Abstract

The Late Miocene sedimentary sequence within Serang Field in the offshore portion of the Kutei Basin, East Kalimantan is dominated by fluvial/deltaic and shallow-marine siliciclastic deposits, including hydrocarbon-bearing sandstone reservoirs. Also present within this ancient Mahakam River delta depositional sequence are numerous carbonate units indicative of coral reef growth in a delta-front, marine-shelf setting. One such carbonate unit, the 80-6 Limestone, was cored in its entirety and is the subject of this IPA Carbonate Core Workshop contribution.

The 88.5 feet of full-diameter material in cores 6-8 of the Serang 3RD2 well include the entire 67 feet of the 80-6 Limestone unit as well as 11.5 feet of underlying non-calcareous, shallow-marine delta-front deposits and 10 feet of overlying calcareous shale. A "lower reef" buildup (25 ft thick) displays abundant platy corals (with argillaceous/silty carbonate mud matrix) at the base grading up to a mix of massive and branching coral fragments and mud matrix. An "upper reef" buildup (42 ft thick) begins with a lower, platy-coral-bearing, highly silty and argillaceous carbonate wackestone unit that changes upward to a non-porous, partly-argillaceous coral rubble deposit and a highly porous and permeable, 10 ft-thick, reef-core type coral rubble deposit. The upper 13 feet of the "upper reef" is a non-porous, reworked mix of a variety of skeletal fragments with an upward-increasing amount of silt/sand and detrital clay. The reef is overlain by slightly calcareous and shelly, silty shale representative of shallow marine shelf deposition within a delta-front setting.

Based on subsurface correlations within Serang Field and on observations of similar coral-reef buildups in front of the modern Mahakam River delta, it appears that the 80-6 Limestone represents a coalescent cluster of reef buildups with a lateral extent of at least 2.5 km and possibly up to several tens of kilometers. Modern coalescent clusters of coral reefs have been observed in water depths of 50-100 m (165-330 ft) in front of a less active (partly abandoned) portion of the Mahakam River delta. Individual buildups up to 1.5 km across occur within a coalescent cluster that extends for greater than 30 km in both depositional strike and dip directions.

Post-depositional modifications within the 80-6 Limestone has resulted in significant degradation of its reservoir quality.. Compactional effects have been minimal; however, recrystallization of both skeletal fragments (especially coral and molluscan fragments) and carbonate mud matrix have been extensive. The only part of the reefal buildup with favorable reservoir quality is the 10 ft-thick reef-core facies with up to 25% porosity. Permeability, however, is developed only to a few tens of millidarcies; and, because of the isolated nature of the moldic/vuggy porosity system, permeability does not increase much with a significant increase in porosity.

Because of the probable lateral discontinuity of the porous and permeable reef-core facies within the laterally-extensive delta-front coral-reef buildups, such carbonate reservoirs will continue to be secondary hydrocarbon targets in the Kutei Basin Late Miocene section that is dominated by numerous, favorable sandstone reservoir units.