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

DOI:10.17014/ijog.3.2.127-137The 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.


Introduction
The Nam Con Son Basin (NCSB) is located in the south-eastern continental shelf margin of Vietnam with an area of circa 110,000 km 2 , bordered by Con Son swell in the northwest, Khorat -Natuna Arch in the southwest, and Tu Chinh High in the east (Figure 1).
Multiple institutions and geoscientist teams have been studying the basin, with many articles that have been published. However, there has not been any significant update on the tectonic evolution and basin history despite a large amount of newly acquired seismic and well data. This causes high uncertainties in the evaluation of petroleum system trap integrity and HC generation and migration. The probability of petroleum exploration success for commercial discovery in the basin is rather low (16% -unpublished data) probably because of the complex tectonic settings of the basin that has not fully been understood. In this article, the authors present a revised interpretation of the tectonic development in the basin scale based on the entire set of seismic and well data. Basin tectonic evolution is critical to improve understanding of the evaluation of petroleum system in the Nam Con Son Basin.

Regional Geologic and Tectonic Settings of The Basin
In Southeast Asia, by Paleocene, the southeast extrusion of the Indochina Block and southward drift of the proto East Vietnam Sea associated with the collision of Luconia micro continent and Borneo caused a series of rightlateral transform faults in the East Vietnam Shelf extending to the East Luconia. This combination possibly derived N -S extension in the Sunda Shelf (Hall, 2002(Hall, , 2009(Hall, , 2013Hutchison, 2004;Clift, 2008). Matthews (1997) and Fyhn et al. (2009) proposed the onset of rifting in the proto East Vietnam Sea (EVNS) as well as the NCSB initiated in the Eocene and lasted to the Early Oligocene (about 30 Ma). This rifting period is controlled by N-S extension, associated with E -W oriented faulting and deposition of rift-fill sediments in local W-E trending subbasins ( Figure 2).
The rifting phase in Eocene -Early Oligocene then was followed by the spreading of East Viet-nam Seafloor, which propagated initially from E to W then WSW. At about 25 Ma, the axis of the seafloor spreading shifted from WSW trend to SW trend (Andrew, 2010;Morley, 2007;Pubellier and Morley, 2014).
At the end of Early Miocene, southwestward propagation of the seafloor spreading continued by a continental breakup, caused the second extension phase at the SW of the rift tip including NCSB, associated with NE -SW normal faults and deposition of syn-extension sediments in NE -SW grabens. The second extension phase is derived by NW -SE regional extension as a slab-pull of the SE drifting Dangerous Ground and the subduction of the proto EVNS beneath NW Borneo.
Well pronounced NW -SE central graben extension and accompanied progressive large scale listric faulting were probably initiated from the middle of the basin to its margins creating a unique depocentre with large scale roll-over structures with their crests in the middle of the grabens. The large scale faults penetrate the whole sedimentary cover to the basement. tally ceased at about 17 -16 Ma, regional sea level fell during the late Middle Miocene leading to the erosion (up to several hundred meters) of the central part of the roll-over structures. The second rifting phase was then followed by a thick post-rift sequence (Upper Miocene -Pliocene -Quaternary) due to the increase in sediment supply with respect to onshore uplift and magmatism (Fyhn et al., 2009;Spackman, 2015, Nguyen Hiep, 2007;Tri and Khuc, 2011).
Stratigraphically, various rock sequences consisting of pre-Tertiary fractured granite and clastic sediments of Oligocene, Lower Miocene, Middle Miocene, and Upper Miocene age fill in the basin. Those Tertiary clastic sequences underlain unconformably by the pre-Tertiary fractured granite are source rocks and reservoir rock of potential hydrocarbon (Figure 3).

Datasets and Methods
This study is based on a number of reflection seismic profiles, totally 54,147 km. They cover Bu ki t M er sin g Li ne L u p a r L i n e W e s t B a r a m L i n e P a l a w a n T r o u g h   Figure 4). The quality of seismic data is fair to good in overall, with only a poor quality area around the central basin of Block 06-94, 05-2, and 05-3, where the basement is too deep. 3D seismic data available in Block 06-94, 05-2, and 05-3 were used to improve the quality of seismic interpretation in these areas.  Figure 3. General stratigraphic column and petroleum system summary of Nam Con Son Basin.

I J O G
Age control is provided to the seismic stratigraphy through a biostratigraphy mode from selected wells located on the different blocks, as shown in Figure 4. These ages were transferred to the seismic profiles after converting drilling depth to two-way travel time (TWT) using the stacking velocities derived from processing the MCS data. Those dated horizons were then correlated across the entire study area. The oldest age pick from the biostratigraphic data was the top of Oligocene. The base of (?) Eocene-Oligocene strata is inferred by the top basement reflector, which can be followed over long distances across the study area. Age resolution is best for Miocene strata because a number of wells have penetrated these formations. In contrast, the Plio-Pleistocene was not cored by industrial wells, so there is no detailed date subdivision of those units.

Results and Discussion
The regional tectonic settings, data, and methods described above are the background guidelines for detailed data interpretation in this study.  Figure 5). After gridding, the time structure maps were generated, then were converted to depth domain. Several time depth conversion methods have been considered for generating depth structure maps of key sequences in the basin. After a careful quality test, the depth structure maps of these key sequences have been generated from the time interpretation by the layer cake method. Four structure maps of Top Basement, Top Oligocene,     6 and 9). The E -W fault trend in Nam Con Son Basin during the Eocene-Early Oligocene is consistent with the oldest magnetic lineation (~32 Ma) oriented ~E -W in the East Vietnam Sea caused by the slab-pull from the southward movement of Dangerous Ground due to the proto East Vietnam Sea subducting beneath Borneo (Hall, 2013). 2. The NE -SW trend faults developed strongly in the northeastern part of the basin with listric geometry during the Middle Miocene sequence as a consequence of SW propagation of East Vietnam Seafloor spreading ( Figure   10). In the central part of the NCSB, the NE -SW faults gradually bend to: 3. NNE -SSW trend implying that the faults was influenced by a N -S previously existed basement rock fabric. We suggest the weak basement fabric possibly formed earlier than the first rift phase in NCSB as exhibited in the tectonic model proposed by Hall (2009). The model implies that in the Early Eocene, the Sundaland region went into regional N -S compression as Australia attempted to move north. As a consequence, approximately E -W extension caused N -S basement fabric to form (Hall, 2009). Most of the normal listric faults in the basin dipping to the NW until WNW, implies that during the second rift phase the basin extended to the SE. The Con Son swell to the NW of the basin acted as a rigid block prohibiting further basin propagation to the W and NW (Figure 2). These faults created grabens, half grabens, and roll-over structures which are typical and easy to recognize on   mainly developed during the second extensional phase in Early -Middle Miocene, but being further to the west their trends reflect the reactivation of pre-existing weak fabric and not affected by the main trend of the East Vietnam Seafloor spreading like the fault systems in the east of the basin. Several of them are still being active until today which make N -S faults distinctive from the rest ( Figure 12).

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