Abstract

The Barito Basin in Southeast Kalimantan is located along the southeastern edge of the stable Sundaland continent. Extensive exploration has been conducted in the basin since late in the 19th century and has been rewarded with the discovery of four commercial oil and gas fields located in the northeastern end of the basin (Tanjung Raya area). The last commercial field was discovered in 1967. Since then not a single commercial discovery has been made. Nonetheless, recent studies indicate that a substantial amount of oil has been generated and expelled in this basin. The basin creates a dilemma. Why has a basin with good source rocks, good reservoirs, and multiple trap-forming tectonic events so far produced so little oil?

This study contributes one of the many ideas needed to solve this "Barito Dilemma". The idea focuses on the tectonic implication of the Paleogene unconformities in the basin to infer the occurrence of tectonic events which, in turn, reveal the occurrence of Paleogene structural traps. The study recognizes three unconformities:

(1) in the latest Eocene.

(2) in middle Oligocene.

(3) in the latest Oligocene.

These unconformities are contemporaneous with distinct faunal changes, plate readjustment around Sundaland and with eustatic sea level drops. The middle Oligocene unconformity is major and coincides with structure-forming tectonics. The unconformity reveals the presence of paleo-structures involving the Tanjung Formation. These may play a role as paleostructural traps (paleo-anticlines) and were favorably positioned for the entrapment of early migrating hydrocarbons. Isopach maps indicate that these paleotraps occur in areas where the Tanjung Formation is thin. The preserved paleo-structural traps, as yet undisturbed by the ongoing basin inversion, are thought to have major importance for the hydrocarbon potential of the area.