Karst in Miocene Carbonate Buildups of Central Luconia Province, Offshore Sarawak, Malaysia

Abstract

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.