Sedimentary provenance of the Cenozoic deposits of Natuna Island, Indonesia: Implications for onshore to offshore correlation and CCS in Sundaland

The benefits of onshore analogue studies for understanding the stratigraphy of offshore basins have long been known and used as a way of determining depositional pathways and environments. Onshore studies are of particular importance in SE Asia, where the links between onshore and offshore geology can be observed in present day systems as well as in the geological record. Few locations in SE Asia exemplify this intrinsic relationship more than the island of Natuna, off the northwest coast of Borneo. Natuna lies within the Indonesian Riau Archipelago, South China Sea and sits atop the Natuna Arch, a structural high that straddles the East and West Natuna basins, representing a type example of how we can use fieldwork campaigns to better understand subsurface geology. This research records a well preserved source-to-sink package, including an uplifted granitic basement that eroded into interbedded conglomerates and quartz-rich sandstones that were deposited in deltaic and paralic environments throughout the Cenozoic. U-Pb zircon geochronology, heavy minerals, and light minerals record dominantly local sourcing for Cenozoic sediments in the region with periodic influx of more diverse sources during the post-rift basin interlinkage phase (e.g., the Peninsular Malaysia and West Borneo). Uplift of the Natuna Arch during the two basin inversion events since the Oligocene has led to the erosion of the granitic basement into a series of sandstones and overlying siltstones that now form the lowlands of the island (the Raharjapura and Pengadah formations, respectively). These onshore sediments correlate directly in both age and sedimentology with Oligocene to Pliocene successions in the West and East Natuna basins that flank the island (e.g., the Gabus, Arang, and Muda formations). The exposure of these sediments onshore with direct access to their source material provides us with an unparalleled analogue for studying the potential for CCS projects in the subsurface.