Revisiting structural styles around northern Malaysia, Indonesia, and Brunei offshore basins

March 17, 2022

We summarize preliminary results of the structural styles obtained from a bibliographic review and our expertise related to the tectonic analysis, the performance of quality controls, and the balanced cross-sections throughout these basins. The seismic interpretation and the review of the surface geology also included in this work allow delimiting several structural domains. This paper will be published at the 83rd EAGE Annual Conference & Exhibition

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Introduction

Hydrocarbon exploration challenges along south-east Asia basins are associated with a wide variety of geologic plays: tectonic, stratigraphic, and combined. In consequence, understanding the structural styles along prolific basins, such Malay, West Natuna, Sarawak and Sabah are fundamental for assessing ongoing exploration campaigns.

Here we summarize preliminary results of the structural styles obtained from a bibliographic review and our expertise related to the tectonic analysis, the performance of quality controls and the balanced cross-sections throughout these basins. The seismic interpretation and the review of the surface geology also included in this work allow delimiting several structural domains. A second phase would provide a good understanding of deformations timing as well as possible relationships between different structural styles and hydrocarbons migration and trapping.

Methodology

One of the first activities to carry out before performing the analysis of structural styles of any sedimentary basin is to evaluate the geodynamic setting and its associated tectonic stress regime. It´s recommended to carefully read Heidbach et al. (2018) and Mustafar et al. (2017) to acquire and optimum understanding of geodynamic settings and associated stress regime along the lithosphere of southeast Asia.

Maximum horizontal stress orientations can be separated into two main areas (Figure 1)

NE-SW west of Natuna high, and
WNW-ESE east of Natuna high.

*Figure 1* *Studied area is located inside yellow rectangle. Dotted lines indicate maximum horizontal stress orientation along top of basement (based on Mustafar et al., 2017, and Heidbach et al., 2018)*.

Isopach maps are also important to compile when performing regional basin assessment. In many cases, depocenters would indicate main tectonic trends and potential hydrocarbon kitchen areas when performing early stages of hydrocarbon exploration. Sedimentary thickness along the studied area is shown on Figure 2 (PETRONAS, 2021) and regional stratigraphy has been recently updated by Morley et al., (2021).

The Malay basin depocenter oriented NW-SE in associated to Paleogene extensional faulting whereas West Natuna basin main trend is NE-SW likely extending towards the north-east (Nan Con Son basin, Vietnam).

The most prominent depocenter is located east of Natuna High, along Sarawak and Sabah basins, with an approximate thickness of 7 to 9 kilometres. The geometry of this thick and elongated depocenter is shaped like a “boomerang”. In its central part it extends towards onshore, mostly around Brunei territory. As for the tectonic control of this depocenter, little or no clear evidence have been discussed or postulated in public domain references.

Figure 2 Tertiary isopach map (modified from PETRONAS, 2021). Malay and West Natuna are located to the west of Natuna high whereas East Natuna, Sarawak and Sabah basins are located to the east.

Structural styles domains

Following a public domain data review and our own experience, six major structural domains have been mapped across northern Peninsular Malaysia (2 domains) and Borneo Island (4 domains), which are separated by Natuna basement high (Figure 3), also known as Natuna arch.

From west to east, these are major structural domains:

Domain 1: Malay basin. Paleogene Extension - Miocene contraction (“Thick-skin” tectonics). Oriented NW-SE.
Domain 2: West Natuna basin. Paleogene Extension - Miocene contraction (“Thick-skin” tectonics). Oriented NE-SW.
Domain 3: Paleogene Extension - Miocene contraction (“Thick-skin” tectonics). East Natuna basin. Oriented NNW-SSE.
Domain 4: Neogene mud-diapirs and Gravity – spreading features (extension vs. contraction /
"Thin-skin” tectonics). From Sarawak to Sabah basins. “Boomerang” shape.
Domain 5: Neogene gravity-gliding features (“Toe-thrusts” / “Thin-skin” tectonics). Sabah basin. Oriented NE-SW.
Domain 6: Deep-seated extensional faulting (“Thick-skin” tectonics), associated to south China sea Miocene rift over a highly extended continental crust. Deep-water Sarawak and Sabah basins. ENE-WSW oriented.

Figures 4a and 4b indicate main structural styles along domain 2 (West Natuna basin) and domain 3 (East Natuna basin): Miocene inversion of Paleogene grabens (modified from Dajczgewand, 2004, and PETRONAS, 2021).

When interpreting seismic images, it remains unclear the behaviour of deep-seated faults along domain 4 depocenter: are they inverted or not?

Figure 3 Structural domains along northern peninsular Malaysia and Borneo Island: 1, 2 and 3: Paleogene Extension - Miocene contraction; 4: Neogene mud-diapirs and Gravity – spreading features; 5: “Toe-thrust” and Neogene gravity-gliding systems; 6: Deep-seated extensional faulting. Stars indicate approximate locations of figures 4a, 4b and 5.

Neogene mud-diapirs have been documented from Sarawak to Sabah basins along domain 4. It´s important to note that this area coincides with the thickest and elongated “boomerang” shape depocenter along those basins, including onshore areas (Figure 5). Active mud-volcanoes are also included in this structural domain, some of them along Sarawak and Brunei onshore.

Figure 4 Two examples of Miocene tectonic inversion: (a) West Natuna Basin (Dajczgewand, 2004); (b) East Natuna basin (mod. from PETRONAS, 2021;1:1 scale). See Figure 5 for locations.

John et al (2016) document more examples of gravity-spreading features, such as extensional faults and "toe thrust" systems along domain 4. In general, the over pressured shales of the Setap Formation (Miocene) are considered responsible for triggering mud-diapirs and mud-volcanoes along Sarawak and Sabah basins.

Figure 5 Example of mud-diapir structures along Sarawak onshore, immediately west of Brunei (mod. from John et al., 2016). This example corresponds to domain 4 (Figure 3).

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Conclusions

A wide variety of structural styles and tectonic processes have been documented around the northern offshore basins of Malaysia, Indonesia, and Brunei.

Malay, West Natuna and East Natuna basins are dominated by structures associated with Paleogene extension (“thick-skin” tectonics) and Miocene inversion. Across Sarawak and Sabah basins gravity-driven processes (“thin-skin” tectonics) are very important: Neogene “toe-thrust” systems and structures associated to mobile shales and mud-volcanoes. Deformations along deep-water Sarawak and Sabah basins are related to extensional faulting contemporaneous to south China sea Miocene rifting, along a highly extended continental crust.

Further studies are needed to refine relationship between structural timing and hydrocarbons generation-expulsion phases to make oil and gas exploration more efficient along these prolific basins. A more detailed structural study can also be very helpful in the risk evaluation of the different elements of diverse petroleum systems.

Acknowledgements

Thanks to PETRONAS colleagues for fruitful discussions between 2015 and 2020. The ideas presented herein, however, are solely the author’s interpretation.

References

Dajczgewand, D. [2004] Tectonic evolution and structural styles of deformation of southern Kakap block, West Natuna basin, Indonesia. Congreso de Exploración y Desarrollo de Argentina, 12 p.

Heidbach, O., Rajabi, M., Cui, X., Fuchs, K., Müller, B., Reinecker, J., Reiter, K., Tingay, M., Wenzel, Fr., Xie, F., Ziegler, M.O., Zoback, M.L. and Zoback, M. [2018] The World Stress Map database release 2016: Crustal stress pattern across scales. Tectonophysics, 744, 484 - 498.

John, J., Kessler, Fr., Noon, S. and Tan, Tr. Q. [2016] Structural Development, Deposition Model and Petroleum System of Paleogene Carbonate of the Engkabang - Karap Anticline, Onshore Sarawak. Berita Sedimentologi, 34, 5 – 46.

Morley, R.J., Salwa,S., Morley, H.P., Hardi, J.M., Mansor, A., Aripin, M.R., Nordin, M.H. and Rohaizar, M.H. [2021] Sequence biostratigraphic framework for the Oligocene to Pliocene of Malaysia: High-frequency depositional cycles driven by polar glaciation. Journal of Palaeogeography, Palaeoclimatology, Palaeoecology, 561, 1, 37 p.

PETRONAS [2021] Malaysia Bid Round 2021 (MBR 2021). https://www.petronas.com/mpm/ (website accessed Jan 8, 2022).

Mustafar, M.A., Simons, W.J., Tongkul, F., Satirapod, C., Omar, K. M. and Visser, P.N. [2017] Quantifying deformation in Northern Borneo with GPS. Journal of Geodesy. 91, 1241–1259.

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This paper will be published at the 83rd EAGE Annual Conference & Exhibition

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