GRC Transactions, Vol. 41, 2017

Insights Into the Apas Kiri Geothermal System from AK-1D – ’s First Geothermal Exploration Well

Ryan Libbey1, Ian Bogie1, Wong Fui Peng2, Jonathan Duasing2, Andryno Payer2, Zafirah Zamhari2, Daniel Lojingau2, Luis Urzua1, Greg Ussher1, Jane Brotheridge1 1Jacobs, 2Tawau Green Energy

Keywords Borneo, Slim well, Hydrothermal Alteration, Waning Volcanic Center

ABSTRACT

This paper presents the findings of the first geothermal exploration well in Malaysia, which was drilled at the Apas Kiri prospect in , on the island of Borneo. The well was drilled vertically to a depth of 1449 m and encountered bottomhole temperatures of ~200°C. Petrographic, XRD, and fluid inclusion analyses of the core from AK-1D have revealed a prograding sequence of hydrothermal alteration from argillic to phyllic assemblages, providing indications that the geothermal reservoir is associated with near-neutral pH, low-moderate salinity fluids. The observed thickness of the argillic zone in the downhole samples is in close agreement with the thickness of the <20 ohm-m conductor in the 3D MT inversion model – a valuable verification that has helped to refine the conceptual model of the system by extrapolating the initial results to other parts of the prospect. The available data suggest that AK- 1D intersects a deep outflow of the Apas Kiri geothermal system, and that temperatures of >200°C are likely to be encountered in the central upflow region closer to the summit of Mt. Maria. The findings from Apas Kiri may serve as a useful analog when prospecting for other geothermal systems situated around waning volcanic centers.

1. Introduction Slim well AK-1D is the first exploration well for the Apas Kiri geothermal prospect in SW Sabah, Malaysia, on the island of Borneo. This operation also represents the first geothermal well drilled in Malaysia. The well is situated on the southeast flank of the Mt. Maria at an elevation of +590 mrsl. The well was vertically drilled to a total depth of 1449.38 mMD (relative to ground level; -859.38 mrsl). The upper 310 m of the well were drilled using percussion-air and tricone methods and the remainder of the well was drilled using a diamond core bit. Libbey et al.

Well AK-1D was drilled to assess the commercial viability of the prospective Apas Kiri geothermal resource. Some key goals of the well were to: • To provide further verification of reservoir temperatures, pressures, and chemistry; • To better understand the lithologies and stratigraphy that host the geothermal system; • To gain insight into the hydrothermal alteration systematics at depth; • To provide a cross-correlation of hydrothermal alteration systematics with resistivity aiding future well targeting initiatives; • To gain insight into shallow, intermediate and deep aquifers to refine the conceptual model; • To gain insight into permeability characteristics of the reservoir; • To gain insight into the system evolution by comparing information from alteration mineralogy with measured static equilibrium conditions; • To gain insight into local stress regime and structural controls on permeability; • To aid in the refinement of a development plan for the geothermal resource.

2. Geologic Setting The Apas Kiri geothermal project is located at the southwest corner of Sabah, Malaysia, on the Island of Borneo (Figure 1). Northern Boreno is situated on an ophiolitic basement of Cretaceous to Eocene age. The region has experienced two phases of volcanism (Balaguru and Hall, 2009). The first phase was associated with the subduction of the Proto China Sea beneath present day northern Borneo in Late Eocene to Middle Miocene, which resulted in the formation of a near the Semporna peninsula. The direction of subduction changed in the Middle Miocene as the plate began to subduct beneath the Semporna and Dent peninsulas. This NW-directed subduction generating the NE-trending arc of Miocene to Quaternary /dacitic volcanic centers that comprise the Dent and Semporna peninsulas. On the Semporna Peninsula these centers include (from west to east): Mounts Magdalena, Lucia, Maria, Wullersdorf, and Pock. The influence of fluids derived from the subducted lithosphere diminished from Mount Pock, through , to the Dent volcanic centers (Hutchinson, 2005). This is consistent with the Dent volcanic center lying farthest away from the volcanic front and suggests that the Miocene volcanism of SE Sabah resulted from NW-directed subduction of the Celebes Sea from a trench lying parallel to the Sulu Archipelago. The present-day maximum principal horizontal stress in this part of Borneo is expected to be WNW-directed. This stress is largely accommodated by regional-scale NW-striking sinistral faults (Hall and Wilson, 2002; Hutchinson, 2005). The Apas Kiri geothermal system is expected to be associated with a heat source beneath the Pleistocene Mt. Maria volcano (youngest age 27 ka, 14C; Siong et al., 1991). Recent(?) monogenetic basaltic centers, including Gunung Bomabalai and Quoin Hill are situated around the base of the volcano (Figure 2). The geothermal system is manifested at the surface by neutral pH Na-Cl warm (<60°C) and hot (max of 78°C) springs (with appreciable SO4 and HCO3; “Conical/Terrace” springs), steam-heated sulfate warm springs (“Upper Tawau” and “Balung” springs), travertine cones and terraces, and altered ground (Table 1 and Figure 2). The results of the initial surface exploration of the resource are summarized in Daud et al. (2010) and Barnett et al. (2015). Libbey et al.

Figure 1. Geology of Sabah, Malaysia, showing the location of the Apas Kiri geothermal field. Map redrawn from Balaguru and Hall (2009) and overlain on a DEM created from the SRTM dataset.

Table 1. Generalized representative chemistry of thermal springs associated with the Apas Kiri geothermal system (from data presented in Barnett et al., 2015 and supplied by Tawau Green Energy). Representative Thermal Spring Parameters/Chemistry Elev. Temp pH Ca Mg Na K HCO3 SO4 Cl SiO2 Name mrsl °C ppm ppm ppm ppm ppm ppm ppm ppm Conical 190 75 7 210 18 840 90 750 320 1300 84 Terrace 170 68 6.5 210 18 820 85 720 300 1270 84 Balung 250 56 6 440 22 32 - 70 1050 1 34 Upper 300 34 4.5 206 8 19 - 20 490 11 20 Tawau Libbey et al.

Figure 2. Lithology and surface manifestation map of the Apas Kiri geothermal field showing the location of slim hole AK-1D. Figure 6 cross-section location shown as dashed line.

3. Temperature and Pressure Measurements Interim downhole temperature surveys were conducted during pauses in the drilling of AK-1D using a HOBO U12-015 temperature data logger and maximum registering thermometers. These surveys were conducted after heating intervals of 1 to 5 days of shut-in conditions. The HOBO U12-015 temperature logger only provided data until 1030 mMD, after which the device’s auto shut-off limit of 150°C was exceeded. Thermochem conducted a formal PT log roughly three months after the well completion. Data collected from maximum registering thermometers, the HOBO data logger, and the formal PT tool were in fairly good agreement with one another, however the formal temperature profile for the upper 1300 m of the well is shifted by +10 to +20°C in comparison to the other data sets as a result of the longer heating period (Figures 3 and 4). The data from the formal PT after 3 months of heating is believed to represent static subsurface conditions. Libbey et al.

The following conclusions are derived from the obtained pressure/temperature data for AK-1D: (1) A ~60°C warm water aquifer is present directly at the water table depth of 260 mMD and extends down to ~340 mMD (note that these warms fluids were flowed to the surface while air- drilling through this section). The inversion of the temperature profile in this section suggests that this is an outflow of potentially steam-heated fluid; (2) From ~400 to ~1100 mMD, the temperature follows a consistent 15°C/100 m conductive gradient to ~1100 mMD; (3) the gradient switches to near isothermal conditions from ~1100 to ~1200 mMD; (4) the temperature from ~1200 mMD to the base of the well (1449 mMD) increases with a gradient of 8°C/100 m; (5) the highest recorded temperature in the formal PT log is 196°C (203°C was recorded by maximum registering thermometers), which occurs at the base of the well. The steeper thermal gradient that begins at ~1100 mMD likely represents a transition to more convective conditions at these depths – a notion that agrees with observed losses during drilling, open fractures, and nature of hydrothermal alteration, and the top of the reservoir as suggested by the base of the conductor in the 3D MT inversion model.

4. Lithologies and Stratigraphy The lithologies encountered from the surface to 442 mMD (vertical meters below the rig floor) include andesite and dacite extrusives (i.e., lavas) and minor crystal tuffs. The dacites occurring at the surface and down to a depth of 110 mMD likely belong to the Pleistocene-aged Maria Dacite (Figure 2). Those occurring in alternating layers with andesite extrusives down to a depth of 298 mMD may belong to the Miocene-Pliocene-aged Magdalena/Lucia unit, however, there is no clear petrographic distinction for this unit and radiometric age-dating of least-altered samples would be required to properly assess the stratigraphic affinity of these dacitic rocks. Poorly-sorted, polymictic, matrix-supported (mud-rich) breccias occur intercalated with andesite extrusives between 442 and 564 mMD. These volcaniclastic sequences are interpreted as lahar deposits, and it is likely that some of the “interlayered andesite extrusives” are in fact large boulders within these deposits. Some volcaniclastic intervals contain organic carbon fragments. The sequence between 564 to the bottom hole of 1449.38 mMD is dominated by plagioclase-, clinopyroxene-, orthopyroxene- (± hornblende-) phyric andesite extrusives. Numerous brecciated zones (5 to 25 m thick) are found in this section. Some of these likely represent paleo-talus deposits, whereas others are more consistent with zones of autobrecciation (i.e., andesite clasts are contained within coeval effusive volcanic rock). Rocks below 647 mMD likely belong to the Miocene-aged Andrassy Andesite. Thin clay-rich volcaniclastic units (usually <3 m thick) occur intercalated with this andesite. The top of the geothermal reservoir (marked by the transition from an argillic to a phyllic alteration assemblage) was intersected by AK-1D at approximately 1120 mMD. It is hosted in fractured and altered andesite extrusives and breccias of the Andrassy Andesite unit. Secondary (fracture-based) permeability is dominant over primary permeability in this unit, as initially it would have been poorly permeable. Partial and total losses of drilling fluid circulation corresponding to permeable zones were experienced at multiple intervals while drilling through the Andrassy Andesite.

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Figure 3. AK-1D well profile, showing lithologies, alteration zonation, results of methylene blue analyses (measurement of swelling clay%), casing layout, downhole measured temperatures, and the resistivity profile in the vicinity of the well (based on a 3D MT inversion model).

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5. Hydrothermal Alteration, Brecciation, and Veining Hydrothermal alteration in the upper 1120 m of the well is prograde with depth from a lower to upper argillic assemblage. The alteration intensity is commonly >40% and there is common to intense calcite (± quartz, ± pyrite, ± clay) veining and hydrothermal brecciation. Clay-rich volcaniclastic units have a notably lower alteration intensity than andesite extrusive units from similar depths. This phenomenon is seemingly related to the low primary and secondary permeability in the volcaniclastic (lahar) sequences. Images of the observed hydrothermal alteration are provided in Figure 5. There is a decrease in the occurrence of zeolites and smectite with depth, and an increase in chlorite, illite (or the illite component in mixed layer clays), pyrite, quartz, anhydrite, titanite, albite, and adularia (the latter three only identified in thin section). The transition from the argillic zone to the phyllic zone occurs at approximately 1120 mMD. This alteration boundary is delineated in AK-1D by the drop-off in the methylene blue index below 10% swelling clay and the concurrent increase in chlorite, disappearance of brown smectite-rich mixed-layer clays (MLC), and the prevalence of white illite clay (confirmed by XRD). There is a close agreement between the base of the conductor in the 3D MT model and the documented downhole transition from argillic to phyllic alteration assemblages in AK-1D, which occurs at a depth of ~1120 mMD (Figure 4). This validation is highly valuable for understanding the geometry of the Apas Kiri geothermal resource and will aid in future well targeting initiatives, as this correlation can be extrapolated to other parts of the system. A minor amount of fine-grained epidote is observed with chlorite as a replacement of hornblende/pyroxene in a low-permeability, moderately-altered andesite extrusive from 1256 to 1275 mMD. The present-day natural state temperature at the depth of epidote occurrence in AK- 1D is 184°C, and the epidote crystals appear to be partially replaced by chlorite. The epidote is thus inferred to be a relic phase, formed during a period when >200°C isotherms occurred at shallower depths in the system. Minor pyrophyllite, which occurs sporadically between 1334 and 1407 mMD, also appears to be a disequilibrium, relic phase. Hydrothermal mineralogy and fluid inclusion data (from quartz and calcite; Figure 4) suggest that the deep geothermal fluid at Apas Kiri is a near-neutral pH, low to moderate salinity fluid. Weakly acidic, low salinity, CO2-rich, steam-heated fluid is likely present in the upper ~800 m of the sequence intersected by AK-1D. A comparison of measured downhole temperatures with hydrothermal mineralogy and fluid inclusion data suggest that elevated temperatures above those currently measured in AK-1D may have occurred in the past. This may help to explain the thickness of the conductor (i.e., clay cap/argillic zone) in this region, which may be a result of deepening isotherms and the progressive downwards extension of argillic alteration with time.

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Figure 4. Temperature/pressure profile of AK-1D, showing downhole measurements, mineral temperature indicators, and fluid inclusion homogenization temperatures. Libbey et al.

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Figure 5. (Previous page) Core photos, binocular microscope images, and photomicrographs of core collected from AK-1D. (A) Coarse illite-smectite (75% illite interlayers) occurring in a vein from 1356.38 mMD; (B) Andesite extrusive with phyllic alteration showing primary magnetite and relic epidote being replacing by chlorite, 1260.38 mMD; (C) Pyrite precipitating at the boundary of an organic fragment in a lahar, 513.2 mMD; (D) Pyroxene phenocryst pseudomorphed by chlorite and minor epidote and prehnite, 1264.78 mMD (ppl); (E) Pinched pyrophyllite veins in altered lahar, 1334 mMD; (F) Chl-clay altered andesite with cal-qtz-chl-hem veining, 760 mMD; (G) Chl-clay altered andesite with patchy hematite overprint and late hydrothermal brecciation with cal-qtz-pyrite veining, 1080 mMD; (H) Phyllically altered andesite, 1430 mMD.

6. Structures Conspicuous shear fractures are abundant in the AK-1D core samples. Whilst oriented core was not available, the dip angle and kinematic indicators of these features and their association with veining provides some insight into the nature of these fractures in the subsurface. These features are important to understand as they provide secondary permeable pathways in the Apas Kiri geothermal system. Of significance is the common association of hydrothermal veining (quartz, calcite, clay, chlorite, chalcedony, and pyrite) with ~60°-dipping shear fractures that display normal kinematic indicators (as revealed by the orientation of slikenlines and chattermarks). The dip and kinematics of these fractures are consistent with shear fractures formed in an extensional environment, i.e., where σ1 (the maximum principle stress direction) is vertical and σ3 (the minimum principle stress direction) is horizontal. This may be an indication that the local extension, perhaps hosted in a regional strike-slip regime, may play a role in controlling the hydrology of the Apas Kiri geothermal system. Steeply-dipping (near-vertical) shear fractures that display oblique kinematic indicators are also present; however, their association with veining is less consistent. Rare ~60°-dipping shear fractures with reverse kinematics were also observed. A detailed structural study is currently being contemplated and will assist with future well targeting strategy. Formation imaging surveys are also being considered for the upcoming large- diameter wells, which will be useful for characterizing the distribution of fractures and their geometrical and kinematic characteristics.

7. Conclusions and Implications for the Conceptual Resource Model Data derived from the AK-1D exploration slim well and previous geophysical, geochemical, and geological surveys have provided valuable information pertaining to the Apas Kiri geothermal resource. As a result of AK-1D, there is now concrete evidence to indicate that the resource temperature at Apas Kiri is in excess of 200°C. Additionally, the observed alteration mineralogy in the deep (>~800 m) region of the well is consistent with that formed from water-rock interaction under near-neutral pH fluid conditions. Fluid inclusion data also provides an indication that the fluid salinity is ~0.7 to 2 wt.% (within the range of typical volcanic-hosted geothermal systems). The observed mineralogy also suggests that a weakly acidic, CO2-rich, steam-heated water is present in the shallower regions of the system. Such steam-heated fluid is common in many moderate- to high-temperature geothermal systems around the world, and is typically the product of the condensation of geothermal gases into perched aquifers. Libbey et al.

The observed transition from mixed-layer illite-smectite to end-member illite (i.e., the transition from the argillic to phyllic alteration assemblage) at ~1160 mMD in AK-1D corresponds closely with the depth of the 20 ohm-m resistivity contour in the 3D MT inversion model, i.e., the base of the conductor. The measured downhole temperatures at this depth are ~170°C. There is a distinct updoming and thinning of the conductor (the geophysical distinction of the clay cap) with an apex that is centered on the northwest region of the Mt. Maria summit caldera (Figure 6). Such conductor geometry is common to upflow zones of medium to high enthalpy geothermal systems in volcanic settings. An approximation of the system geometry and temperatures can be made by extrapolating the measured isotherms along the resistivity contours provided by the 3D MT inversion model (Figure 6). This conceptual model is supported by the chemistry of SO4 thermal features in Balung and Upper Tawau hot springs and the likely presence of steam-heated waters in the upper 800 m of AK-1D.

Figure 6. Cross-section of 3D MT inversion model for Apas Kiri, including measured/extrapolated isotherms and arrows indicating inferred fluid migration patterns in the subsurface. Cross-section location shown in Figure 2.

REFERENCES

Balaguru, A. and Hall, R. “Tectonic evolution and sedimentation of Sabah, North Borneo, Malaysia.” Proc. AAPG International Conference and Expedition, Cape Town, South Africa (2008). Barnett, P.R., Mandagi, S., Iskander, T., Abidin, Z., Amaladoss, A., Raad, R.. “Exploration and Development of the Tawau Geothermal Project, Malaysia.” Proc. World Geothermal Congress, Melbourne (2015). Libbey et al.

Daud, Y., Javino, F., Nawawi, M., Nordin, M., Razak, M., Amnan, I., Saputra, R., Agung, L., Sucandra. “The first magnetotelluric investigation of the Tawau geotermal prospect, Sabah, Malaysia.” Proc. World Geothermal Congress, Bali (2010). Hutchinson, C.S. “Geology of North-West Borneo.” Elsevier Science (2005), ISBN: 978-0-444- 51998-6, 444 p. Siong, L.P., Intang, F., and On, C.F. “Geothermal prospecting in the Semporna Peninsula with emphasis on the Tawau area.” Geo. Soc. Malaysia, Bulletin, 29 July, 1991, 135-155.