Radioactive apatite-rich “Hot Sands” of the Tenggol Arch: Stratigraphic curiosity or sub-seismic reservoir correlation tool?

A review of Late Oligocene to Early Miocene reservoir sandstones in the Tenggol Arch area has identified a number of intervals with anomalous radioactive levels within the K, J, and I Tertiary sequences. A correlation between spikes on the spectral gamma ray logs and petrographic analysis of cores and cutting samples points to Thorium-bearing, apatite-rich sandstones and siltstones as the main source of radioactivity. The elevated radioactivity levels occur within meandering channel sequences with sediment derived from the Malay Peninsula, the Johor Platform and perhaps some local cuesta ridges nearby on the Tenggol Arch. Commonly, the anomalous radioactive intervals are represented by flasersand/silt and clay associations. Given the relatively thin radioactive intervals, measuring at most a few metres, these sandy intervals cannot be imaged on seismic data, but could be used as a tool for correlating reservoir units at reservoir or field scale. Furthermore, the high radioactivity due to Thorium (Th) and Uranium (U) could lead to wrong estimates of clay and sand percentages, unless they are corrected with the help of spectral gamma ray logging.