Abstract

Union Oil Company, acting as contractor to Pertamina, undertook exploration of the Sibolga Basin for a period of ten years, beginning January 26, 1968. These activities discovered gas accumulations in six different localities, five of which were from limestone reservoir rocks.

The Sibolga fore-arc basin lies between Sumatra and a partially subaerially exposed outer-arc ridge. The Sibolga Basin is filled with a moderately deformed Neogene sedimentary section 1000 to 15000 feet thick overlying a strongly folded Paleogene sedimentary and volcanic sequence of unknown thickness. The angular relationship between the Paleogene and the Neogene is most noticeable on seismic in the northern part of the basin (Meulaboh area).

Miocene carbonate rocks are located primarily beneath the present-day shelf areas on the eastern side of the Sibolga Basin. The oldest unit in the northern part of the basin is Middle Miocene and possibly Early Miocene. It consists of a widespread shelf limestone with scattered reefs throughout. Gradationally overlying this unit is a deepwater and outer to middle shelf sequence of clay-mudstone-siltstone. This is succeeded by an Upper Miocene carbonate and clastic shelf sequence with localized reefs.

In the southern part of the basin, carbonate deposition began during the late Middle Miocene and continued into the Late Miocene (Singkel area). This produced localized shelf deposits with fewer reefs than to the north. Elongate, narrow shelf carbonate accumulations were formed on the shallow eastern side between the north and south parts of the basin during Late Miocene time.

In the study area methane gas was discovered in Upper Miocene rocks in the Keudepasi 1 and Singkel 1 wells. Both accumulations are in reef to reefshoal deposits.