Publications

Lowstand deepwater sands comprise prospective reservoirs in the Middle-Late Miocene sections of the Makassar Strait area, offshore Kalimantan, Indonesia (Figure 1). These sands are envisioned to have been derived from Mahakam delta highstand deposits that were reworked or displaced basinward during sealevel lowstands (Figure 2). Depositional patterns and correlations on the slope and basin floor have been modified by regional compressional folding and faulting, wi1ish are most evident in the Middle Miocene and older sections. These areas were also deformed bj. growth faulting and shale diapirism in much of the Late Miocene and younger section (Figures 3, 4).Sequence stratigraphic techniques involving seismic, well log? and biostratigraphic data provided an initial stratigraphic framework. Because of the structural complexity and related difficulties in seismic imaging, biostratigraphy was a key tool. It was used not only for guiding seismic correlations, but also for developing insight into depositional processes and reservoir distribution (Figure 5). Faunal abundance and diversity curves were used along with age versus depth plots to identify intervals of lowstand sand influx. These also helped validate system tract interpretations made from seismic and log data.Interpretation results indicate that major lowstand influxes occurred within the upper Middle Miocene to lower Late Mioccne interval, a time when eustatic curves also indicate a major second-order lowstand event (Figure 6). The unconfonnity surface at approximately 10.5 Ma associated with this lowstand shows distinctive seismic truncation and onlap, and a blocky gamma ray character occurs at the base of sands that are present (Figures 7, 8). With the sedimentary conditions associated with lowstand processes, planktonic and benthic diversity is low. Diversity increases markedly in the upper Late Miocene and Pliocene sections where generally distal highstand conditions or only small-scale lowstands are thought to have prevailed.Well to well variations in microfossil abundance and diversity profiles have also been useful in identifying spatial variations in sand distribution and relating them to structural controls on sedimentation as expressed by seismic structure and isochron maps (Figures 9, 10). Well control within two faultseparated intra-slope basins shows contrasting biofacies distributions that are consistent with seismic facies and lithofacies observations. These appear to be linked to the timing of faulting and shale mobilization in the area.