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.

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.