IPA14-G-068

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Eighth Annual Convention & Exhibition, May 2014

SATELLITE INTERFEROMETRY AND THE DETECTION OF ACTIVE DEFORMATION

ASSOCIATED WITH FAULTS IN SUBAN FIELD, SOUTH SUMATRA BASIN, INDONESIA

Richard A. Schultz* Xiaopeng Tong**, ***

Khalid A. Soofi* David T. Sandwell** Peter H. Hennings*

ABSTRACT Suban field in southern Sumatra, Indonesia, is a fractured carbonate/crystalline wet-gas reservoir located in a tectonically active island-arc setting. Reservoir-scale right-oblique reverse faults and folds that have trapped the hydrocarbons have been related previously to deformation in the back-arc setting of southern Sumatra associated with oblique subduction of the Indo-Australian plate at the Sunda trench (Hennings et al., 2012). Increased well productivity in some parts of the field was inferred to correlate with completing wells in the damage zones of critically stressed faults. Satellite interferometry acquired over the three-year period between mid-2007 and early 2011 involving specially stacked and filtered interferograms, following prior applications to heavy-oil fields and Arctic sea ice by Soofi and Sandwell (2010), reveals active deformation within Suban field. Several areas of localized subsidence potentially exceeding ~5 mm/yr were identified in the field. Areas of uplift and horizontal movements of comparable magnitude were resolved across the approximate position of the inferred surface trace of the major right-oblique, critically-stressed fault zone in the southwestern part of the field, corroborating wellbore-based inferences and independent observations of active surface deformation. Block-tectonic models constrained by GPS measurements across the entire Sumatra contractional orogen predict comparable magnitudes and directions for horizontal motions observed locally at Suban.

The combination of InSAR and GPS-based plate-tectonic models provides a robust tool for

* ConocoPhillips Company ** Scripps Institution of Oceanography *** Now with University of Washington

monitoring the deformation of oil and gas fields located in tectonically active areas. STRUCTURAL AND TECTONIC SETTING OF SUBAN FIELD Suban field is a ConocoPhillips gas producing asset located in the South Sumatra Basin, one of several sedimentary basins in the back-arc domain of Sumatra, Indonesia (Barber and Crow, 2005; Hennings et al., 2012). The field was discovered in 1998 and began producing in 2003. The major structures at Suban, oblique-slip faults and related folds, strike northwest-southeast, parallel to the principal Neogene tectonic fabric of Sumatra and to the subduction zone ~400 km to the southwest (Figure 1). The principal element uplifting the overall structure is a basement-rooted fault-propagation fold on the northeast flank. Damage zones of several smaller sub-parallel faults in the uplift provide good fluid-pressure communication across the field. Wells with the highest flow capacity relate to critically stressed faults and related fractures along the southwestern bounding anticline which is interpreted using 3-D seismic data and wellbore stress analysis to be the result of strike-slip and reverse offsets along the southwest bounding fault.

Comparison of stress states inferred from wellbores, seismically interpreted stratigraphic offsets, and structural architecture (in both map and cross-sectional views) with the tectonics on Sumatra and the direction of plate subduction (e.g., McCaffrey, 2009) suggested to Hennings et al. (2012) that the faults and deformation that control well productivity in Suban field are currently active and closely related to the subduction zone tectonics.

At the latitude of Suban field in south-central Sumatra, oblique subduction of the Indian and Australian plates beneath Southeast Asia occurs at a

rate of 50–70 mm/yr (DeMets et al., 1994, 2010; Prawirodirdjo and Bock, 2004). The oblique convergence is partitioned into a trench-normal component, primarily accommodated along the Sunda megathrust, and right-lateral shear accommodated primarily along the Great Sumatran Fault. A series of large to great earthquakes along the Sunda megathrust have been documented since 1797 (e.g., Subarya et al., 2006; Chlieh et al., 2008; Prawirodirdjo et al., 2010). Rupturing along these slip patches has uplifted, tilted, and deformed Sumatra during these and previous events during at least Quaternary time. At least four large-magnitude (> 7) strike-slip earthquakes have occurred on-land along the Sumatran fault (Sieh and Natawidjaja, 2000) since 1900 in the vicinity of Suban field (Sørensen and Atakan, 2008). The deformation that produced Suban field, including its stratigraphic and structural traps as well as its productivity, thus appears consistent with contemporary Sumatran seismicity and tectonics. Conversely, monitoring of deformation at Suban could potentially inform seismic hazard analysis in southern Sumatra. INSAR AND GPS IN SOUTHERN SUMATRA Satellite or geodetic data (see overviews by Segall and Davis, 1997; Bürgmann and Thatcher, 2013) are currently available for much of Sumatra, including the Sumatran continuous Global Positioning System (cGPS) array that was established and monitored by Caltech’s Tectonics Observatory through 2008 (data including station locations are available at http://sopac.ucsd.edu/ projects/sugar.html). Prawirodirdjo et al. (2010) describe observations mostly from northern Sumatra. Early work reported by Prawirodirdjo et al. (1997) showed that strain accumulation rates in northern Sumatra, 23–24 mm/yr, were comparable to those inferred from slip rates along the Sumatran fault by Sieh and Prawirodirdjo (2000), but twice as large as those along the fault in central Sumatra, the southernmost part of the geodetic survey network. Because slip rates increase along the Sumatran fault from SE to NW (e.g., Huchon and Le Pichon, 1984; Bock et al., 2003), in association with increasing obliquity of subduction in that direction (e.g., DeMets et al., 2010), the discrepancy between low measured slip rates and higher shear-strain rates in the network may suggest the occurrence of either strain accumulation or slow aseismic creep along the Sumatran fault near the latitude of Suban. Although Meltzner et al. (2006) analyzed data from the ASTER, SPOT, IKONOS, QuickBird, and Landsat satellites to investigate uplift and subsidence patterns of the northern part of Sumatra,

around the epicenter of the 2004 great earthquake, satellite interferometry (InSAR) techniques had apparently not been utilized prior to our study to measure current deformation in southern Sumatra. The present conference proceedings paper builds upon the more concise overview by Schultz et al. (2014). InSAR Analysis The synthetic-aperture radar (SAR) data (L1.0) were obtained from the Advanced Land Observing Satellite (ALOS aka Daichi; http://www. satimagingcorp.com/satellite-sensors/alos.html) via an L-band (23.6-cm wavelength) radar, which enabled temporal coherence with acceptable quality of the interferograms. Previously, Stancliffe and van der Kooij (2001) successfully used L-band radar to monitor production at the forested Cold Lake heavy-oil field in Alberta, Canada. Twenty-six raw images (19 ascending and 7 descending acquisitions) from the middle of 2007 into early 2011 were used in the present study to perform the interferometry analysis following the methods of Soofi and Sandwell (2010) and Tong et al. (2013). We combined the Line-of-Sight (LOS) motion measured in the two look directions to derive vertical and horizontal deformation maps of Suban field and vicinity. The data were processed with GMTSAR software (Sandwell et al., 2011; http://topex.ucsd.edu/gmtsar) on a Linux platform. In the interferometry analysis, the raw binary radar images were pre-processed and focused to form full-resolution SAR images. All the SAR images were then aligned to one single image with 2D cross-correlation techniques. The image alignment needed to be accurate within 1 pixel (~10 m) to construct the interferograms. The phase due to topography was removed with the shuttle radar topography mission (SRTM) digital elevation model (Farr et al., 2007; http://www2.jpl.nasa.gov/srtm). The residual phase was unwrapped with SNAPHU software (Chen and Zebker, 2000; http://www. stanford.edu/group/radar/softwareandlinks/sw/snaphu/). Twenty high-quality interferograms were thereby obtained (Figure 2). The unwrapped phase maps were individually filtered with a high-pass Gaussian filter, then stacked to yield an average LOS velocity map of Suban field. The filtering step reduces the error sources associated with any ground motion at greater than 20 km wavelength, isolating smaller-scale motions and reducing long-wavelength noise

from the interferograms. The standard deviation of the LOS velocity was calculated to evaluate the uncertainties caused by various error sources such as clouds in the troposphere, ionosphere delay, inaccurate orbital ephemeris, unwrapping errors. The standard deviations of the velocity map range from 0 to 100 mm/yr with a mean of 12 ± 10 mm/yr. This error reflects the location of Suban field in a highly vegetated tropical area. Fourteen filtered interferograms along the ascending track and 6 along the descending track were stacked to derive an averaged LOS velocity map over the Suban field, assuming the deformation rate is linear. We found that removing 6 short-time (e.g. 1- or 2-month interval) interferograms along the ascending orbits helped reduce noise in the LOS velocity map. The standard deviation of the ascending LOS velocity exceeds 20 mm/yr over certain regions in the Suban Field. This larger error is expected because Suban field is located in a highly vegetated tropical region. Areas in the LOS velocity map with large noise (i.e., standard deviation > 20 mm/yr) were masked out. The final stacked and masked LOS velocity map ranged from approximately –23 to 33 mm/yr with a standard deviation of ~1 mm/yr. All the results derived from the above analysis were co-registered in ArcGIS. Accurate registration of the ALOS interferograms was verified by overlaying a 2.5-m-resolution SPOT image. For any ground point, the LOS velocity is a projection of the east-north-up velocity components. The projection is based on the azimuth of the satellite trajectory (8° counter-clockwise from North for ALOS) and the look angle of the radar. Taking the look angle of the radar to be constant (i.e., 34° for ALOS), the ascending (Vasc) and descending (Vdes) velocities are given respectively by Vasc=0.61VEast+0.13VNorth-0.79Vup Vdes=- 0.61VEast+0.13VNorth-0.79Vup These equations were used to calculate the vertical and east components from the ascending and descending LOS velocity maps. The north component contributes only 13% to the LOS velocity. Because the LOS velocity is not as sensitive to the north component as to the east and vertical components, the north component of the velocity was neglected in this analysis. A sensitivity test was conducted to investigate how significantly assuming the northward velocity to be zero would influence the results. Instead of taking VNorth to be

zero, it was assumed in the test to be the same as VEast, so that the horizontal motion would be in the northeast direction. The result was similar to the previous results, indicating that the east and vertical rates obtained in this investigation are reliable. Calibration of the InSAR results by using GPS stations within the area of the interferograms could further refine the horizontal (VNorth and VEast) components of the velocity field. Tectonic Model Constrained by GPS The block-tectonic model of Prawirodirdjo et al. (1997, 2010) was used to investigate the regional velocity field near Suban. The model represented Sumatra by three interacting elastic plate-tectonic blocks: the Sundaland continental shelf (including Sumatra and adjacent submarine portions; Simons et al., 2007), the forearc sliver to the southwest (Diament et al., 1992; McCaffrey, 2009), and the Indo-Australian plate (Figure 3). Block motions were described as rigid-body rotations about their respective poles of rotation, and block boundaries were defined by three-dimensional fault surfaces with displacement discontinuities associated with the earthquake rupture distribution along the Sunda (or Java-Sumatra) subduction zone and the onshore Sumatra strike-slip fault zone. GPS velocities measured at stations throughout Sumatra were utilized as boundary conditions. Interseismic (post-2007) displacement vectors in the horizontal plane were calculated by using the Fortran program DEFNODE (McCaffrey, 1995) which uses the solution for dislocations in an elastic half-space. The model was verified by reproducing the results of Prawirodirdjo et al. (2010). RESULTS AND IMPLICATIONS The InSAR results suggest that uplift and horizontal contractional motions occurred between mid-2007 and early 2011 along the SW bounding fault in Suban field (Figure 4). Three areas of subsidence, perhaps as much as 5 mm/year over that interval, were inferred over Suban field. Although unlikely due to high stiffness of the producing interval, these displacements may be related to the pressure depletion of production. This potential effect has not yet been evaluated in detail. Significant horizontal contraction, and uplift, each as much as 5 mm/year over the same time interval, were also inferred in the InSAR data. The pattern of uplift rate across the bounding fault is consistent with the deformation of the ground surface above blind or surface-breaking reverse or thrust faults in

other contractional tectonic settings (e.g., King et al., 1988; Okubo and Schultz, 2004). The area of interest overlies the approximate surface projection of what may be interpreted from subsurface seismic data as a contractional oblique-slip relay zone along the major right-oblique bounding fault in the southwestern part of the field. The pattern of uplift (Figure 5a) generally correlates with the contemporary topography, as shown by a SW to NE transect (Figure 5b), implying that the current topography across this part of Suban field is related at least in part to reverse offsets along the subjacent faults (Figure 5c) and related deformation. This result is consistent with the inference drawn by Hennings et al. (2012) that major faults in Suban field are currently active and related to the subduction tectonics across Sumatra. We also modeled the interseismic displacement and strain field across Sumatra by using the block-tectonic model described above. The model predicts an average velocity of ~4 mm/year to the NE near Suban field (Figure 6). This NE-directed displacement is generally consistent with other locations across Sumatra east of the Great Sumatran Fault, although the predicted magnitudes, orientations, and rates vary somewhat depending on location. As noted by Prawirodirdjo et al. (2010), the displacement velocity vectors south of approximately 3–5°S are consistent with little contemporary coupling on the subduction zone (i.e., the megathrust is slipping aseismically there), whereas vectors between 0.5°N and 5°S are rotated counterclockwise from the Indo-Australian plate-convergence direction and were interpreted to result from a greater degree of contemporary coupling (i.e., locking) of the subduction zone over that latitude range. Prawirodirdjo et al. (2010) compared the interseismic velocity fields from before and after the subduction zone rupture sequence of 2001–2007. The main difference is a modest counter-clockwise rotation of the velocity vectors above the subduction zone, south of approximately 3–5°S latitude, and reduction in their magnitude, both consistent with a small degree of coupling since 2007 in that area. The apparent stability of the velocity vector orientations east of the Sumatra fault along with modest reductions in their magnitude suggest that the general pattern of deformation at the surface on southern Sumatra did not change appreciably as a result of coseismic slip along the subduction zone, implying longer-term stability of the local displacement orientations.

The available SAR data for Suban field were limited to an approximately 3-year observation interval. A new L-band SAR satellite, ALOS-2, is scheduled to launch in 2014. The accuracy and quality of InSAR velocity maps of Suban field and vicinity could be improved by incorporating observations from this satellite. For example, a shorter repeat time of 14 days provided by ALOS-2 could enable InSAR time-series analysis so that any temporal changes of the surface displacement could potentially be related to local deformation. Combining such InSAR measurements with independent observations from nearby GPS sites would provide a more complete and accurate description of the deformation field across southern Sumatra. CONCLUSIONS

Satellite interferometry reveals that Suban field is actively deforming in association with oblique subduction southwest of Sumatra, supporting previous inferences that were based on subsurface 3-D seismic, wellbore, and production data. Although InSAR has been used to monitor production activities at heavy-oil fields such as those in Alberta, Canada (Dubucq et al., 2008; Soofi and Sandwell, 2010; Gu et al., 2011), this study demonstrates its utility for oil and gas fields in tropical and tectonically active areas as well as its potential for contributing to seismic hazard assessment (e.g., Abidin et al., 2010) and geothermal resources (e.g., Muraoka et al., 2010) in Indonesia.

ACKNOWLEDGMENTS Lynette Prawirodirdjo, Rob McCaffrey, and Yehuda Bock advised on the block-tectonic model; Scott Keough helped with the figures. The ALOS PALSAR L1.0 data were obtained through the Alaska Satellite Facility (ASF). Wenjie Jiao kindly provided Figure 1a. ConocoPhillips, ConocoPhillips Indonesia, MIGAS, Talisman (Corridor) Ltd., PT Pertamina Hulu Energi Corridor, and PT Pertamina EP are thanked for granting permission to publish this paper. REFERENCES Abidin, H.Z., Subarya, C., Muslim, B., Adiyanto, F.H., Meilano, I., Andreas, H., and Gumilar, I., 2010, The Applications of GPS CORS in Indonesia––Status, Prospect and Limitation, in FIG (International Federation of Surveyors) Congress 2010––Facing the Challenges––Building the

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Figure 1 - (a) Digital elevation model of southeast Asia region showing tectonic earthquakes with magnitudes ≥ 5.0 recorded for Sumatra during 2013; data from

the Global Centroid-Moment-Tensor (CMT) catalog (http://www.globalcmt.org; Dziewonski et al., 1981; Ekström et al., 2012). (b) Locations of InSAR images (blue square outlines) and Suban field (red dot) shown on shaded relief map of Sumatra (after Hennings et al., 2012 and Schultz et al., 2014)

.

Figure 2 - (a) Interferograms for ascending track T442. Eight of 14 long-time interferograms are of

acceptable quality and were used in conjunction with 12 short-time interferograms in the stacking analysis. (b) Interferograms for descending track T117. Five of 7 interferograms are of acceptable quality. Figure after Schultz et al. (2014).

Figure 3 - Block-tectonic model used for GPS calculation. (a) Outlines of Indo-Australia block (pole of rotation, blue circle) and forearc block (pole of rotation, red circle). (b) Comparison of computed (red) and observed (black) GPS velocities for 1991–2007 period (velocities after Prawirodirdjo et al., 2010). Location of Suban field (right-hand panel) shown for reference.

Figure 4 - (a) Vertical and (b) Horizontal (Easting) velocity maps obtained from the InSAR analysis for

Suban field and vicinity; contours are at 5 mm/yr interval, dashed line is approximate position where the SW right-oblique fault might be extrapolated to the near-surface region. (c) SRTM topography; (d) Structure contours of Suban reservoir top at depth, corresponding approximately to the top of the pre-Tertiary units (pre-T in Figure 5) with local highs in warm colors, after Hennings et al (2012); (e) Surface geology after Suwarna et al. (1992) and Crow and Barber (2005). Note increased values of vertical velocity change (uplift) and horizontal velocity change (shortening) along SW oblique-slip fault; the location of a profile (A–B) is shown by solid line on (a–c). White dots are gas wells, black dot, oil well; North arrow and scale bar common to all panels; figure after Schultz et al. (2014).

Figure 5 - (a) Deformation associated with reverse or thrust faulting, after Okubo and Schultz (2004), showing pattern of surface uplift and subsidence associated with reverse faulting at depth; displacements calculated for fault dips of 15°, 25°, and 40°. (b) Comparison between InSAR vertical velocity change (positive values, uplift; negative, subsidence), (c) Surface elevation from SRTM digital elevation model, and (d) Geologic section across part of Suban field (after Hennings et al., 2012), part of section line A–A’ shown on Fig. 1; Qtk, Pleistocene Kasai Formation; Tma, Middle-Upper Miocene Air Benakat Formation; Tmpm, Muara Enim Formation; pre-T, pre-Tertiary crystalline and metamorphic basement. Location of profile A–B shown on Figures 4a–4c, 4e; figure after Schultz et al. (2014).

Figure 6 - Horizontal interseismic velocity field for southern Sumatra predicted by the block-tectonic

model (after Schultz et al., 2014).