Unlocking Hydrocarbon Potential in South East Asia

Geological fieldwork is important to help us gain a better understanding of the hydrocarbon development and potential productivity of the offshore Natuna basins of the South China Sea.
This article appeared in Vol. 18, No. 1 - 2021


Unlocking Hydrocarbon Potential in South East Asia 

Conducting geological field work in South East Asia can be challenging for multiple reasons, including often remote and difficult to access locations and tough conditions. Despite this, it can also be incredibly rewarding, providing a wealth of geological knowledge in an underexplored region. Fieldwork-based studies in South East Asia have already improved our understanding of the world’s deepest forearc basin (Pownall et al., 2016), developed detailed tectonic reconstructions of the region (Hall, 2012), and even helped discover when and how fossil hominid species lived (e.g., Homo floresiensis; Sutikna et al., 2016). However, what makes continued fieldwork in South East Asia so exciting, and indeed very profitable both scientifically and in terms of hydrocarbon exploration, is that there is still so much left for geologists to discover.

  • Granitic Basement of Natuna erodes away into white sand beaches. © Max Web.

The Link Between Onshore and Offshore 

Just as Charles Lyell famously said ”the present is the key to the past”, it could be said that the onshore geology of a basin holds the key to sediments deposited off shore. Nowhere is this truer than in South East Asia, where the links between onshore and off shore geology can be observed in present day systems as well as in the geological record. 

South East Asia has been one of the most tectonically active parts of the world for about the last 40 million years, with consistent high sediment yields leading to deposition of thick successions that form productive hydrocarbon plays throughout the abundant basins of the region. One such example is the Salin sub-basin in Myanmar, with over 18 km of sediment deposited throughout the Cenozoic (Hall and Morley, 2004). The relatively recent formation of these basins means that both the source region lithologies and tectonic regimes that deposited sediment into them can still be observed in adjacent onshore areas.

Recent studies focussed on characterising the geology of Myanmar and its potential for sourcing the Nicobar Fan and other regional basins (Gough et al., 2020; Webb et al., 2021), have shown the complex link between onshore source regions and the off shore subsurface. They have also demonstrated the importance of a fieldwork-based understanding of basin bounding highs for the reconstruction of both sedimentary pathways and basin evolution, especially in South East Asia. Few places in this region exemplify this intrinsic link between the onshore and off shore realms more than the small Indonesian tropical island of Natuna, off the northwest coast of Borneo, where the links between sediment source, depositional environment, and tectonic evolution of surrounding basins can be studied in outcrop all within a few miles of each other.

  • Fieldwork in SE Asia often involves boat travels through the tropical seas. © Max Web.

Natuna Island: Complete Source to Sink Package

As you fly into the island’s capital, Ranai, you can see long stretches of white sand beaches fringing a dramatic central peak, surrounded by turquoise seas hosting a series of actively producing hydrocarbon basins. These features provide an early indication that Natuna Island is a perfect location to observe the links between onshore and off shore geology. 

The island lies within the Indonesian Riau Archipelago in the South China Sea. It sits atop the Natuna Arch, a structural high that straddles the East and West Natuna basins and represents a typical example of how we can use fieldwork campaigns to better understand subsurface geology. A recent South East Asia Research Group (SEARG) fieldtrip to the island, for instance, revealed a perfectly preserved source-to-sink package including an uplifted granitic basement shedding sediment into conglomeratic and clean quartz sandstones deposited in deltaic and beach environments throughout the Cenozoic.

Ranai Mountain (Gunung Ranai), the central granitic peak of the island, formed as part of an extensive subduction-related arc in the Cretaceous. Periodic uplift of this mountain and the Natuna Arch during basin inversion events since the Oligocene has led to the erosion of this granitic peak to create a series of quartz-dominated sandstones that now form the lowlands of the island, known as the Raharjapura and Pengadah Formations. These onshore sandstones correlate directly in both age and sedimentology with Oligocene to Pliocene successions in the West and East Natuna basins that flank the island, such as the Grabus, Arang, and Muda Formations (Darman, 2017).

  • Cross-bedded clean quartz sandstones can be found throughout the onshore Cenozoic successions. Inset: Onshore field studies allow the direct targeting of clean sands vs. organic-rich muds on Natuna.

The exposure of these sediments onshore with direct access to their source material at Ranai Mountain provides us with an unparalleled analogue for studying the hydrocarbon producing sequences in the subsurface. A detailed fieldwork-based understanding of the depositional environments, sedimentary provenance, and uplift history of this fascinating island would provide key insights into targeting the clean quartz sandstones shed off of the Natuna Arch and into the surrounding basins. Indeed, whilst both oil and gas are being actively produced from the West Natuna Basin (via the Grabus and Arang formations, respectively) vast natural gas reservoirs of the underexplored East Natuna Basin are yet to begin production. These include the East Natuna Gas Field, which is one of the largest fields in SE Asia (see insert). With this in mind, can a better understanding of the onshore analogues exposed on Natuna Island, including records of both the sedimentology and structural history of the region, aid future production and exploration offshore?

The Future of Fieldwork Natuna

  • Natuna Island with the large granitic peaks of Gunung Ranai in the distance. © Max Web.

Natuna Island provides a snap-shot of the potential of comparatively cheap fieldwork studies for de-risking exploration throughout this region, especially in comparison to expensive seismic and drilling campaigns in frontier regions. As such, fieldwork remains a vital skill in the exploration of offshore basins where the onshore geology so faithfully mimics the rocks, structural styles, and depositional systems found at depth. Universities in the region, such as Institut Teknologi Bandung, among many others, are renowned for training excellent field geologists through immersive fieldwork programmes who will be leading the future of exploration in the region. Furthermore, as we shift into the global energy transition, fieldwork still has a vital role to play throughout the world. In South East Asia, the continued exploration of rare earth mining projects, for example along Vietnam’s eastern coast and northern border, will assist the discovery of new rare earth element resources that will produce renewable energies. Fieldwork can also aid our understanding as to where these renewable projects can be best utilised, including, for example, in the placement of hydroelectric dams. 

‘Boots on the ground’ geologists will always be vital to understanding this geologically complex but incredibly exciting part of the world.


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