Abstract

Hydrocarbons within the North Sumatra Basin are produced mainly from both clastic and carbonate reservoirs of Miocene age (i.e. the Peutu/Belumai, Baong and Keutapang Formations). Currently, hydrocarbon exploration in the area is becoming more and more difficult since the basin has been intensely explored since the beginning of the century. The objective of this paper is to describe hydrocarbon potential in the Tampur Formation, a new and promising objective present below the existing "zone of interest" and an alternative drilling target for future exploration in North Sumatra.

The Tampur Formation was deposited as a thick and uniform shelf carbonate sequence over a broad shallow shelf during the Late Eocene. The formation is widely distributed over the Tampur Platform. The western margin of this shelf carbonate was apparently marked by the major north-south oriented Lokop-Kutacane Fault zone. East of this fault zone, reefal buildups occurred on the shelf edge.

The Tampur Formation carbonates have been described at outcrop as being of potential reservoir quality. Dolomitisation during diagenesis may have resulted in the formation of potential reservoir rocks. Areas where limestones are present within the dolomites may contain primary porosity. Secondary porosity may be present as fractures or vugs.

Since the early structuring of the basin, Tampur Formation limestones and dolomites have formed basin highs, structurally above and adjacent to, thick shale sequences laid down in the intervening troughs. These shales have been mature and generating hydrocarbons since the Miocene.

Significant quantities of gas have already been tested from the Tampur Formation where it occurs beneath the Peutu carbonates at Alur Siwah, Peulalu and from beneath certain of the Malacca Limestone Member reefs offshore. Strong gas shows are also recorded from the Tampur Formation at the Sembilan-A1 well in Aru onshore area.

Although the Tampur Formation could be considered as an additional drilling target, it does contain some risks. It is also postulated that a substantially untested hydrocarbon play exists within the Tampur limestones/dolomites and these could be tested where the formation is structurally high within the basin.