Abstract

Exploration for oil in reef traps of the Miocene Kais Formation has been undertaken in the Salawati Basin since before the Second World War. At least 500 exploration and development wells have been drilled to date and these well data, combined with seismic data, have allowed a model of reefal development within the Kais Formation to be constructed.

Log markers have been utilized to define time-equivalent horizons in the section overlying the Kais Formation. Constructing isopach maps between these markers and the Kais Formation allows the height to which Kais reefs grew to be determined. The reefs grew on a widespread carbonate platform during transgressive episodes in the Miocene. The Kais reefs developed in a series of stages evidenced by the concurrence of groups of reefs with common thicknesses of section between the overlying marker and the tops of the reefs. In the main productive part of the Salawati Basin three such stages can be recognized. Each stage consists of a period of reef upgrowth during a transgressive expisode followed by termination of reef growth during minor regressive phases. Renewed transgression led to renewed upgrowth on some reefs while on others growth was not re-established. Towards the end of the Miocene, reef growth and carbonate deposition was terminated by an influx of fine and coarse clastics and rapid basin subsidence.