Abstract

Interpretation of recently acquired seismic data in conjunction with borehole and outcrop data in Pertamina Unit 1 area, North Sumatra has revealed inconsistencies in stratigraphic nomenclature. Sequences presently called Middle Baong Sandstone are broadly time equivalent to, and contiguous with, sediments of the lower parts of the Keutapang Formation in the south of the area.

By identifying sequence boundaries on seismic sections, a complex facies boundary can be mapped which explains these inconsistencies in nomenclature, running from the Telaga field south-east towards the Pantai Pakam boreholes. This boundary marks the division between the dominantly deltaic sequences of the Keutapang to the south and the marine Upper Baong Shale to the north. Three separate phases of delta progradation can be identified with source directions ranging from NW to SSW. Successions encountered in boreholes either side of this boundary have been correlated using the seismic as a control.

The Middle Baong Sandstone appears to be a complex of several thin stratigraphic sequences. The most prospective division of the Middle Baong Sandstone comprises a stratigraphic sequence that was deposited only to the north-east of this boundary. It appears to have been deposited in a different environment and sourced from a different direction to the overlying and underlying generally finer-grained divisions of the Middle Baong Sandstone which are related to deltaic sequences prograding from the south-west.

The implications of these findings for the geological evolution of the North Sumatra Basin and its hydrocarbon prospectivity are discussed and an alternative stratigraphic nomenclature proposed which should clarify relationships.