Abstract

The Tanjung Formation, which is the oldest Tertiary sedimentary section in the Barito basin, has been producing oil since the early 1960's.

Geologic history of the basin can be divided into 3 sedimentation cycles, i.e.:

a. Eocene-Oligocene cycle (Tab-Tcd)

b. Oligocene-Miocene cycle (Tcd-Tf)

c. Miocene-Pliocene cycle (Tgh)

Each cycle is distinctly separated by tectonic activities. During the Eocene-Oligocene cycle the Tanjung Formation was deposited over almost the whole basinal area and overlies the Pre-Tertiary basement unconformably.

The lower member consists of terrestrial and paralic clastics and red beds and the middle member of marine and deltaic shales, sandstones and coals. The upper member consists of neritic shales and silty shales, with a series of thin limestone beds and paralic clastics at the top. There is thinning and pinching out to the west and south onto the north ward tilted basement surface.

A Plio-Pleistocene orogeny caused a westward movement of the Meratus block, which folded and thrust the basin's rocks into a series of north-northeast to south-southwest trending, tight anticlines that probably are controlled by basement features.

Because of their transgressive and deltaic nature and due to the tectonic history of the basin, the Tanjung Formation clastics form highly potential reservoirs for structural and stratigraphic traps.

A structural trap was proven by the discovery of the Tanjung oil Field, which had a cumulative production of 14,836,835 cubic meters by December, 1979, from both pre-Tertiary and Lower and Middle Tanjung reservoirs. A primary stratigraphic (lens) trap is supposed to occur in the Kambitin structure, while secondary (unconformity) traps still remain to be discovered.

So far, in the subsurface this formation is only recognized from well log data, except in the Tanjung Raya area where it is also seismically identifiable.

Integrated exploration efforts, using modern exploration concepts and tools, should undoubtedly be able to identify the formation in the subsurface in other areas and locate more structural and stratigraphic traps.