Abstract

The TUNU gas field is located in the Mahakam Offshore area, East Kalimantan, on the Bekapai-Attaka structural trend.

Three wells were drilled in 1973 and 1974 on the eastern part of the TUNU structure: Tanjung Bayor 1, 2 and 3, but all were dry or non-commercial. Tunu 1 drilled in 1977 found a significant thickness of gas bearing sands, but the existence of important reserves was proved by six wells drilled later in 1982–1983.

The structure is a very gentle asymmetrical anticline elongated North-South and over 19 kms in length (distance between wells Tunu 5 and Parangat 2).

The areal and vertical closures of the structure are more than 100 km² and 40 m respectively, with the apex shifting gradually westward with increasing depth. Seismic isochron maps are not entirely satisfactory because of static correction problems and velocity anomalies, therefore the lateral continuity of seismic markers is not sufficient for confident mapping.

Sand percentage decreases progressively from West to East. Hydrocarbon bearing reservoirs encountered between 2200 m and 4000 m are, in general, of delta front to shelf bars, consisting of thin bedded sand intercalating with shale, whilst some scarce channels are present.

Due to numerous thin reservoirs, a comprehensive test programme has to be carried out. Gas produced during DST is important by considerable condensate production. Reserves are estimated to be significant and of the order of several Tcf.

A delineation program including additional seismic and intensive drilling is underway in order to investigate the extension of the TUNU field and improve reserve estimates.