Abstract

The primary objective of the Vanda-1 well, in Sceptre Resources's Bunyu PSC, Northeast Kalimantan, proved to be a 90 metre thick, predominantly clean but well cemented and partly shaly limestone. Good oil fluorescence and cut encountered while drilling initiated a coring programme with recovery of four cores composed mainly of limestone, in which four depositional facies were identified. These facies are: coral framestone (Facies A), coral rudstone (Facies B), argillaceous coral floatstone-rudstone (Facies C) and laminated silty claystone (Facies D). Facies were correlated with four cleaning-up cycles recognized from the gamma-ray wireline log, each cycle representing a sequence of reef-growth and progradation, the ideal cycle commencing with Facies D claystones, and ending with Facies A coral framestones.

Although the precise interplay between tectonic and eustatic controls for reef initiation and development is not known, a mechanism is proposed in which continued shallowing due to local tectonics (by generation of a rollover anticline basinward of a large growth-fault system) caused shoaling and hence reef initiation. Later exposure of the reef (relative sea-level fall) is also thought to be due to tectonic uplift of the rollover anticline rather than eustatic change. Therefore, during development of each reef cycle, uplift worked in the opposite sense to, and exceeded the rate of, the background of slow but progressive early Pliocene eustatic gain. Fast relative sea-level rise (either eustatic or as a result of subsidence) rapidly terminated each cycle, as shown by the sharper bed contacts at the tops of the early cycles. Carbonate production ended at the top of cycle 4 as a result of a relative sea-level rise. Absence of back-reef sediments lends support to the proposed mechanism, since reef progradation down-slope during relative sea-level fall would theoretically generate an extensive tract of elevated and exposed reef-crest carbonates, eliminating lagoonal areas.

None of the cored limestones demonstrated good reservoir potential. Porosities and permeabilities are poor due to argillaceous matrix in the lower reef-front facies and to major calcite cementation in other facies. However, secondary porosity may well have been preserved elsewhere in the Vanda-1 build-up, as suggested by mass-balance requirements for the extensive calcite cementation. Also, the hydrocarbon shows encountered indicate migration via permeability pathways which must exist both within and beyond the reef complex. Delta-front reefs are therefore still considered to be potential reservoir targets in the Tarakan Basin. Although this early Pliocene reef model infers the absence of back-reef facies, other younger Pliocene or Pleistocene reefs, which grew under different eustatic or structural conditions, may have developed lagoons containing back-reef facies in which porosity could be preserved.