BERAI CARBONATE DEBRIS FLOW AS RESERVOIR IN THE RUBY FIELD, SEBUKU BLOCK, MAKASSAR STRAITS: A NEW EXPLORATION PLAY IN INDONESIA

摘要

Most of the productive carbonate reservoirs in Indonesia are shallow water, high energy carbonate banks and reefal build-ups. The main porosity development in these reservoirs was developed by sea level fluctuations that generated secondary porosity. However, the reservoir in the Ruby Field, Sebuku Block, South Makassar, was deposited under very different conditions and is a good example of a less well known carbonate play type.The Oligo-Miocene Berai Limestone in the Ruby Field consists of re-deposited carbonate located on the outer slope of a large carbonate platform, which has been folded into a structural trap. The limestone clasts range from pebble-size to boulders within a matrix of lime mud and fine abraded bioclasts. The transported packstone and wackestone clasts contain diverse bioclasts that originated in shallow water, mixed epi-reefal and back-reefal environments. The reefal limestone was subject to erosion and re-deposition in a fore-reef toe of slope environment as a debris flow fan, recognizable on seismic. The pore system is contained in both the clasts and matrix, and is enhanced by fractures associated with the folding. Porosity in both the transported clasts and matrix is mostly mouldic and vuggy, resulting mostly from dissolution of foraminifera and red algae after transportation, although some pre-transportation porosity also survives.The carbonate in the Ruby Field has proven to be of good reservoir quality in the Makassar Straits-1 (MKS-1), Makassar Straits-3 (MKS-3) and Makassar Straits-4 (MKS-4) wells, but of lower quality in the Makassar Straits-2 (MKS-2) which can be seen on seismic to be the only well thatpenetrated carbonate between fan-lobes. Excluding MKS-2, these wells penetrated 288-330 feet gross carbonate with net pay between 122-279 feet, with average porosities between 15-17%. MKS-3 well flowed 39 MMSCFGPD sweet gas during an open-hole test and MKS-4 well flowed 39 MMSCFGPD combined from two cased hole tests.