Abstract

Sandstone reservoirs at Attaka field offshore, E. Kalimantan, have produced more than 500 million barrels of oil and 880 billion cubic feet of gas during the past 20 years. Based on analyses of cores, wireline logs and subsurface maps, we interpret the Attaka reservoirs as deposits of fluvial and distributary channels and delta-front bars that are preserved within a series of Upper Miocene depositional sequences. Reservoirs as thick as 55 ft, that we believe to be fluvial and distributary deposits, are coarse- to fine-grained, cross-bedded sandstones that have sharp, erosional bases. Rocks that we interpret as distributary channel sandstones differ from fluvial sandstones in that they contain burrows and/or muddy and coaly "drapes," possible indicators of tidal influence. We interpret 1-15 ft thick, bioturbated to laminated, fine- to very fine-grained sandstones as delta-front bars. These numerous, dominantly thin-bedded reservoirs commonly have gradational bases and become coarser grained and less muddy upward, but they typically display a high degree of vertical and lateral heterogeneity (as indicated by high-resolution probe permeametry of cores). The primary controls on porosity and permeability in Attaka reservoirs are depositional: Mean porosity and median permeability for fluvial sandstone cores are 30% and 1040 mD; for distributary sandstones, 27% and 390 mD; and for high-energy, delta-front sandstones, 18% and 21.5 mD. However, sand provenance and diagenesis also influence reservoir quality and performance. Most of the shallow sandstone reservoirs at Attaka field contain abundant volcanic rock fragments, feldspar grains, and smectite pore-lining cement. These reservoirs have lower porosity and permeability than the more quartz-rich sandstones from underlying zones, and they commonly suffer from completion difficulties, including sand production and fluid sensitivity.