Abstract

The Bungalun Area lies on the southwestern side of the Mangkaliat Peninsula. Geologically, it is more related to the Mangkaliat Province than to the Kutai Basin. Oil was first recognized here in the late 1800s. Several shallow wells were productive between 1900-1941 from thin layers of sand of Late Miocene to Pliocene age, although geochemical analysis indicate that the source rocks are of Eocene o Early Miocene. The problem of the exploration in the Bungalun Area is to find reservoirs of sufficient size to be economical for development.

The stratigraphy of the Bungalun Area is underlain by oceanic basalt or melange (Danau Formation), and "unconformably" overlain by two main facies. The western facies, belonging to the Mangkupa and Beriun Formations of Eocene to Early Oligocene age, have moderate to good porosities of more than 1000 m thick. The correlative eastern facies is dominantly tuffs of the Sembulu Formation having poorly developed primary porosities.

The Oligocene-Early Miocene to younger sequences consist of two facies. The peripheral zones are dominated by limestones which are progressively younger towards the southeast, beginning with the Oligocene Kedango limestones (700 m), Early Miocene Tabalar limestone (500 m) and Middle Miocene Sekerat limestone (200-300 m). The deeper parts of the basin are dominated by fine elastic sediments, shales and silts.

The Structural configuration of the Bungalun Basin is on nearly the same north-south trend as the Kutai basin, except those near the Sangkulirang Bay, which have an east-west direction. This change of trend was primarily caused by the rotation effect of the Palu-Koro Fault further east.

This paper will give a general outline of the possible extent of the Mangkupa and Beriun Formations as well as attempt to clarify the natural boundary of these units to the Sembulu tuffs further east. This will be based on the regional tectonic history of the Mangkaliat Peninsula as well as a reinterpretation of the existing seismic data.