Structural Model Update on the Patuha geothermal system, West Java, Indonesia

The Patuha geothermal field is a steam-dominated geothermal system lying on a quaternary volcanic unit in West Java Province, Indonesia. The existence of manifestations in this field is interpreted from the structure control and rock deformation that generate steam production in reservoir zone. This updated study is aimed to further describe the geometry and kinematics of the geological structures model to estimate the permeable areas. The model also will explain the link between the larger tectonic framework and the detailed structure model scale in Patuha. The analysis consists of reassessment the LiDAR data, surface structure features measurement from fieldwork, and review the updated subsurface data such as feed zones, new borehole images, and re-visit the geophysical data such as microearthquakes and gravity. The surface structural interpretation is drawn from a larger area to understand the regional tectonic framework and the relation with small detail from faults and non-tectonic features. Later, the new structural framework from surface data is correlated with subsurface data that indicates permeable features, including feed zones and conductive fractures, then coupled with the geophysical data. The latter part, which concerns validating surface fault traces using subsurface data, is currently being developed and will be included in the subsequent paper. An integrated review of the geological structure of the lineament found that a strike-slip system developed in the larger Patuha area with the main fault trending NE-SW, while the accompanying fault is en-echelon faults trending relatively NNE-SSW.


Introduction
The Patuha Geothermal Field is located in West Java, about 50 kilometers southwest of Bandung.The field is situated within a northwest-trending volcanic mountains range, such as Mt.North Patuha, Mt.South Patuha, Mt.Urug, Mt.Puncaklawang, Mt.Pungkur, and other surrounding peaks [1].Surface thermal manifestations distributed in Patuha field are fumaroles at K. Putih, K. Cibuni, and K. Ciwidey, thermal springs are located on the south, west, and northwest flank of the volcanic highland (Figure 1).
Previous structural interpretation was conducted in 2016 [2] and 2020-2021 prior to drilling campaign [3].However, the latest structure exhibits distinct tectonic regimes on a larger scale that does not correspond to the deformation history of field-scale structures.Thus, some initiative was done to reassess both the existing and new data, and try to link with the regional structure scale for the structure model and interpretation.The current focus of study was concentrated on characterizing faults from LiDAR and surface grounds based on regional and detail scales, which subsequently will be integrated with subsurface data.These structural measurements are important parts of the field data and will be analyzed based on the present-day tectonic model.However, fracture measurement is limited by the accessibility and quality of the outcrops.The number and uneven distribution of data points compared to the scale of the mapped area can be compensated for by the diversity of the regional dataset.These datasets are correlated with field data points, allowing for consistency of interpretation within areas where direct observations are unavailable.

Methodology and Data Availability
DTM airborne LiDAR and surface geological mapping coupled with drilling and geophysics data has been reassessed to build the structural framework of Patuha geothermal field and its surrounding area.In this study, an integrated method of fault interpretation utilizing surface and subsurface data was performed allowing holistic field scale structural frameworks consistent with the regional tectonics setting in West Java.The detailed workflow can be seen in Figure 2.This analysis also involved several new data from Patuha Unit-2 drilling campaign, including the feedzone location from wells.Some notable fractures from borehole images may be considered in this study but due to operational problems, most of the new wells were uncovered with the data.

Remote Sensing Analysis and Interpretation
The main purpose of remote sensing analyses is to evaluate the fractures zone or faults from the lineament using LiDAR data and the relationship that might exist between the observed field data as well as geothermal surface manifestations or alteration patches.Structural lineament mapping consists of scanning any linear structure observed on the processing images.This structure indication is based on the direct tracing of lineaments detected according to various directions of filters [4].
For the first step, ridge and drainage lineament analyses were used for evaluating lineament patterns in Patuha as input for the next analysis.Ridge lineament analysis shows that Patuha area has major NE-SW and relative N-S trends, which control structural deformation in Patuha, similar to the result of the drainage analysis, see the detail in Figure 3.
Then, the lineament analysis was done by comparing Digital Elevation Model (DEM) surface from LiDAR with eight different hill shade angles, N0°E, N45°E, N135°E, N180°E, N225°E, and N315°E in azimuth with 45° angle of altitude.The eight hillshade angles are expected to diminish doubts from one angle to another.If the lineament is still observed in other different shade angles, then the lineament existence will be considered major patterns.This alignment interpretation from LiDAR will subsequently be overlaid with other analytical methods.Based on the result, there are three major patterns observed in this method.

Surface Mapping
Patuha surface mapping was conducted by Gadjah Mada University (UGM) in 2019 with 145 observation points, 48 outcrops with structure geology features observed, such as slickensides, shear fractures, brecciations or fault alignments, minor fault planes, and joints.Geometry and kinematic analysis were calculated to observe the fault existences, their strike-dip, and fault regime.The study findings suggest NE-SW trend has transtensive (normal fault/dip slip dominant movement) with dip angle around 60-80°.However, the interpretation is considered inconclusive due to the limited data available to confirm the measurement [3].
The following study on surface structural geology features was conducted in 2022 with 127 observation points, 14 outcrops with structure geology features, and 34 altered rock locations observed.Geometry and kinematic analysis were calculated in this study to determine potential fault existence and fault trends in Patuha.
The result shows that structural features in the study area can be classified as fractures and faults commonly found as dominantly planar shear fractures.Most fractures have gentle-steep dip angles, predominantly by shear fractures.Meanwhile, the azimuth is slightly scattered and has several peak trends around East and South direction (Figure 5).Kinematics analysis in Figure 6 shows that NE-SW, NW-SE, and relatively N-S trends are present in Patuha.

Surface data integration
From two surface data analyses utilizing structure geology features from field mapping and remote sensing interpretation, Patuha area is structurally controlled by at least three major fault patterns, NE-SW trend that bounding the field, relative N-S trend located within the area, and circular patterns in the northern part of Gunung Patuha.
In comparison with the regional scale in West Java, the NE-SW and relative N-S trend is aligned with relative N-S Java's main regional horizontal maximum compressional stress [5].Whilst, NE-SW and relative N-S trends possibly created from strike-slip deformation [6], see Figure 7.The circular pattern in the northern part of Gunung Patuha does not match the current structural arrangement in Java, so it is suspected most likely caused by local deformation or non-tectonic events around Gunung Patuha.Based on the comparable model from [7], the structure model in Patuha is found to be a sinistral strikeslip system developed in the larger Patuha area with the main fault trending NE-SW, while the extensional duplex of accompanying fault is en-echelon faults trending relatively NNE -SSW lying within the geothermal system.
Later, these major patterns will be overlayed with subsurface data such as gravity data, MeQ data, and feedzone data from Patuha's drilling activity to be analyzed further for the presence of the effective fracture permeability related to the structure model.

Subsurface Data 4.1. Geophysics Data
The microearthquake MeQ survey was conducted in 2019 over 1 year period.The result is from the eight seismometer stations captured more than 1000 events, then based on the even location, the cluster density map was made to see the event density population (Figure 8).In this study, MeQ supports surface data interpretation of which areas have intense movements that are related to structural geology.Later, MeQ density data will be divided into several intervals to give better interpretations.
The dense MeQ clusters started to be seen around -250 mASL up to 750 mASL.This elevation range corresponds to the area over which the feedzones are distributed.It means that the MeQ distribution were form from the reservoir activities.Subsequently, some of the MeQ clusters at 250 mASL show intense movement focused on the area within N-S pattern of the en-echelon structures.Therefore, these events may indicate that the fluid flow movements along the fractures are well distributed along the N-S fracture/fault trend.
Then it concluded that all MeQ density data show that the intense movement of Patuha area focused on in between of N-S pattern (Figure 9-10) consistently within the interval around 250 m and confirms that Patuha's N-S pattern could exist.
In the other side, the gravity data is used to see the influence of the density contrast related to the deep or shallow structure that may align with the surface interpretation.In this study, the regional anomaly, complete Bouguer anomaly and residual anomaly gravity from 150 stations that were acquired in 2020 are used for the analysis.
Based on the regional gravity trend in Figure 11, shows that density contrast is aligned with the NE-SW regional trend as it is similar to the tectonic model in Patuha.It can be seen around the northern part of Gunung Patuha area with lower gravity anomaly to the existing of Ciwidey area with a higher gravity anomaly.While, based on the complete Bouguer anomaly (CBA) and residual anomaly, some of the higher gravity values on Eastern part are separated along the en-echelon of the NNW-SSE trend faults in the middle of the geothermal system.Therefore, the separation may be related to the extension and duplexing of these en-echelon systems.In summary, the regional structural geology framework on Patuha has a strong alignment with the gravity data pattern.Further studies are still in progress since the drilling campaign still completed around 7 wells with the remaining wells to be drilled in 2023 -2024 campaign and the final evaluation will be updated once all the data is collected for interpretation.

Conclusion
The current finding of the study by an integrated evaluation of structural geology framework using data from field measurements, LiDAR lineament, geophysical data, and drilling data in Patuha, demonstrate that the field is controlled by NE-SW, N-S trends and circular structural patterns in the northern part of Gunung Patuha.The model is similar to strike-slip systems developed by [7] with the main fault trending from NE-SW, while the accompanying fault is en-echelon faults trending relatively NNE-SSW.Meanwhile, subsurface data, such as geophysical interpretation, also support the structure model, particularly in regional trends that are dominant with NE-SW direction and NNW-SSE trend in the middle of geothermal system.The separation may be related to the extension system from strike-slip regime.While MeQ data doesn't clearly represent the interpretation, the densely clustered events are commonly located within the N-S of en-echelon system.Additionally, it may also be influenced by the steam production from existing wells.Further study is still being conducted to investigate from advanced processing to strengthen the interpretation.Lastly, the hard data from feedzones shows the dip angle of faults may confirm the hypothesis of strike-slip regime which has a common steep dip angle.The conclusion from subsurface data shows the effective fracture/permeable fracture is well distributed in between the en-echelon fault system with a specific fault angle or elevation that can be used for welltargeting input.
Structural geology frameworks will continuously be updated, taking account into new data from new drilling activity, including borehole scale fracture analysis from images logs or other data related to this research to make it more robust models.

Figure 1 .
Figure 1.Patuha field location with Digital Elevation Model (DEM) from LiDAR as background.

Figure 6 .
Figure 6.Kinematic analysis shows NE-SW, NW-SE, and relatively N-S trends exist in the Patuha, purple dots represent the measurement location.

Figure 12 .
Figure 12.Vertical feedzone distribution in Patuha from existing and new wells.