This paper describes the deposition of Miocene carbonates around Sarawak in a tectono-stratigraphic framework. The onset, termination, and location of the two main carbonate units, the Subis or Lower Cycle II limestones and the Luconia limestone, were controlled by tectonic processes, each beginning with a subsidence event, and terminated by influxes of siliciclastic sediments due to hinterland uplift. New data are presented on the intra–late Miocene decline of Luconia Limestone platforms that is correlated to the uplift of onshore Sarawak (Tinjar Province) and renewed siliciclastic sedimentation, which is dated as being at the same time as major uplift in northern Borneo. Miocene sedimentation around Sarawak was controlled mostly by extensional tectonics with several rapid subsidence events, which produced transgressive unconformities with mappable focal areas. Away from these focal areas, the contrast in facies, before and after the event, gradually diminishes in a predictable manner. This property of the unconformity is governed by Walther’s Law in that one well or field section cannot be exempt from the mappable trends in facies contrast observed in surrounding wells. This relationship constrains the interpretation of seismic, mapping, and analytical data, as illustrated by an example of a misdated unconformity that previously violated this balance of facies change in space and time. The tectono-stratigraphic model is a refinement of an existing empirical scheme devised in the area, with units called “Cycles” (Cycles I to VIII). This evidence-based framework is argued to be a genetic description of depositional units that developed in a dynamically evolving depocenter, subject to geographic rotation and relative variations in sea level that were dependent on location. This shifting basin configuration precludes use of a passive margin sequence stratigraphic approach, which assumes and requires a constant proximal to distal sedimentary direction and steady basement subsidence.

A new compilation of data suggests aragonitic coral reefs were already common in Southeast Asia by the mid-Oligocene. A gradual change from calcite to aragonite seas through the Oligocene and early Miocene appears to be related to a gradual expansion of the importance of scleractinia, along with green algae and mollusks, and an associated decline in the abundance of calcitic larger foraminifera. The larger foraminifera had been important rock-forming bioclasts in the early part of the early Miocene, but were a minor component of carbonate faunas by the end of the middle Miocene. This gradual decline in abundance included a few extinction events that reduced diversity, and these extinctions appear to correlate with periods of tectonic change. The K-selection evolutionary pressure impacted carbonate facies, but foraminifera maintained their taxonomic diversity until the abrupt faunal extinctions. Changes in sea-surface temperature, or the regional change from seasonal to ever-wet climate, do not appear to have impacted larger foraminiferal diversity or caused extinctions, only modified their latitudinal range. Some extinction events can be recognized across the whole Tethys Ocean, as can some of the times of tectonic activity and possible climate change. These correlations tentatively point to a link between large-scale changes in plate motion, oceanography, and foraminiferal extinctions. In contrast, the change from seasonal to ever-wet conditions around the Oligo–Miocene boundary around the South China Sea does not appear to have been caused by a wider tectonic event, and this event does not impact larger foraminifera diversity. A combined tectonic unconformity and mass extinction of larger foraminifera in middle middle Miocene times might have been due to the plate tectonic constriction of a throughflow between the Pacific and Indian Oceans.

Central Luconia is a geological province on the Sarawak Shelf characterized by a widespread occurrence of carbonates of (largely) middle to late Miocene age. These carbonates have been a target of petroleum exploration since the late 1960s, leading to the discovery and development of a world-class gas resource mainly supplying the global liquefied natural gas (LNG) market. Carbonate growth in Central Luconia was initiated during a major regional transgression related to accelerated subsidence from crustal stretching associated with the formation of the South China Sea. Similar carbonate developments are seen elsewhere along the margins of the South China Sea, but the scale of Central Luconia, in terms of the large number of carbonate edifices, is unique. After a short “learning” phase, exploration in Central Luconia readily became extremely successful in the early 1970s, although the hoped-for “big oil” did not materialize; instead, large quantities of, almost exclusively nonassociated, gas were found. Being an export gas play, exploration in Central Luconia has been dictated strongly by market demand and therefore has been discontinuous over time, with fairly long periods of only piecemeal activity or even complete inactivity. In recent years, through growing LNG demand and improved commercial incentives, the play has seen a remarkable revival in terms of both activity and success. Despite its maturity with over 100 exploration wells drilled, the play still has important gaps in understanding, notably with respect to prospect specific charge and retention issues, and as a result, some very significant late-stage discoveries were possible. The carbonates proper have been the least of the concern in the total exploration effort to date; virtually without exception, wells drilled found carbonate reservoir rock of adequate quality for production of gas.

Middle to late Miocene carbonates from Central Luconia, offshore Sarawak, Malaysia, contain significant hydrocarbon reserves. However, the complex pore system of the carbonate reservoir poses drilling and production challenges, such as water coning. Moreover, capturing and storing CO 2 in depleted carbonate buildups requires the pore type architecture to be well understood. The aim of this study was to investigate pore types in a stratigraphic context and to propose a 3D conceptual model of the pore type distribution. The case study discussed here is the E11 Field. E11 is considered the type location for Central Luconia carbonates because of its unique, almost complete core coverage. The data used for this study included a 3D seismic volume, core descriptions, together with petrographic and petrophysical data. The workflow used involved partitioning the buildup into specific lithofacies, pore, and cement types within stratigraphic sequences and depositional environments. Results show that the E11 Field represents a coral and foraminifera-dominated isolated carbonate platform. Fifteen lithofacies and ten microfacies were identified. Paragenetic alterations include five stages of calcite cement, three stages of dolomite cement, one stage of dedolomite, and a minor stage of pyrite mineralization. Diagenetic changes took place in various environments ranging from early marine phreatic, to mixed meteoric-marine, to meteoric realms. Minor burial diagenesis led to the formation of late-stage cements. Early diagenetic alterations closely resemble the primary facies arrangement in distinct environments of deposition and stratigraphic sequences. Interestingly, these sequences mimic in places distinct changes of the seismic geomorphology of buildups. In particular, the middle to upper Miocene boundary (TF2/TF3) coincides approximately with a major reduction in buildup diameter. This backstep corresponds to a meter-thick, low-porosity flooding interval observed in the core of the E11 buildup. Tight (low-porous) layers in the E11 buildup mark the upper and lower boundaries of stratigraphic sequences and are partially traceable on seismic reflection data across the buildup. A lithological correlation across the E11 field showed that wells located near the inner, lagoonal part of the buildup are more prone to dolomitization and attract higher thicknesses of low-porosity flooding interval. The combination of depositional sequences, diagenetic phases, and seismic geomorphology allowed the buildup to be divided into six stratigraphic sequences, each approximately 50–70 m thick. These sequences can be compared to neighboring buildups and to regional stratigraphic sections using biostratigraphic and chemo-stratigraphic data. Larger benthic foraminifera; i.e., Miogypsina and Austrotrillina , are restricted to the middle Miocene stage “TF1” and “TF2” (where TF is a stage of the Tertiary Period), (19–11.1 Ma), whereas Amphistegina and Cycloclypeus are more indicative of the late Miocene stage TF3 (11.1–7.1 Ma). The biostratigraphic boundary TF2/TF3 was correlated with its strontium isotope signature. This allowed the age of the middle to late Miocene boundary to be estimated. These observations from the E11 buildup were synthesized in a conceptual depositional and diagenetic model. The description of E11 may serve as an analog for carbonate buildups elsewhere in Southeast Asia (Vietnam, Indonesia, and Philippines) and aid in the proposed CO 2 storage project.

The Central Luconia Miocene carbonate platform represents one of the largest regions of Liquified Natural Gas (LNG) production in the world. Although several studies have been conducted, the reservoir diagenesis of this gas-producing region remains poorly understood. To address this issue, a comprehensive and systematic diagenetic study has now been undertaken. Methodologies used included petrography, X-ray diffractometry (XRD), scanning electron microscopy (SEM), backscattered electron microscopy (BSEM), and cathodoluminescent microscopy (CL). Other technologies included elemental analysis using electron probe microanalyzer (EPMA), fluid inclusion microthermometry (FIM), and stable C, O, S, and Sr isotope analyses. The resulting datasets have been integrated so that the paleodiagenetic fluid flow, cementation history, and potential late-stage high-temperature hydrothermal corrosive fluids can be assessed with respect to the effect on reservoir potential. The results show that the reservoirs have undergone a complex diagenetic evolution over time. Six stages of calcite cementation (Cal-1 to Cal-6), four stages of dolomitization (Dol-1 to Dol-4), and one stage of dedolomitization (Ded-1) have occurred. Three phases of major dissolution and several minor late burial diagenetic events, such as fluorite and anhydrite replacement, pyritization, and kaolinite bridging have also been recognized. Each stage is characterized by different crystal habits, cathodoluminescent characteristics, elemental compositions, and isotopic signatures, indicating their precipitation took place at different temperatures and diagenetic environments. The early surface to shallow burial calcites (Cal-1 to Cal-4) and dolomites (Dol-1 to Dol-2) were mainly precipitated in marine, phreatic, and possible mixing water environments at relatively low temperatures (<50° C). The late calcites (Cal-5 and Cal-6), dolomites (Dol-3 and Dol-4), and dedolomite (Ded-1) were precipitated at higher temperatures (85–130° C). The late stages of dolomite (Dol-3 and Dol-4) have narrow distribution of δ 18 O[−5.29 to −6.03‰ Peedee Belemnite (PDB) scale], and δ 13 C (0.64 to −3.65‰ PDB) isotope values have been interpreted as precipitating from dolomitizing fluid that may be associated with deep burial and hydrothermal conditions. Fluid inclusion homogenization temperatures (Th) range from 125° to 130° C, and the melting temperatures of ice (Tm) range from −2.60° to −3.30° C, corresponding to a salinity of 4.34 to 5.41% weight NaCl equivalent. This interpretation also is consistent with the presence of large saddlelike dolomite and high-temperature minerals in the deeper part of the reservoirs. Three main phases of dissolution that enhanced the porosity occurred during the subaerial exposure of the platforms. The reservoir properties were enhanced further by early dolomitization, followed by hydrothermal-related corrosive fluids at high temperatures (>130° C) that possibly migrated upward from deep-seated areas underneath the reservoir via faults prior to hydrocarbon migration. This late diagenetic fluid flow was constrained by porous and nonporous layers formed during deposition and early diagenesis. These fluids created high porosity (up to 40%) and permeability (exceeding 1000 mD) within the hydrocarbon reservoirs.

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.

Estimating the geometry and sedimentary heterogeneities of carbonate platforms is a very challenging task. In recent decades, numerical stratigraphic forward models have been developed to provide a quantitative view of sedimentary processes with the resulting stratigraphic architecture. Although used in many academic and industrial studies, this numerical approach is rarely used in everyday exploration and appraisal workflows. The calibration of these models on available seismic and well data is indeed a challenging and time-consuming process. Here we use the DionisosFlow model to simulate a carbonate platform typical of the Miocene formations observed in Central Luconia, South China Sea. We build a reference simulation or virtual ground truth from literature data, using for instance the Haq et al. (1988) , Laskar et al. (2011) , and Bosscher and Schlager (1992) curves, which prove indications on sea-level variations, orbital parameters, and carbonate production as a function of water depth, respectively. We analyze the sensitivity of the simulation results to the parameters controlling the physical processes: accommodation space creation, carbonate production, and sediment transport. Results show that stratigraphic modeling makes it possible to test the concepts of sequence stratigraphy in a virtual digital world, thus opening the possibility of testing the sensitivity of the different characteristics of a sedimentary system to physical parameters. We also propose an innovative approach to using this stratigraphic modeling in operational cases. The first step is to identify a diachronous geological body such as the carbonate platform sensu stricto, which is easily identifiable using seismic data. A comparison of the geometry of this geological body with the thickness maps derived from the seismic interpretation provides a first regional metric controlling the shape of the studied sedimentary systems. The second step is to validate the simulation results with well data, and, in particular, facies from the log interpretation. The computation of vertical proportion curves (VPC) in the vicinity of wells facilitates understanding the local variability of facies in the digital world as well as in the real world. This second metric, based on a comparison of VPCs, makes it possible to assess sequences and sedimentary heterogeneities and to define much smoother cost functions, thus facilitating the use of automatic optimization algorithms. In conclusion, this work shows that numerical stratigraphic forward modeling is a tool that reproduces stratigraphic concepts using simple physical laws. Through the use of well and seismic metrics adapted to the resolution of the model, use of this numerical approach in daily exploration work to complement the interpretation of seismic and well data provides a quantitative stratigraphic view of the studied area to better access uncertainties and risk in exploration.

A generic strategy for modelling gas fields in Miocene isolated carbonate platforms leverages insights from over 20 analogue fields in the Central Luconia Province in Malaysia, some with 30+ years of development and production history. The strategy advocates customizing modelling efforts to answer the questions required to make business decisions at hand (decision-based modelling) while keeping the models as simple as possible (fit-for-decision models). Four presented examples vary from (1) a conceptual, scaled simulation model that is used to investigate various host tie-back options (screening development options—two examples), through (2) a static reservoir model with seismically constrained reservoir properties combined with a simulation that uses local multipliers to test the impact of key heterogeneities on well-placement options (development optimization), to (3) a full-field static and dynamic reservoir model that is matched to a long and detailed production history to accurately forecast late-field-life gas and water production (wells, reservoir, and facilities management). In combination with standardized work flows for reservoir property modelling, the strategy shifts the modelling focus from elaborate base-case models with uncertainty ranges to testing multiple scenarios that span the range of possible outcomes. The use of this generic modelling strategy has led to faster turnaround of subsurface work, enabling faster development and asset management decisions. Although the examples presented in this paper are specific to Miocene carbonates in Central Luconia, a similar strategy could be suitable for other geological settings.