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.

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.

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.

Abstract The Central Luconia province in the South China Sea, offshore Sarawak, features extensive development of Middle to Late Miocene isolated carbonate platforms. This study presents a regional seismic architecture – seismic facies review of platform flank geometries and off-platform depositional styles, with the goals of understanding their patterns and exploring the controlling processes. The information on flank steepness and predominant shedding direction is contextualized with respect to extrinsic and intrinsic factors of carbonate platform growth, including tectonics, eustatic sea-level fluctuations, hydrodynamics and regional palaeogeography. Results reveal that flank geometries are consistent throughout the Middle to Late Miocene. In the north of the province eastern flanks are dominantly aggradational, steep and sediment-starved escarpments, whereas western flanks show more progradation and are accretionary in nature. Discrepancies from this pattern are observed among closely spaced platforms. The eastern-aggrading flanks are interpreted to have been highly influenced by monsoonal wind-driven currents. Further south in the province tidal currents, antecedent topography and syn-depositional tectonics were more important controls on platform architectural development. This study of regional processes and geomorphic products provides a basis for enhanced conceptual facies models and reservoir quality predictions.

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.

Abstract The central part of offshore Sarawak comprises a smooth continental shelf approximately 300 km wide that forms the easternmost part of the northern Sunda shelf. Geologically, it includes a large part of the younger Tertiary Northwest Borneo sedimentary basin, exposed in the Miri zone of Sarawak. Coastal-plain to deeper marine sediments of Oligocene to Holocene age form the basin fill and are developed in a series of eight regressive depositional cycles. Deformation of the area is most severe in the nearshore part or Balingian Province, where a wide belt of anticlinal structures is developed. The outer shelf area, or Luconia Province, is characterized by an extensive complex of Miocene reefal carbonate rocks formed during a period of maximum transgression. Exploration of this shelf area has been carried out during the last 15 years by Sarawak Shell Berhad, since 1976 under the terms of a production-sharing agreement with Petronas. To date, many encouraging indications of hydrocarbons, including one oil field and several gas discoveries, have been found. Objectives include sands of coastal-plain and coastal origin in Balingian and reefoid carbonate rocks in Luconia.

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.