Several oil fields have been discovered in the Mahakam River Delta including Attaka, Badak (1971), Semberah, Nilam, Sanga Sanga, Bekapai (1972), Handil (1974), Samboja, Jakin and Sepinggan. The Handil, Badak and Bekapai fields are anticline structural traps with oil reservoir sandstones between 450 and 2900 m. The delta is in the Kutei basin, bounded by the Mankalihat and Paternoster carbonate arch, containing Eocene shales overlain by Oligocene fluvial deposits during marine regression, culminating in the formation of the delta in the late Miocene.