accurately to identify this formation in the TLC zone. HGI has an extremely higher value in resistivity and slightly brighter colour expression in borehole static image, compared to other rock types.

SM is the only subsurface rock formation with known relative age. This formation is older than the volcanism in Hululais. Hence, SM is interpreted as basement blocks fragments that were uplifted when the volcanism activities in this area. This formation consists of discontinue lenses slate of metasediment rock that are found in wells of clusters A, C, D, P and Q (see Figure 10) in lower Suban Agung Volcanic (LSAV) and Hululais Volcanic (HV) formation layers.

2.2.2. Production Geochemistry. Geochemistry of reservoir fluids was taken from the wells which in the reservoir water is neutral chloride as parent fluid with chloride containing about 5000 – 8000 ppm.

It shows in the Enthalpy-Chloride graph in Figure 11. It is seen clearly that the mixing process occurs along the mixing line of parent fluid from wells and surface water from manifestation. Manifestation of Semelako hot spring in the northern part shows less mixing with chloride contained in the range of 500 – 2600 ppm compared to another manifestation on the southern part with maximum chloride containing 5 ppm. The exception is seen in the hot spring of Suban Agung Hilir 1 which has high chloride and contains about 800 ppm.

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Figure 12. The gas sample in FT-HSH diagram

(circle symbol as fumarole and square symbol as wells sample)

The gas sample from the fumarole in Suban Agung and Suban Gregok then plotted in FT-HSH (see Figure 12) diagram indicates above zero fluid fraction line, it means that the fluid in the reservoir has steam. But the gas sample from the wells indicates below zero fluid fraction line, it means that the fluid in the reservoir is compressed liquid.

3. Discussion

The updated conceptual model of Hululais Geothermal System is the result from the integration studies of geology, geochemical, geophysical, exploration and exploitation/development drilling, see Figure 13.

Four main factors that form a geothermal system are cap rock, heat source, temperature, and permeability.

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Granite-granodiorite intrusion beneath the mountain ranges of Suban Agung, Beriti, and Gedang is interpreted as the heat source of Hululais Geothermal System which this intrusion is associated with regional pluton rock formation from Sumatra basement. The intrusion heated the fluids in the reservoir and fluids through the fracture zone form surface manifestation as fumarole, mud pool, hot springs and ground alteration. The existence of heat source was encountered by the borehole of wells in cluster A, C, E and G.

The heat influences the rock to form a conductive clay layer as well as cap rock. This cap rock feature is seen clearly from MT anomaly with low resistivity <10 Ohmmeter. From borehole geology shows that this cap rock has high smectite content.

Studies of geology in the Hululais area suggest the evolution of the geothermal system is affected by a transtensional tectonic regime of Musi and Ketaun Segment as part SFZ which the transtensional regime manifests itself as a transtensional pull-apart like opening basin structure. The structural map of the field shows a pattern with dominantly NE-SW trending fault planes, i.e., Suban Agung Fault, and some across faults, i.e., Cemeh, Semelako and Gregok Fault, as permeability control and maintains flow paths for geothermal fluids.

The Suban fault is encountered by wells in cluster A, C, E, G and H. The permeability in the well C and E is good permeability that indicated by kh-value from 15 to 30 Darcymeter but in the well in cluster H shows low permeability with kh value less than 5 Darcymeter [13]. Probably the lowest permeability in cluster H is caused by the secondary mineral filling. The Gregok fault that were encountered by well in cluster P has low-medium permeability with a kh value range in 5 to 10 Darcymeter. The same condition with the wells in cluster D that encounter the Cemeh fault.

The Sulphate water found as fumarole in Suban Agung in the southern part is interpreted as the upflow zone and the outflow zone is situated in the northern part of Semelako area as chloride- bicarbonate water hot springs which the chloride is flowing from deep reservoir. The sulphate- bicarbonate water is situated in the area between the upflow and the outflow which is steam-heated water. The exception is seen in hot spring of Suban Agung Hilir 1 which has high chloride and contains about 800 ppm. This condition is interpreted that chloride presence in hot springs is controlled by the deep fault of the Suban Agung fault because there is no indication of chloride gas found in reservoir fluids samples.

By plotted in FS-HSH diagram in Figure 12, the gas sample taken from the fumarole and the wells indicate that the reservoir is a water/liquid dominated system. Reservoir has high temperature reaches 300^oC in the southern upflow zone and high permeability that shown by the high kh-value in cluster A, C, E and G. Meanwhile in the northern outflow zone has a temperature below 215^oC that encountered by wells of B1 and D1 which the permeability is very low.

4. Conclusion and recommendation 4.1. Conclusions

භ Hululais Geothermal Field (HGF) is one of the water/liquids dominated geothermal systems in Sumatra that has a high reservoir temperature reaching 300^oC and neutral fluids. It is situated in a cluster volcano of The Musi Segment part as part of pull-apart in the southern side of The Ketahun-The Musi Segment.

භ The upflow zone as the main reservoir is situated in the southern high terrain of Beriti-Gedang- Suban Agung area and the outflow zone lies in the northern low terrain of Semelako area as a basin margin of pull-apart basin of Musi – Ketaun Segment in SFZ.

භ The high permeability in the upflow zone is controlled by the Suban Agung fault as the main fault in NW-SE direction and low permeability in outflow zone is controlled by the Cemeh and the Gregok fault as secondary fault in N-S direction and probably caused by the secondary mineral filling.

භ The magnetotelluric, gravity and microearthquake method are effective in the identification of reservoir boundaries that are proven by well drilling. The upflow zone is proven by the borehole of wells in cluster A, C, E and G and the outflow zone by wells in cluster B, D, P and Q.

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4.2. Recommendation

The permeability as one of the significant factors in geothermal system development needs to be identified accurately. It is important to find the appropriate method to identify subsurface permeability distribution to make it more accurate in well targeting.

Acknowledgments

The authors thank PT Pertamina Geothermal Energy for permission to publish this study in the 11^th ITB International Geothermal Workshop 2022 (IIGW 2022). Thanks and appreciation for PGE’s Exploration and Exploitation Division in supporting the data, information, and discussion.

References

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Geology Fault Network of Hululais Geothermal System, Bengkulu, Indonesia, Proceedings 42nd New Zealand Geothermal Workshop, 24-26 November 2020.

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Volcanostratigraphy of Hululais geothermal field, Bengkulu, Indonesia. Proc. 42th New Zealand Geothermal Workshop, 24-26 November 2020

[8] Sastranegara, T., Nainggolan, S. S., and Raharjo I. B. (2015): The Application of a Triangular Mesh for Gravity Inversion to Reconstruct Subsurface Geological Structures in the Hululais Geothermal Prospect. Bengkulu. Proceedings World Geothermal Congress. Melbourne.

Australia. 19-25 April 2015.

[9] PT Pertamina Geothermal Energy. (2014): Feasibility Study of Hululais Development Geothermal Field Project – Unit 1 and 2 with 2 x 55 MW Capacity. Internal Report.

[10] Juanda, A. A., Wardhani, A. D., and Raharjo, I. B. (2015): Microearthquake (MEQ) Investigation Reveals the Sumatran Fault System in Hululais Geothermal Field, Bengkulu, Indonesia, Proceedings World Geothermal Congress, Melbourne, Australia, 19-25 April 2015

[11] PGE internal report. (2019)

[12] Nusantara, V. D. M., Pratama, G. P., Nurseto, S. T., Arifin, M. T., and Thamrin, M. H. (2020):

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[13] Arifin, M. T., Nurseto, S. T., Nusantara, V. D. M., Pratama, G. R., Marastio, F. E., and Thamrin, M. H. (2020): Feedzone Evaluation in Hululais Geothermal Field, Bengkulu, Indonesia, Proceedings 42^nd New Zealand Geothermal Workshop, 24-26 November 2020.

From Exploration to Exploitation

IOP Conference Series: Earth and Environmental Science

Hululais Field Review from Geoscientific Exploration to Exploitation Stage

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Hululais Field Review from Geoscientific Exploration to Exploitation Stage