The Subis Platform is considered one of the very few outcrops in Malaysia which records remarkable changes in the growth history of a carbonate system. The Subis Platform is located near Batu Niah, Sarawak. Stratigraphically, the Subis Platform is named the Subis Limestone, a member of the Tangap Formation. This article discusses the older succession of the Subis Limestone at the Subis-2 well and the Hollystone Quarry. Both well and outcrop indicate a slightly older succession based on the occurrence of larger benthic foraminifera and calcareous nannofossils. The age of the Subis Limestone ranges from Oligocene to Miocene, based on the occurrence of the larger benthic foraminifera Miogypsinoides sp. (late Oligocene, Te4) and Miogypsina sp. (early Miocene, Te5), as well as on the calcareous nannofossils Sphenolithus capricornutus and Sphenolithus conicus (Te4). The boundary between the late Oligocene and the early Miocene coincides with a sharp change from foraminifera-dominated facies to coral-dominated facies, shown at the Hollystone Quarry. The Subis Limestone records a transgression event from mixed siliciclastic–carbonate (Subis-2 well) to clean biohermal carbonates as shown in the outcrops of the Subis quarries. Our findings on the Oligo–Miocene boundary were then compared with those from other carbonates around Southeast Asia. It is clear that coral reefs existed in Southeast Asia earlier than was first thought, by Oligocene times. The role of localized tectonic events, siliciclastic influx, oceanic mineralization, and Indonesian Throughflow are the main controls to determine the biota changes from foraminifera to coral-dominated facies.

Karst is a common phenomenon in carbonate buildups worldwide. It has significant economic effects on exploration, drilling, field development, and secondary recovery mechanisms. In Malaysia, over 250 Miocene carbonate buildups were mapped offshore in the Central Luconia province of Sarawak, and roughly 65 carbonate buildups have been drilled. Almost every field has encountered indications for high permeability zones likely associated with karst, such as mud losses and drill bit drop during drilling activities; some fields were left abandoned due to mud losses that could not be controlled. Hence, it is important to predict and avoid karst features while drilling, during field operations, and for future carbon dioxide storage, to reduce development costs. The geometry, distribution, and dimension of karst structures in Central Luconia fields remain poorly known, as there have been very few systematic studies conducted. This paper provides a comprehensive characterization of karst features typical of Miocene carbonate buildups. The analysis was based on core and thin-section description, well-log characterization, and seismic spectral decomposition attribute interpretation. Importantly, the subsurface description is supplemented with the analysis of drilling parameters of some 68 wells from 36 carbonate buildups located across the Central Luconia province. A total of 51 wells encountered mud losses of varying intensities, which have been classified and plotted on maps. Loss circulation depths were recorded and compared to core samples that showed characteristics indicative of dissolution and exposure, such as chalkified texture, and were marked on well logs, which often showed unusually high porosity readings in the karstified intervals. All possible karst intervals were documented for further detailed seismic interpretation. Seismic spectral decomposition attributes techniques were applied to seismic data of Jintan and F6 platforms in order to map karst features, which were observed to be dendritic, round, and elongated patterns of several hundred meters in diameter and tens of meters deep. These features are particularly well-developed below backstepping external buildup geomorphology. The analysis showed that particularly strong losses occurred in stratigraphic intervals located toward the center of buildups. These were predicted using seismic spectral decomposition attribute signatures of karst features. The most extensive buildups with the strongest karstification were observed in the central part of the Central Luconia province.