Cross-Section Restoration for Validating Interpretation in Poor Seismic Image, Study Case: Betara High, Jambi Sub-Basin Abstract

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CROSS-SECTION RESTORATION FOR VALIDATING INTERPRETATION IN POOR SEISMIC IMAGE, STUDY CASE: BETARA HIGH, JAMBI SUB-BASIN

Rahmadi Hidayat*, Jasmin Jyalita Jurusan Teknik Geologi, Fakultas Teknik, Universitas Gadjah Mada Jl. Grafika No.2 Bulaksumur, Yogyakarta, Indonesia * corresponding author: [email protected]

ABSTRACT Study area is located at Betara High of Jambi Sub-basin, northernmost area of South Sumatera Basin. Seismic re-study become quite active recently, especially in less-explored area surrounding Betara Complex as main hydrocarbon field in the area. However, poor seismic data due to Plio-Pleistocene syn-inversion requires better justification for seismic interpretation to reduce uncertainty and geological risk. This research proposed an alternative validation method for seismic interpretation, especially in horizon picking, utilizing cross-section restoration based on geometrical point of view. Fault-horizon network and flexural slip unfolding were carried out in interpreted regional NNE-SSW seismic line to define cross-section restoration. Slip movement of fault-horizon network, controlled in Hz-4, of Fault 1 (1055 m) and Fault 2 (400 m) produced near perfect joint-bed of all horizons on each segments. Flexural slip unfolding also depicted similar true vertical thickness (TVT) across segments of all intervals (Near GUF, UTAF and LTAF). This method indicated good geometrical validity in seismic interpretation. In visualizing TVT map as validation, cross-section restoration gives better image than conventional flattening method because it can resolve thickness ambiguity in fault zone. In tectonically inverted area, it is critical to resolve this problem in order to avoid irregular interpolation value near to fault zone.

I. INTRODUCTION well control. If there is no control for validation (fully assumption- Study area is located in Betara High, Jambi based/uncontrolled interpret-ation), Sub-basin, northernmost part of South uncertainty in subsurface mapping would Sumatra Basin (SSB). There are proven and definitely increase and cause higher geological prolific hydrocarbon fields, mainly gas with oil risk in trap definition. rim, near to this area, such as Betara Complex and Geragai Deep (Figure 1). Betara Complex In this condition, multiple interpretation itself consists of several closures such as North scenarios of targeted horizon should be Betara; Northeast Betara; and Gemah Field conducted and have to be validated using that are included in Jabung Area in South alternative method. Cross-section resto-ration Sumatra (Saifuddin et al., 2001; Chuanli et al., is proposed validation method in this research 2007). This fact implies to active exploration for validating seismic inter-pretation in poor studies recently, such as seismic re- image and limited well control. Utilizing this interpretation in surrounding proven Betara method, it will have better interpretation than Complex that mainly focusing on less explored uncontrolled interpretation and minimize both areas. uncertainty and geological risk by concerning geometrical point of view. Compare to Betara Complex, research area has experienced more severe deformation of II. GEOLOGICAL SETTINGS syn-inversion phase. Consequently, image Structural features in the Betara High are quality of seismic data is generally poor and produced by three main tectonic phases: (1) difficult to trace the horizon continuity on Early Eocene to Early Miocene extensional, (2) each faulted segments. Exploration wells are relative quiescence during Early Miocene to also limited for validating horizon Early Pliocene, and (3) Plio-Pleistocene oblique interpretation because not all segments have 479

PROCEEDING, SEMINAR NASIONAL KEBUMIAN KE-8 Academia-Industry Linkage 15-16 OKTOBER 2015; GRHA SABHA PRAMANA compressional (Argakoesoemah & Kamal, III. METHOD 2004). Several horst-grabens as well as Northeast-southwest trending seismic line inverted structures are obviously seen in perpendicular to fault system was utilized for present day structural condition (Figure 2A). performing cross-section restoration. Horizon Early Eocene to Early Miocene extensional interpretation was mainly focusing on four phase is believed as result of subduction along deeper targets, referred to Betara Complex. the West Sumatran Trench, the continental Basement (HZ-1, purple) was picked as the crust in the South Sumatra area was subjected lowest horizon to determine basin to a major extensional event from Eocene to configuration. Talangakar was analyzed above Early Oligocene times. This extension phase basement that consists of Lower Talangakar created several half-grabens in SSB with (HZ-2, yellow) and Upper Talangakar (HZ-3, various geometry and orientation due to pale green). Lower Talangakar is predicted as basement heterogeneity. Initially, extension main reservoir target within study area. Near appears to have been orientated east-west Gumai (HZ-4) was picked as the uppermost producing a sequence of north-south horsts horizons (Figure 2B). and grabens. Rotational moving by After conducting all horizon interpretation, approximately 15 degrees clockwise since the cross-section restoration could be applied. The Miocene was resulting present-day graben concept is adopted from Dahlstrom (1969) orientation (Ginger & Fielding, 2005). NE- that states interpretation of deformed trending faults as well as NS-trending faults structures can be restored back to control the development of this extension undeformed to provide a validation for initial phase (Saifuddin et al., 2001). interpretation. Two schemes were essential to Early Miocene to Early Pliocene (Post-Rift build this restoration: (1) Fault-horizon Megasequence; Ginger & Fielding, 2005) is network and (2) Restoring fold. Fault-horizon marked as the end of rifting phase. However, network is basically to move initial hanging- the thinned continental crust under the SSB wall horizon back to non-fault condition continued to subside as lithospheric thermal (modified horizon) with rigid-body equilibrium was re-established. High displacement. In order to do this scheme, subsidence rates and high relative sea level hanging-wall horizon should be slipped via resulted in a long-lived transgression of the fault with given length (slip length) to match basin which reached its maximum extent precisely (joint-bed) to footwall horizon. approximately 16 Ma ago with flooding in the Restoring fold attempts to unfold the horizon entire basin (Ginger & Fielding, 2005). with a model of flexural slip. Compare to common flattening method, flexural slip Plio-Pleistocene oblique compression is the considers restoring folding horizon while last major tectonic events to have affected the maintaining its bed length. In most cases, it geologic development of Sumatera and it is provides a better correct view for interpreting this event that is responsible for massive the faulting during the time of deposition inversion in entire basin. Elongate northwest- (Jamaludin et al., 2015). Bed length southeast orientated trans-pressional folds- consistency also is one of requirements to faults of varying magnitude were formed make valid restoration (Dahlstrom, 1969) across the basin and cut across much of the along with constant surface area and volume underlying syn-rift fabric. Most of (Stewart, 2012). hydrocarbon-bearing structural traps in the SSB were formed at this time (Ginger & For validating interpretation, true vertical Fielding, 2005). thickness map (TVT) of each interval was built. Detection of unrealistic or uncommon 480

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thickness (rapid change of thickness without Contacts between segments were relatively any geological explanation), especially at edge smooth (357 m – 368 m in S2 – S3 and 526 m – of fault segment, can be considered as bad 509 m in S2 – S1). Similar to Near GUF, UTAF interpretation. interval with TVT in range 230 – 561 m, also suggested smooth contacts (347 m – 366 m in IV. DATA ANALYSES S2 – S3 and 425 m – 436 m in S2 – S1). High Initial seismic interpretation depicted two contact anomaly occurred in LTAF TVT map in major northeastward facing thrust faults (F1 S2 – S3 contact (550 m to 179 m). However, and F2) that divided this area into three this area was expected as paleo-graben during segmentations (S1, S2 and S3). Initial bed Early Eocene to Early Miocene extensional length is 27.7 km. First attempt was moving S2 phase which inverted in last tectonic episode. and S3 via F1 fault controlled by HZ-4. Furthermore, it would not count as validation Calculated slip length of HZ-4 was 1055 m. due to geological reasoning. In this case, all After conducting this attempt, HZ-4 matched horizons were categorized as good perfectly in between S1 and S2 as well as interpretation. other lower horizons. Bed length extended to To compare the usability of this method, 28.5 km (2.88% from initial condition). Second ordinary flattening also established in this attempt was delivering S3 via F2 fault to research (Figure 6). Near GUF TVT map established S1 and S2. Calculated slip length in utilizing flattening method, contact of S2 – S3 HZ-4 was 400 m. In this attempt, 3.97% bed has an extremely high value around 669 m and length was extended from initial condition as well as S1 – S2 (1505 m). It also occurred in (28.8 km). Simulation of fault-horizon network UTAF (S2 – S3: 913 m and S1 – S2: 1481 m) schemes can be seen in Figure 3. and LTAF (S1 – S2: 1239 m). These numbers Fold restoration with flexural slip model could were more than maximum TVT in each interval be done after doing fault-horizon network has and could be categorized as unrealistic been established (see Figure 4). In first number. By applying this number to grid map, attempt, it managed to restore of unfaulted especially in limited or sparse seismic data, it HZ-4 as active restored horizon to datum would give residual interpolation and created plane while others are acted as passive “thickness ambiguity” for interpreter. Cross- horizons. It generated an extension 4.69% section restoration method attempted to from initial condition (bed length was 29 km). avoid unrealistic number and give better Second attempt was restoring unfaulted HZ-3 imaging than ordinary flattening method. as active and it extended bed length as long as 29.1 km (5.05% from initial condition). Last VI. CONCLUSIONS movement was attempted to restore Working in poor and sparse seismic data with unfaulted HZ-2. It produced bed length around tectonically inverted area must be taken 29.2 km or 5.41% of extension from initial carefully to minimize speculative condition. interpretation. It needs to be validated, at least qualitatively, by reasonable method. V. DISCUSSIONS Flattening method can be offered to do this but it doesn’t resolve thickness ambiguity in In order to depict clearer image of thickness contact between two segments. In making TVT anomaly, TVT maps of each interval were maps, residual interpolations will appear in made (Figure 5). In validation purpose, the certain area and create difficulties to validate main focus point should be in between two interpretation. This research proposed an segments. Near GUF TVT has TVT range 237 to alternative method, known as cross-section 647 m, appeared thinning towards Tiga Puluh restoration, as validation. Contrast to Mountain as basement high in SSW area. flattening method, it can clarify thickness 481

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ambiguity and gives clearer image of TVT grid (Lembaga Kerjasama Fakultas Teknik) Gadjah for validation. Mada University. and Ditjend MIGAS. Author would like to thank to geological team VII. ACKNOWLEDGEMENTS members of LKFT Gadjah Mada University for This research is the result of a Joint Study supporting analysis and technical discussions. project carried out by collaboration of LKFT

REFERENCES Argakoesoemah, R. M. I., & Kamal, A., 2004. Ancient Talang Akar Deepwater Sediments in South Sumatra Basin : A New Exploration Play. Proceedings, Deepwater and Frontier Exploration In Asia & Australasia Symposium. Chuanli, X., Haofan, M., Honggang, L., Dongmei, L., Xiuli, Q., & Benzhong, X., 2007. Alluvial fan facies and their distribution in the Lower Talang Acar Formation, Northeast Betara Oilfield, Indonesia. Petroleum Science, 4(2), 18-28. Dahlstrom, C. D. A., 1969. Balanced cross sections. Canadian Journal of Earth Sciences, 6(4), p. 743- 757. Ginger, D., & Fielding, K., 2005. The petroleum systems and future potential of the South Sumatra basin. Proceedings of the Indonesian Petroleum Association, 30th Annual Convention, Jakarta. Groshong Jr, R. H., 2006. 3-D Structural Geology: A Practical Guide to Quantitative Surface and Subsurface Map Interpretation. 3-D Structural Geology: A Practical Guide to Quantitative Surface and Subsurface Map Interpretation 2nd Edition, Berlin: Springer. Jamaludin, S. F., Latiff, A. A., & Ghosh, D. P., 2015. Structural Balancing vs Horizon Flattening on Seismic Data: Example from Extensional Tectonic Setting. IOP Conference Series: Earth and Environmental Science (Vol. 23, No. 1). Saifuddin, F., Soeryowibowo, M., Suta, I. N., & Chandra, B., 2001. Acoustic impedance as a tool to identify reservoir targets: A case of the NE Betara-11 Horizontal well, Jabung block, South Sumatra. Proceedings of the Indonesian Petroleum Association, 28th Annual Convention, Jakarta. Stewart, S. A., 2012. Interpretation validation on vertically exaggerated reflection seismic sections. Journal of Structural Geology, 41, p. 38-46.

FIGURES

Figure 1. Location of study area (red box) and Betara Complex as proven fields (green box). 482

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Figure 2. (A) Structural elements and (B) Regional stratigraphic column of South Sumatra Basin (Ginger & Fielding, 2005) as well as seismic horizons utilized in this study.

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Figure 3. Fault-horizon network steps.

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Figure 4. Restoring Fold steps.

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Figure 5. True Vertical Thickness (TVT) maps.

Figure 6. Comparation of flattening method to cross-section restoration.

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