Abstract

Exploration has been carried out over the Barito Basin for nearly 100 years, and in 1938 was rewarded with discovery of the Tanjung Oilfield. The search for another major deposit has thus far been unsuccessful. In 1993 Permin Tracer Petroleum Ltd. signed a PSC for the North Tanjung Block which lies in the northern end of the Barito Basin. As a result we have completed an in-depth study which offers several new opinions which differ from the "conventional wisdom" that has prevailed in the literature to date.

Our findings indicate the Tanjung Field is not a thrust-induced fold. Instead we believe there is strong evidence suggesting it is a long-lived anticlinal structure which first experienced folding and initial trap formation in the early Oligocene and again in the late middle Miocene. Oil migration began in the early Miocene and had ample opportunity to fill the proto-Tanjung structure. The strong Plio-Pleistocene thrust faulting episode most likely lifted Tanjung structure as a nearly intact oilfield to its present position. A method of identifying similar pre-existing or "relic" fold structures later disturbed by Plio-Pleistocene thrusting was developed through detailed seismic interpretation.

We recognize three manifestations of the Plio-Pleistocene tectonic pulse. First is a series of west-dipping thrust faults which are cut by a younger series of east-dipping thrust faults. Both the west and east dipping thrusts are then cut by a series of strike slip faults.

The pervasive swarm of east-dipping thrust faults evident in the North Barito Basin appear to be related to the uplift of the Meratus Mountains. The older west-dipping swarm of often imbricated thrust faults appears to be associated with the Kuching Uplift of Central Borneo. A description and explanation for the Barito Basin presence of this latter west-dipping thrust swarm has not previously appeared in the published literature although it has been described in various company confidential reports.

We map a distinct sub-basin for which we propose the name Beraspapan Sub-Basin. The Beraspapan Sub-Basin is clearly separated from the northern Barito Basin by the Ajuh and Kasale Ridges.