SPE 165848

INTEGRATED REGIONAL INTERPRETATION AND NEW INSIGHT ON PETROLEUM SYSTEM OF SOUTH SUMATRA BASIN, INDONESIA

A. Carrillat, D. Bora, A. Dubois, Schlumberger, F. Kusdiantoro, S. Yudho, E. Wibowo, M. Musri, J. C. Tobing, MedcoEnergi, T. P. Gomez, F. Xue, D. Balasejus, T.D. McDonald, and P. Audemard, Schlumberger Copyright 2013, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Asia Pacific Oil & Gas Conference and Exhibition held in Jakarta, Indonesia, 22–24 October 2013. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract South Sumatra Basin is an inverted post-arc Tertiary basin, which has a complex evolution history from late Eocene-Oligocene extension to late Miocene and Pliocene compression. To evaluate the overall basin prospectivity, a regional analysis is conducted at 8 stratigraphic levels from pre-Tertiary unconformity to Pleistocene. An integrated interpretation including more than 1400 2D seismic lines, 4 seismic 3D surveys, and formation evaluation from 80 key wells is used to run the basin analysis. A series of regional seismic transects are defined through key wells and major structural elements to capture the characteristics of structural styles, lithostratigraphy and hydrocarbon distribution across the basin.

Structural restorations unravels the timing of fault activity showing basin rifting until ~23 Ma with main depocenters in Benakat Gully, Limau Graben, Central Palembang and Lematang Depression followed by sagging until 14.6 Ma. The compressive event is recorded from 5 Ma to present day. The buckling of syn-rift sediments suggests shortening expressed by inversion and fault reactivation rather than thrusting. Review of the source rock data, reservoir distribution, hydrocarbon phase and source to reservoir correlation data are evaluated in perspective of the basin configuration in order to select sections for basin modeling. The modeling results show onset of expulsion varying from ~10-15 Ma from Lemat Fm. and Talangakar Fm., and 5 Ma from Telisa Fm. Modeling suggests that Talangakar Fm. reservoirs are completely filled, whereas Lemat Fm. reservoirs are partially filled due to limited lateral and downward migration. Baturaja Fm. reservoirs in proximity to depressions are filled, and partial charge risk away from kitchen area. Most of the hydrocarbon are generated, expelled and accumulated between sedimentation of Lower Palembang Fm. to inversion time (10-5 Ma). The subsequent inversion is likely to have re-migrated hydrocarbon in Talangakar and Baturaja reservoirs along Benakat Gulley and associated fault bound folds.

Introduction From an exploration and production stand point, the South Sumatra basin is considered as a mature basin with more than 100 years of exploration and oil and gas production. However, the mature plays are essentially limited to the Miocene carbonates of Baturaja Fm. and the shallow clastics of Palembang and Airbenakat formations. The last compressional event formed the inverted rift-basin anticlines by reversed normal faults resulting in monoclines and anticlines. These obvious structures identified from surface geological mapping were the first targets of exploration as they trapped hydrocarbon that migrated from mature source rocks in adjacent structural lows such as Lematang Depression and Benakat Gulley.

On the other hand, the syn-rift plays of the Lemat and Talangakar formations are rather underexplored except over the Pendopo-Limau anticline. The exploration of the Baturaja Fm. started in the 1980’s and still remains a challenge from an exploration success ratio. Nevertheless, the Baturaja Fm. offers significant remaining potential as a number of stratigraphic traps have been drilled successfully and are now producing fields.

The study area is focused on South Sumatra Extension, Rimau and Lemantang blocks located in the South Sumatra basin (Fig.1).

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Geological Setting The South Sumatra Basin consists of Tertiary half-graben sub-basins filled with mixed terrigenous, volcaniclastic and carbonate rocks unconformably overlying pre-Tertiary sedimentary, metamorphic and igneous rocks. It is traditionally divided into three sub-basins: Jambi, Central Palembang and Lematang (or South Palembang) sub-basins. This study covers partially the Central Palembang Depression, Benakat Gulley and Lematang Depressions (Fig. 1).

Five plays account for the majority of discoveries to date. These are found in pre-Tertiary fractured basement, Oligocene to Early Miocene (Lower Talangakar Formation) fluvio-deltaic sandstones, Early Miocene (Baturaja Formation) carbonates and Early Miocene (Gumai Formation) and Middle Miocene (Airbenakat Formation) shallow marine sandstones. Oligocene-Early Miocene age lacustrine and deltaic source rocks are recorded or implied by discovered oil characteristics. Source-rock type and distribution have influenced the presence of hydrocarbons in reservoirs across the basin.

Discoveries and producing reservoirs range from pre-Tertiary basement through upper Miocene sandstones and carbonates deposited as syn-rift strata and as fluvial-deltaic, marine shoreline to deeper water sediments. Carbonate and sandstone reservoirs produce oil and gas primarily from anticlinal traps of Plio-Pleistocene age. Stratigraphic trapping and faulting are important locally mostly within Miocene carbonates. Furtermore, the production is compartmentalized due to numerous intraformational seals. The regional marine shale seal, deposited by a maximum sea level highstand around 15-16 Ma, was faulted during post-depositional folding allowing migration of hydrocarbons to shallow reservoirs and locally to the surface.

Figure 1: Location map of South Sumatra Basin and study area

The Tertiary history of the South Sumatra basin can be divided in the four tectono-stratigraphic stages (Ginger and Fielding, 2005). The main structural elements in the basin are described in Longley (1997) with the respect to their relation to plate tectonic events controlling the structural history of the South Sumatra Basin. These four tectonos-tartigraphic stages include a pre-rift ranging from circa 54 to circa 30 Ma (Paleocene to Early Oligocene) (Ryacudu, 2005a), a syn-rift stage from circa 40 to circa 29 Ma, a post rift stage from circa 29 to circa 5 Ma and a syn-orogenic/inversion stage from circa 5 Ma to present.

Above the basement, the pre-rift is referred to as the Lahat Formation (Kamal et al, 2008). The formation refers to pre-rift sedimentary sequence composed of high content of volcanic material such as volcanic breccias, agglomerates, and interbedded of tuff and tuffaceous sandstone (Ryacudu, 2005a).

The syn-rift is represented by the Lemat Formation, deposited unconformably above the Lahat Formation (Fig. 2). It is

(ft)

South SumatraExtension Block

Lemetang Block

Rimau Block

N

LematangDepression

MusiHigh

Palembang High

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described as sandstones, conglomeratic sandstones, interfingering with Benakat Member which is composed of grey-brown shales, tuffaceous shales, siltstones and sandstones with occasional thin coals, irregular carbonate bands and glauconitic units. Sedimentation is mostly controlled by fault, ranges from scree, alluvial fan, fluvial to fresh or brackish water lacustrine in the central part of the basin. Based on palynomorph data, the age of the Benakat Member is between Late Oligocene to Early Miocene (Ryacudu, 2005a). Following the change in the tectonic regime in the Late Oligocene, the whole region underwent regional subsidence in sag phase. The Talangakar Formation was deposited during the early transgressive stage (Late Oligocene to Early Miocene). The rocks are described as greyish-brown channel sandstones, siltstones and shales, grading basin wards into light brown carbonaceous shales with coal seams deposited in fluvial-lacustrine to lagoonal-shallow marine environment (Barber et al., 2005).

The post-rift stage is characterized by marine sediments related to sagging and early marine transgressive stage (Late Lower to Middle Miocene), which are represented by Gumai and Baturaja Formations (Fig. 2). The Baturaja formation is composed of platform limestone with local carbonate banks and isolated build-up reefs situated at basement highs. Basinwards the massive limestone pass into limestone beds intercalated with open marine shales of the Gumai Formation. The limestone formation was named earlier as Basal Telisa Limestone (de Coster, 1974).

The Gumai Formation comprises a series of foraminifera-bearing grey shales and siltstone with intercalation of fine grained glauconitic sandstone and siltstone, and lenses of tuff. Glauconitic sandstones and tuffs become more significant towards the Barisan Mountains (Barber et al., 2005). The name of Telisa Formation was widely used for this formation in Jambi and Central Palembang Sub-basins. Kamal et al. (2008), proposed to classify glauconitic sandstone deposited in the Rimau area as Gumai Sandstone Member.

The syn-orogenic stage developed during the Middle Miocene onwards, the Barisan Mountain uplift was faster than regional basin sag and caused further subsidence along back-arc and fore-arc basin, and ceased the regional transgression. These movements coincide with the inversion of the basin sediment with faults re-activation, folding of basin sediments and development of unconformities in the sequence. In the backarc basin, from Mid to Late Miocene, turbiditic sandstones become an increasing component of the Airbenakat Formation (Fig. 2). By the Late Miocene and Early Pliocene these deposits had passed upwards into shallow marine, sub-littoral and deltaic sediments of Muaraenim Formation (Fig. 2). By Late Pliocene, the dominant deposits are terrestrial sands and clays with abundant volcanic debris, which is distinguished as Kasai Formation (Barber et al., 2005).

Figure 1: Lithostratigraphic chart for South Sumatra basin (Central Palembang, Benakat Gulley and Lematang Depression) and key

regional seismic horizons marked in dashed lines (modified after Kamal et al. 2008)

Baturaja, 17MaTop Seq 11, 15Ma

Top Seq 13, 14Ma

Coal B, 5Ma

Coal A, 10Ma

Top Seq3, 21Ma

Top Basement (PTU)

Top Seq1, 23Ma

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Workflow The regional study workflow consist in interpretation of 1400 2D lines of seismic data, 4 seismic 3D surveys and stratigraphic correlation of approximately 80 key wells. The integrated stratigraphic and structural interpretation of seismic data and wells lead to generation the framework for basin analysis from the pre-Tertiary unconformity to the Upper Miocene Palembang Fm. (Coal B – Muara Enim Fm.). The regional stratigraphic framework is based on observation and interpretation of stratigraphy and structures inherited from tectonic history. The stratigraphic interpretation model is constructed using regional transects covering the key areas of the basin from Pigi Trough, Musi Platform to Benakat Gulley, from Lakitan, Lematang Depression to Kuang High and from Pendopo High to Palembang High. This stratigraphic model is an update of the existing chrono- and litho-stratigraphic framework taking into account the observations from seismic interpretation across the basin. These stratigraphic markers are identified on the seismic regional transects and represent sequence boundaries validated on the integration of available biostratigraphic studies, key wells and seismic stratigraphic expression. These horizons capture important geological events and relate to the previous lithostratigraphic classification (formation based); Pre-Tertiary Unconformity, top of Sequence 1, top of Sequence 3, top Baturaja Equivalent, top of Sequence 11, top of Sequence 13, Coal A and Coal B (Fig. 2).

Based on the interpretation of regional transects, structural restorations are performed to refine the understanding of the basin evolution and then used as input for 2D basin modeling. The regional interpretation used for basin modeling depicts the basin configuration today, but keeps in perspective the role of large inverted structures (e.g Pendopo High) and major lows such as Lematang depression in the South, Palembang depression in the North and the Benakat Gulley connecting these two lows.

Benefiting from its long exploration history, geochemical data are widely available in South Sumatra Basin. However, in this study, the geomchenistry data is put within a consistent regional stratigraphic framework and evaluated with respect to the structural implications (pre- and post-compressive event). Accordingly, the source rock data (kerogen type), reservoir distribution, hydrocarbon phase data and source to reservoir correlation data (source rock inference from oil samples) is also put in perspective of the basin configuration to select fit for purpose 2D sections for basin modeling.

Basin Modeling Structural restoration and analysis Sequential structural restoration is performed to test the basin evolution in the study area and to validate the seismic interpretation. Structural restoration builds on geometric and kinematic techniques to test the strength of the geological interpretation. The orientation and location of the regional transects is the result of a balance between data availability and orientation of the main structural elements. The regional sections are constructed based on the regional interpretation of 2D seismic lines in time domain and depth converted using average velocities from well ties. The transect illustrated in Fig. 3 is running from the Musi High to Palembang High via the Benakat Gulley and Pendopo-Limau anticline. This transect is 145 km long and strikes roughly SW-NE and close to 4 km in depth. The sedimentary thickness interpreted on seismic represents approximately 12 000 feet. Formations interpreted on this section range from pre-Tertiary Unconformity (PTU) to present day. Six faults are interpreted on seismic. They are former graben faults that initiated during Eocene. The southern fault is the main graben fault. It is dipping NNE and is strongly inverted during the Pliocene compression. The other faults are synthetic and antithetic normal faults associated with the principal normal fault. The Musi Platform displays a regional tilt towards the NE. This can be explained by the thrusting (associated with the inversion) of the Lemat to Coal A sedimentary section over the Musi High. This composite line is depth converted and successively restored until Baturaja deposition time (Fig. 3). The Pendopo-Limau anticlinal is strongly eroded. The eroded section had to be reconstructed before the restoration can be carried out. An estimated section of 1.5 km of rocks has been eroded. This is three times more than the amount of erosion determined by Vitrinite Reflectance from well data available further north on the sourthern flank of the Central Palembang Depression. This regional transect is restored until Coal A deposition time (Late Miocene). As coal deposited in a flat environment, no regional slope is used as a restoration target. The reverse displacement along the inverted graben fault is restored.

Results and observations from restoration The restored section shows that before the inversion, the Musi High and the Palembang High are at the same structural depth. They are separated by a major normal fault dipping towards the NNE. Coal A to Lemat formations display a sedimentary thickening towards this fault plane. This proves that this normal fault is a long lasting fault. The maximum vertical relief measured along this fault (before inversion) is 2.1 km as measured on the pre-Tertiary Unconformity. The percentage of shortening recorded represents 11% of the section after the maximum of extension. The composite section is parallel to the main compression direction. So, this percentage of shortening is considered to represent a representative value.

The sedimentary wedging is observed from Coal A until Lemat Fm. deposition. However, it is more important at Talangakar and Lemat formations level. This shows that the normal fault activity is decreasing from Oligocene until Late Miocene and this is consistent with the late syn-rift and basin sagging phase reported in the literature.

Along the section discussed here, all the shortening is accommodated by fault reactivation and no thrust development is observed. The estimated percentage of extension between Lemat deposition time (Eocene) and Late Miocene would be between 15 and 20%.

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Review of the sections in this study unravel the timing of fault activity showing basin rifting until around 23 Ma with main sediments accumulation in the Pigi Trough, Benakat Gulley, Limau Graben and Lematang Depression followed by sagging until 14.6 Ma. Until that time, both Pigi Trough and Lematang Depresssion experienced the same deepening and sediment filling history. The compressive event is recorded from 4.2 Ma to present day. The buckling of syn-rift sediments indicates that this compressive event is expressed by inversion rather than fault reactivation. Along the restored transect striking SW-NE, all the shortening is accommodated by fault reactivation and no thrust development is observed.

Figure 3: Restoration steps of a regional section from Palembang High in the NE, Pendopo High, Benakat Gulley to Musi High in the

SW and location map. Petroleum System Modeling Data analysis A total of 21 wells across the study area have measured VRo data, of which two wells have multi-maceral maturity analysis. Rock eval pyrolysis and TOC data is available from 76 wells across the basin. Existing basin modeling studies used 3 % TOC and 600 HI as Type I kerogen for Lemat source rock, whereas 3-4% TOC and 250-600HI as Type II and III kerogen for Talangakar source rock. Initial TOC is computed 20-25% higher than measured TOC in mature samples. The source rock in Talangakar formation shows HI range of 0-600 and TOC range of 0.5-10%. Considering the mixed deltaic and coastal plain depositional environment, a mean value of 3% TOC and 300 HI is used while honoring the complete data range as end members in uncertainly analysis (Fig. 4). The source rock in Baturaja formation shows HI range of 0-500 and TOC range of 0.5-4%. Considering the mixed deltaic and coastal plain depositional environment, a mean value of 1% TOC and 200 HI is used while honoring the complete data range as end members in uncertainly analysis. The source rock in Telisa formation shows HI range of 0-400 and TOC range of 0.5-4%. Considering the mixed deltaic and coastal plain depositional environment, a mean value of 1.5% TOC and 200 HI is used while honoring the complete data range as end members in uncertainly analysis (Fig. 4).

There is no kinetic analysis available for South Sumatra source rocks. Therefore, analogue kinetics is selected from the modeling data base using knowledge of depositional environment, facies, TOC and HI data. However, these may not be accurate in particular for carbonates. Selection of analogue kinetics is performed in three steps. First is selection of depositional environment and lithology, second is selection of kinetic type and third is closet match with known HI and TOC.

SW NE

Pre Tertiary U

Batu RajaSeq 11

Lemat Talang Akar

Present day

Present day (eroded section is reconstructed)

Coal A deposition time (Late Miocene)

Seq 13 deposition time (Middle Miocene)

Seq 11 deposition time (Middle Miocene)

Baturaja deposition time (Early Miocene)

25 km

25 km

145 km

4500

m

Location map of the restored regional transect

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Figure 4: HI and TOC range for shaly formations from 21 key wells in the study area of South Sumatra basin Boundary conditions Paleowater depth is inferred from known depositional environment and bathymetric analysis in Morley (2009). Palaeowater depth increases gradually from onset of Talangakar sedimentation and reached maximum during Telisa sedimentation and then decreases gradually again. Sediment water interface temperature is calculated using paleowater depth and temperature variation due to plate movement from 50 Ma to Present (Fig. 5).

Heat flow is estimated using McKenzie crustal stretching model considering syn-riftt phase from 50 Ma to 25 Ma and post rift from 25 Ma to Present. Fig. 5 shows snapshot of boundary conditions. The modeled basement heat-flow is 20-25% lower than previous basin modeling studies.

Figure 5: Example of boundary conditions (sediment water interface temperature SWIT and heat flow) used of the 2D modeling

Modeling Results Four regional transects were modeled, including the geomechanically-restored sections. The modeling results for two transects are presented here (Fig. 6). Modeled maturity is in good agreement with measured maturity available from vitrinite reflectance data in two wells along Section 1 and one well along Section 2. The modeling along regional Section 1 running

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from Lematang Depression to Illiran High (Fig. 6) shows the main oil window below a depth of 8500-9000 ft. The area between faults is modeled as inverted after 5 Ma. Transformation Ratio profile shows 60-80% transformation of Lemat source rock, 40-60% transformation of Talangakar and 20-30% transformation of Telisa source rocks. Baturaja Formation is assigned source rock in basin side and reservoir on highs which shows 70-90% transformation. The onset of expulsion from Lemat, TAF and Baturaja is varying from 15-10 Ma (Fig. 6). This indicates that major reservoir Baturaja were deposited before onset of expulsion from deeper source rock. Onset of expulsion is modeled older than 15 Ma in previous modeling studies which is primarily due to higher heat flow. Modeled saturation profile shows hydrocarbon in Lemat, Talangakar and Baturaja reservoirs (Fig. 6). The accumulation and migration vectors profile is showing hydrocarbon are reaching to then existing topographic highest area towards Illiran High. The hydrocarbons accumulated in modeled uplifted area were trapped before inversion. Therefore, some of the hydrocarbon fields along Benakat Gulley are likely to be charged from this kitchen before inversion which latter underwent remigration due to inversion. Results indicate most of the hydrocarbon fields towards Illiran High area are charged from this kitchen.

The basin model generated along the regional Section 2 (Fig. 6) was established prior to the structural inversion from the restored section flattened at Coal B level. It shows thydrocarbons expelled and accumulated in this area between sedimentation of Coal B to inversion time (10-5 Ma). The subsequent inversion is likely to have re-migrated hydrocarbon in Talangakar and Baturaja reservoirs along Benakat Gulley and associated fault bound folds. However, fault closure is likely to be risky (Fig. 6).

Figure 6: Results of basin modeling along regional: Section 1 from Lematang Depression to Illiran High with present structure (left),

Section 2 from Musi High, over Benakat Gulley, Pendopo High to Palembang High prior to the structural inversion (right), and location map (bottom right)

Conclusions This study provides an integrated regional geological and geophysical interpretation and the foundations of the basin analysis to identify hydrocarbon plays within the late syn-rift and post-rift Late Oligocene to Middle Miocene units as well as the syn-orogenic Middle Miocene to Pliocene units. The main interpretation consist mapping of pre-Tertiary unconformity, and the

SW NE SW NE

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Expulsion onset

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20 km 30 km

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stratigraphic boundaries and disconformities separating the Upper Oligocene to Early Miocene Talangakar Fm., Lower Miocene Baturaja Fm., Lower, Middle to Upper Miocene Telisa Fm., and Middle to Upper Miocene Palembang Fm. and their correlative conformities. The data integration and regional seismic provides a new understanding of the basin stratigraphic and tectonic evolution. The boundary conditions of the petroleum system elements are better established and can be fed into play fairway mapping.

Based on the regional transects, structural restorations are performed to refine the understanding of the South Sumatra basin evolution. The restored sections unravel the timing of fault activity showing basin rifting until around 23 Ma with main sediments accumulation in the Pigi Trough, Benakat Gulley, Limau Graben and Lematang Depression followed by sagging until 14.6 Ma. Until that time, both Pigi Trough and Lematang Depresssion experienced the same deepening and sediment filling history. The compressive event is recorded from 4.2 Ma to present day. The buckling of syn-rift sediments indicates that this compressive event is expressed by inversion rather than fault reactivation. Along the restored sections striking SW-NE, all the shortening is accommodated by fault reactivation and no thrust development is observed.

The modeling results show onset of expulsion varying from ~10-15 Ma from Lemat and Talangakar formations, and 5 Ma from Telisa formation. Accumulation is observed in Lemat and Talangakar reservoirs. Talangakar reservoirs are expected to be completely filled, whereas Lemat reservoirs are partially filled due to limited lateral and downward migration. Baturaja reservoirs in proximity to depressions are filled, however carbonate reservoirs in Musi Platform located further away from kitchen area may be partially filled. Considerable migration distance is observed before inversion (>50 km). Saturation and transformation ratio of Baturaja carbonate is quite sensitive to source rock kinetics, which are unknown in this basin.

Acknowledgements The authors would like to thank MedcoEnergi E&P for authorizing publication of this work and Schlumberger for support.

Reference Barber, A. J., Crow, M. J., & Milsom, J. S. (eds), 2005. Sumatra: Geology, Resources and Tectonic Evolution, Geological Society,

Memoirs 31, London. De Coster, G. L., 1974. The Geology of the Central and South Sumatra Basins, Proceeding IPA 3rd Annual Convention, Jakarta, p. 77-

100. Ginger, D. and Fielding, K., 2005. The Petroleum Systems and Future Potential of the South Sumatra Basin, Proceedings Indonesian

Petroleum Association Thirtieth Annual Convention & Exhibition, Jakarta, Indonesia. Kamal, A., Argakoesoemah, R. M. I. & Solichin, 2008, A Proposed Basin Scale Lithostratigraphy for South Sumatra Basin, IAGI Special

Publication of Sumatra Stratigraphy Workshop, Riau, 2005, p. 85 – 98. ((Description of Eocene- Pliocene stratigraphy of S. Sumatra basin).

Morley, R. J., 2009. Sequence Biostratigraphy of Medco South Sumatra Acreage, MEDCO Internal Report, Jakarta. Ryacudu, R., 2005a. Tinjauan Stratigrafi Paleogen Cekungan Sumatra Selatan, IAGI Special Publication, Sumatra Stratigraphy Workshop,

Riau, p. 99 – 114. Ryacudu, R. 2005b. Studi Endapan Synrift Paleogen Cekungan Sumatra Selatan, Doctoral Dissertation, ITB, Banding, Indonesia.