Indonesian Journal on Geoscience Vol. 3 No. 2 August 2016: 127-137

INDONESIAN JOURNAL ON GEOSCIENCE Geological Agency Ministry of Energy and Mineral Resources

Journal homepage: hp://ijog.geologi.esdm.go.id ISSN 2355-9314, e-ISSN 2355-9306

Seismic Interpretation of the Nam Con Son Basin and its Implication for the Tectonic Evolution

Nguyen Quang Tuan1, Nguyen Thanh Tung1, Tran Van Tri2

1Exploration and Production Centre - Vietnam Petroleum Institute 2Vietnam Union of Geological Sciences

Corresponding author: [email protected] Manuscript received: March 8, 2016; revised: June 17, 2016; approved: July 19, 2016; available online: August 4, 2016

Abstract - The Nam Con Son Basin covering an area of circa 110,000 km2 is characterized by complex tectonic settings of the basin which has not fully been understood. Multiple faults allowed favourable migration passageways for hydrocarbons to go in and out of traps. Despite a large amount of newly acquired seismic and well data there is no significant update on the tectonic evolution and history of the basin development. In this study, the vast amount of seismic and well data were integrated and reinterpreted to define the key structural events in the Nam Con Son Basin. The results show that the basin has undergone two extentional phases. The first N - S extensional phase terminated at around 30 M.a. forming E - W trending grabens which are complicated by multiple half grabens filled by Lower Oligocene sediments. These grabens were reactivated during the second NW - SE extension (Middle Miocene), that resulted from the progressive propagation of NE-SW listric fault from the middle part of the grabens to the margins, and the large scale building up of roll-over structure. Further to the SW, the faults of the second extentional phase turn to NNE-SSW and ultimately N - S in the SW edge of the basin. Most of the fault systems were inactive by Upper Miocene except for the N - S fault system which is still active until recent time. Keywords: seismic, tectonic evolution, Nam Con Son Basin © IJOG - 2016, All right reserved

How to cite this article: Tuan, N.Q., Tung, N.T., and Tri, T.V., 2016. Seismic Interpretation of the Nam Con Son Basin and its Implication for the Tectonic Evolution. Indonesian Journal on Geoscience, 3 (2), p.127-137. DOI: 10.17014/ijog.3.2.127-137

Introduction acquired seismic and well data. This causes high uncertainties in the evaluation of petroleum system The Nam Con Son Basin (NCSB) is located trap integrity and HC generation and migration. in the south-eastern continental shelf margin The probability of petroleum exploration success of Vietnam with anIJOG area of circa 110,000 km2, for commercial discovery in the basin is rather low bordered by Con Son swell in the northwest, (16% - unpublished data) probably because of the Khorat - Natuna Arch in the southwest, and Tu complex tectonic settings of the basin that has not Chinh High in the east (Figure 1). fully been understood. In this article, the authors Multiple institutions and geoscientist teams have present a revised interpretation of the tectonic de- been studying the basin, with many articles that velopment in the basin scale based on the entire set have been published. However, there has not been of seismic and well data. Basin tectonic evolution is any significant update on the tectonic evolution critical to improve understanding of the evaluation and basin history despite a large amount of newly of petroleum system in the Nam Con Son Basin.

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Mae Ping Fault Three Pagodas Fault EAST SEA OCEANIC CRUST Ho Chi Minh City N

Mesozoic Phu Quoc Basin

Cuu Long Basin Truong Sa (Spartly) Con Son Swell

TROUGH West Baram Line

PATTANI PATTANI KHORAT ARCH Nam Con Son Basin DANGEROUS GROUNDS

NORTH LUCONIA Malay-Tho Chu Basin PROVINCE East Natuna Basin

NORTH BORNEO TROUGH West Natuna Basin BARAM DELTA PROVINCE

WESTERN NATUNA CENTRA LUCONIA ARCH L LUCONIA PROVINCE West Balingian Liro PROVINCE

Penyu Basin BALINGIAN TATAU PROVINCE Rajang Accretionary Prism PROVINCE TINJAR PROVINCE Tatau Line

SUNDA SHELF Lupar Line BORNEO

0 60 120 240 360 480 Km

Figure 1. The main tectonic and structural features of the Nam Con Son Basin and adjacent areas.

Regional Geologic and Tectonic Set- nam Seafloor, which propagated initially from E tings of The Basin to W then WSW. At about 25 Ma, the axis of the seafloor spreading shifted from WSW trend to In Southeast Asia, by Paleocene, the south- SW trend (Andrew, 2010; Morley, 2007; Pubellier east extrusion of the Indochina Block and and Morley, 2014). southward drift of the proto East Vietnam Sea At the end of Early Miocene, southwestward associated with the collision of Luconia micro propagation of the seafloor spreading continued continent and Borneo caused a series of right- by a continental breakup, caused the second ex- lateral transform faults in the East Vietnam tension phase at the SW of the rift tip including Shelf extending to the East Luconia. This com- NCSB, associated with NE - SW normal faults and bination possibly derived N - S extension in the deposition of syn-extension sediments in NE - SW Sunda Shelf (Hall, 2002, 2009, 2013; Hutchison, grabens. The second extension phase is derived by 2004; Clift, 2008). Matthews (1997) and Fyhn NW - SE regional extension as a slab-pull of the et al. (2009) proposed the onset of rifting in the SE drifting Dangerous Ground and the subduction proto East Vietnam SeaIJOG (EVNS) as well as the of the proto EVNS beneath NW Borneo. NCSB initiated in the Eocene and lasted to the Well pronounced NW - SE central graben Early Oligocene (about 30 Ma). This rifting pe- extension and accompanied progressive large riod is controlled by N-S extension, associated scale listric faulting were probably initiated from with E - W oriented faulting and deposition of the middle of the basin to its margins creating rift-fill sediments in local W-E trending sub- a unique depocentre with large scale roll-over basins (Figure 2). structures with their crests in the middle of The rifting phase in Eocene - Early Oligocene the grabens. The large scale faults penetrate then was followed by the spreading of East Viet- the whole sedimentary cover to the basement.

128 Seismic Interpretation of the Nam Con Son Basin and its Implication for the Tectonic Evolution (N.Q. Tuan et al.)

Ailao Shan-Red River Fault Zone 90O E 100O E 110O E East Himalayan Syntaxis

China

O 25 N Block Burma West

India Mynmar North Bac Bo Gulf Basin Vietnam Sagaing Fault

Pearl River Mouth Basin Qiongdongnan Bengal Basin Laos Basin

Three Pagodas Fault Song Hong Basin N Thailand Tri Ton Horst Hoang Sa O 15 N Basin EVSF Cambodia Mae Ping Phu Fault Zone Khanh Basin Mesozoic igneous belt THFZ Mergui East Sea Metamorphic Andaman Basin Pattani core complex Sea Basin Phu Quoc Cuu Long Basin Basin Truong Sa Approximate area Basin of Cenozoic Basin Tu Chinh Con Son Swell Oceanic crust Vung May Nam Con Son with magnetic anomalies Basin Trough Basin Luconia Main Cenozoic strike-slip direction East Palawan Natuna West Baram Line Subduction zone West Basin Malaysia Natuna Sumatran Fault Basin Major thrust fault Penyu Basin LuparBukit Line Mersing Line

500 km Borneo

Figure 2. Regional tectonic map of SE Asia, modified after Fyhn et al., (2009), Andrew (2010) and Pubellier (2014). The studied area is marked by green curve. THFZ = Tuy Hoa Fault Zone, EVSF = East Vietnam Scarp Fault.

Moreover, growth-faults and related half grabens Stratigraphically, various rock sequences became progressively younger from the middle consisting of pre-Tertiary fractured granite and part of the basin depocentre. clastic sediments of Oligocene, Lower Miocene, As the East Vietnam Seafloor spreading to- Middle Miocene, and Upper Miocene age fill in tally ceased at about 17 - 16 Ma, regional sea level the basin. Those Tertiary clastic sequences under- fell during the late IJOGMiddle Miocene leading to lain unconformably by the pre-Tertiary fractured the erosion (up to several hundred meters) of the granite are source rocks and reservoir rock of central part of the roll-over structures. potential hydrocarbon (Figure 3). The second rifting phase was then followed by a thick post-rift sequence (Upper Miocene - Plio- cene - Quaternary) due to the increase in sediment Datasets and Methods supply with respect to onshore uplift and magma- tism (Fyhn et al., 2009; Hall and Spackman, 2015, This study is based on a number of reflection Nguyen Hiep, 2007; Tri and Khuc, 2011). seismic profiles, totally 54,147 km. They cover

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BIO STRATIGRAPHY

LITHOLOGIC LITHOLOGY DEPOSITIONAL DESCRIPTION COLUMN ENVIRONMENT EVENTS STAGE EPOCH PERIOD SEISMIC HORIZON FORMATION MAIN TECTONIC TECTONIC MAIN PALYNO zone PALYNO PETROLEUM SYSTEM PLANK. “N” zone NANNO. “N” zone

Shale, claystone interbedded thin layers of sandstone, rich Shallow marine organic matters and fossils. to outer shelf PLIOCENE-QUARTENARY Post-rift 2 Dacridiuom Phyllo

T85

Yellow claystone interbedded Shallow marine with siltstone, average cemented, to slope, deep marine rich organic matters and fossils. Meridionalis Florschuetzia

T65 QUARTENARY QUARTENARY NEOGENE Limestone intercalated thin layers of sandstone, claystone, and mudstone Subzone F. Trilobata Trilobata F. Shallow marine (inner shelf to midle shelf) Syn-rift 2

Interbeds of claystone, siltstone,

sandstone, and sometime limestone Subzone F. Semilobata F. T30

Coastal plain Interbeds of claystone, siltstone, to shallow marine sandstone, and coal layers Late Middle Early N6-N18 N9-N15 N16-N18 N19-N21 N22-N23 NN10-NN11 NN21-NN18 NN19-NN21 NN21-NN18 NN19-NN21 NN12-NN4 NN5-NN9 NN10-NN11 Inter-rift 1 Inter-rift

T20

Sandstone interbedded with claystone, siltstone, and thin layers of coal Lacustrine, Deltaic THONG-MANG CAU NAM CON SON BIEN DONG THONG-MANG CAU NAM CON SON A CAU U to coastal plain PALEOGENE OLIGOCENE MI OCENE F. Trilobata Cicatrico Floschuetzia Levipoli Cicatrico Trilobata F. Syn-rift 1 T00

PRE- - Igneous rocks TERTIARY (fracture granite, ryolite...)

- Metamorphic rocks JURA - CRETA - CRETA JURA IJOG Gas and oil shows Commercial oil and gas Source rock

Figure 3. General stratigraphic column and petroleum system summary of Nam Con Son Basin.

the whole NCSB used for interpretation and ref- 2, and 05-3, where the basement is too deep. 3D erence (Figure 4). The quality of seismic data is seismic data available in Block 06-94, 05-2, and fair to good in overall, with only a poor quality 05-3 were used to improve the quality of seismic area around the central basin of Block 06-94, 05- interpretation in these areas.

130 Seismic Interpretation of the Nam Con Son Basin and its Implication for the Tectonic Evolution (N.Q. Tuan et al.)

Figure 11

Figure 12

Figure 13 Figure 9

Figure 6

Figure 4. Blocks boundary map of NCSB showing the total seismic reflection data coverage analyzed in this study (gray lines). Blue bold lines show those lines presented in this paper. The red dots show the well locations used to provide bio- stratigraphic age control.

Age control is provided to the seismic stra- Results and Discussion tigraphy through a biostratigraphy mode from selected wells located on the different blocks, as The regional tectonic settings, data, and shown in Figure 4. These ages were transferred methods described above are the background to the seismic profiles after converting drill - guidelines for detailed data interpretation in this ing depth to two-way travel time (TWT) using study. Five main seismic horizons have been the stacking velocities derived from process- interactively interpreted in time domain, namely ing the MCS data. Those dated horizons were Top Basement (T00), Top Oligocene (T20), Top then correlated across the entire study area. Lower Miocene (T30), Top Middle Miocene The oldest age pick from the biostratigraphic (T65) and Top Upper Miocene (T85) sequences data was the top ofIJOG Oligocene. The base of (?) (Figure 5). After gridding, the time structure maps Eocene-Oligocene strata is inferred by the top were generated, then were converted to depth basement reflector, which can be followed over domain. Several time depth conversion methods long distances across the study area. Age resolu- have been considered for generating depth struc- tion is best for Miocene strata because a number ture maps of key sequences in the basin. After a of wells have penetrated these formations. In careful quality test, the depth structure maps of contrast, the Plio-Pleistocene was not cored these key sequences have been generated from the by industrial wells, so there is no detailed date time interpretation by the layer cake method. Four subdivision of those units. structure maps of Top Basement, Top Oligocene,

131 Indonesian Journal on Geoscience, Vol. 3 No. 2 August 2016: 139-149

NW NW-SE rifting SE Rollover structure

Early Miocene

Middle Miocene

Late Miocene

Oligocene Eocene

N-S rifting

Figure 5. NW - SE seismic line from the Con Son Swell across Central Basin and further SE showing extension feature and the two distinct rift stages of NCSB. The interpreted horizon in red is top Basement, violet is top Oligocene, green is top Lower Miocene, blue is top Middle Miocene and yellow is top Upper Miocene. Several normal listric faults dipping to the WNW were recognized and associated with rollover anticline structure on the hanging wall side.

Top Lower Miocene, and Top Middle Miocene southward movement of Dangerous Ground sequences were displayed on Figure 6. due to the proto East Vietnam Sea subducting To reconstruct the tectonic evolution of the beneath Borneo (Hall, 2013). basin, the fault systems have been interpreted with 2. The NE - SW trend faults developed strongly great attention. Fault heaves and fault timing were in the northeastern part of the basin with carefully determined to accurately reflect their listric geometry during the Middle Miocene development. As a result, four major fault trends sequence as a consequence of SW propagation developing at different time intervals can be of East Vietnam Seafloor spreading (Figure recognized and described as follows (Figure 7): 10). In the central part of the NCSB, the NE 1. The E - W trending fault system dominated - SW faults gradually bend to: in the southern and centre parts of the Nam 3. NNE - SSW trend implying that the faults Con Son Basin. The faults have extensional was influenced by a N - S previously existed features that are clearly evidenced on the top basement rock fabric. We suggest the weak crystalline basement, but mostly buried by the basement fabric possibly formed earlier than Late Oligocene and Early Miocene sediments the first rift phase in NCSB as exhibited in (Figure 8). The growth sequence of Early the tectonic model proposed by Hall (2009). Oligocene to the top basement against the The model implies that in the Early Eocene, E - W trend faults suggests that the timing of the Sundaland region went into regional N - S E - W faulting activities is about Eocene (?) - compression as Australia attempted to move Early Oligocene during the first regional rift north. As a consequence, approximately E - W stage described in the previous section. This extension caused N - S basement fabric to form observation is also IJOGsupported by the isopach (Hall, 2009). Most of the normal listric faults map of the Oligocene sequence which shows in the basin dipping to the NW until WNW, clear E - W trend of the sediment thickness implies that during the second rift phase the controlled by E - W trending fault (Figures basin extended to the SE. The Con Son swell 6 and 9). The E - W fault trend in Nam Con to the NW of the basin acted as a rigid block Son Basin during the Eocene-Early Oligocene prohibiting further basin propagation to the is consistent with the oldest magnetic linea- W and NW (Figure 2). These faults created tion (~32 Ma) oriented ~E - W in the East grabens, half grabens, and roll-over structures Vietnam Sea caused by the slab-pull from the which are typical and easy to recognize on

132 Seismic Interpretation of the Nam Con Son Basin and its Implication for the Tectonic Evolution (N.Q. Tuan et al.)

a b

5 km 5 km

354.272 c d 795.834 1237.396 1678.959 2065.326 2506.889 2948.451 3334.818 3776.380 4217.943 4604.310 5045.873 5487.435 5873.802 6315.365 6756.927 7198.490 7584.856 8026.419 8467.981 8854.349 9295.911 9737.474 10123.841 10565.403 11006.966 11393.333 5 km 5 km

Figure 6. Total sediment thickness to seabed generated for (a) Top Basement, (b) Top Oligocene, (c) Top Lower Miocene, (d) Top Middle Miocene (Tung, 2015).

NE-SW fault formed in Middle Miocene DBSCL-01

N-S striking faults

NNE-SSW IJOGstriking faults

E-W faults formed 5 km Natuna High in Eocene Early Oligocene

Figure 7. Fault map of the NCSB. Four major fault systems were recognized. The W-E faults initiated in Eocene – Early Oligocene, associated with E-W trending grabens and are buried beneath the Early Miocene sequence.

133 Indonesian Journal on Geoscience, Vol. 3 No. 2 August 2016: 139-149

N S

Middle Miocene

Late Miocene

Oligocene Eocene

Figure 8. N-S seismic line to the S of the basin showing the E-W fault system. The fault ceased to be active by the middle of Oligocene time. The interpreted horizon in red is top Basement, violet is top Oligocene, green is top Lower Miocene, and blue is top Middle Miocene.

130 09-1 16-2

03 09-3 (DM-NR) DBSCL-01 09-3/11

17/11 25

04-2

18 132

26

19 133

27

20 134

06-1

28

21

13/03 22/03 29/03 IJOG 136/03

5 km

Figure 9. Isopach map of Oligocene sequence with overlay of faults activated during the first extentional phase in Eocene - Early Oligocene time. The basin was likely E -W orientation, which is consistent with E - W trending faults.

seismic sections as well as on structure maps 4. N - S trend which is mostly recognized in (Figure 11). The last major fault system is of: the western part of the basin. These faults

134 Seismic Interpretation of the Nam Con Son Basin and its Implication for the Tectonic Evolution (N.Q. Tuan et al.)

mainly developed during the second exten- Vietnam Seafloor spreading like the fault sional phase in Early - Middle Miocene, but systems in the east of the basin. Several of being further to the west their trends reflect them are still being active until today which the reactivation of pre-existing weak fabric make N - S faults distinctive from the rest and not affected by the main trend of the East (Figure 12).

NW SE

Post rift

NW-SE Middle Miocene rifting

Inter-rift Late Miocene

Oligocene Eocene N-S rifting

Pre-rift

Figure 10. NW - SE seismic line in the NE of basin showing NE-SW fault system, which was active during the second exten- sional phase and terminated by Late Mioncene time. The interpreted horizon in red is top Basement, violet is top Oligocene, green is top Lower Miocene, blue is top Middle Miocene.

NW SE

Middle Miocene NW-SE rifting Late Miocene Oligocene Eocne IJOG

Figure 11. NW - SE seismic profile showing the NNE-SSW fault system, which was also active in Early to Middle Miocene and terminated by Late Miocene.

135 Indonesian Journal on Geoscience, Vol. 3 No. 2 August 2016: 139-149

W E

Post rift

Middle Miocene

NW-SE Late Miocene rifting

Oligocene Eocene Pre-rift

Figure 12. The W - E seismic line in the south of the basin showing the N - S fault system.

Conclusion causing an increase in sediment supply to the basin depocentre. NCSB has tectonically From the systematic interpretation of seismic data been quiet, only the weak N - S fault system and its integration with the major geological history continues to be active in some areas of the of Nam Con Son Basin, it can be infered as follows: basin. 1. The preceding morphology of the NCS area 4. The significant Eocene - Early Oligocene underwent a regional erosion until the first rifting episode resulted in the deposition rift phase happened in Eocene - Middle Oli- of ~ 5 km of sediments in E - W trending gocene. The N - S extentional force caused grabens. At a regional scale the N - S trending E - W fault system with local E - W grabens basement fabric, which is oblique to the and half-grabens to develop in the whole basin regional extension direction (NE - SW), has area. In the Inter-Rift Phase (Late Oligocene been reactivated resulting in an oblique rifted - Lower Miocene) the NCSB was relatively basin. At a local scale within the basin, the quiet probably because all extensional effects Eocene - Oligocene structure has passively were accomodated by the NW - SE opening controlled the deformation by creating of East Vietnam Sea. heterogeneity within sedimentary covering 2. The second extensionalIJOG phase (Middle Mio- sequences. cene) resulted from the southwest propagation of the NE - SW seafloor spreading zone, due to the proto East Sea subducting beneath Borneo. Acknowledgements This phase is believed to be the cause of the NE - SW, NNE - SSW, and N - S fault systems This research is funded by PetroVietnam and in the NCSB. Vietnam National Foundation for Science and 3. The post rift sequence (Upper Miocene - Technology Development (NAFOSTED) under Recent) is associated with an onshore uplift grant number 105.04-2015.12.

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