Karstification in carbonate platforms of the Miocene age in Central Luconia province, offshore Sarawak, Malaysia, has been discussed since the onset of exploration and initial discoveries in the region, with over 200 mapped platforms to date. An extensive drilling program over the last decade confirmed the existence of karst during the drilling process where issues such as total loss circulation and bit drops were common. Karst in Central Luconia has been proposed by several authors; however, detailed quantitative description of the observed features have not yet been conducted. This study involves systematic mapping of loss circulation depths, chalkified/rubble/vuggy zones described from cores, and vugs of >2 mm in size and moldic porosity observed on thin sections of the Jintan platform. These data supplement the interpretation of karst from multiple 3D seismic attributes. Seismic interpretation of the Jintan and M1 platforms revealed an extensive dendritic pattern which is on average 70–100 m deep and 3–5 km long, and circular geobodies of 1 km in width that exist on the upper part of the platform. Spectral decomposition, also known as time-frequency analysis, was used to enhance the interpretation of karst features on seismics within a specific wavelength. In this study, a comparison of three spectral decomposition methods applied on the 3D seismic cube of the Jintan and M1 platforms was undertaken to determine the method which allowed for better delineation of the karst features. The results show that the short-time Fourier transform (STFT) method using frequencies of 46, 54, and 60 Hz delineated most of the karst features compared to the continuous wavelet transform (CWT) Morlet and CWT Ricker wavelet methods. This paper aims to discuss the dimensions, evolution and geometry of the karst features quantitatively on three selected karst horizons named “K1”, “K2”, “K3”. Interpretation revealed that the dendritic karst features were found to be most prominent on the K2 horizon which lies below a conspicuous change of the external geomorphology of the platform. Backstepping of the platform margin by 12 km is observed in both platforms. Quantitative seismic interpretation shows that the karst observed in M1 platform is approximately 70–100 m deep, and the dendritic features are around 1–2 km in length and approximately 500 m wide; whereas, in the Jintan platform the dendritic features observed are up to 5 km in length with several 1 km wide circular/sinkhole features. More than 20 dendritic features orientated SE and NS were mapped mainly in the transitional area as well as the center of both platforms. The nature of the karst morphology in Central Luconia remains controversial; however, it is proposed to be of mixing zone karst origin.

Stratigraphic forward modelling (SFM) is a numerical method that simulates the key factors that control depositional processes. A few models have been developed over years for different geological environment (fluvial, turbidite and carbonate). The model for carbonate system is particularly more complex compared to others. This is due to fact that carbonate system is controlled by the interplay between carbonate productivity, eustasy, subsidence/uplift. Carbonate platform’s morphology also strongly influenced by hydrodynamic factors (Wind and Waves). SFM has been used to test the hypothesis on factors that controlled the evolution of carbonate platforms. This technique also a reliable tool for hydrocarbon exploration and development. SFM has been used to predict carbonate facies distribution, petrophysical properties, and architecture of carbonate platforms. In this review paper, four SFMs namely CARB3D+, GPM CARBONATE, DIONISOS, SEDPAK are discussed.

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.

No abstract