Abstract In the 1960s, anticipating the award of concessions in the Gulf of Thailand, BP carried out reconnaissance geological mapping over all of Southern Thailand including its Gulf islands. The resulting eight geological maps at 1:250 000 scale provided a database that underpinned discussions of Thailand's place in global plate-tectonic reconstructions that were emerging at that time. In particular, new light was thrown on the Paleozoic succession, and important differences were found between the Upper Thai Peninsula and the Lower Peninsula (respectively, NW and SE of the prominent bend in the peninsula's outline). The Phuket Group of the Upper Peninsula is a very thick diamictite-bearing succession of possible Devonian to Early Permian age, apparently sourced in the west, a region now occupied by the Indian Ocean. That in turn suggested a possible Gondwana origin for SE Asia, a contentious concept at the time, but one that is now widely accepted. The emphasis of field mapping these days is on obtaining detailed data that necessarily means over areas of limited extent. But a very broad regional mapping project such as BP's in Thailand revealed stratigraphic and tectonic insights that could be revealed only from a wide-ranging regional study of this kind.

Abstract The geodynamic evolution of the Palaeo-Tethyan Ocean is key to understanding the development of Pangaea. The main continents of SE Asia were welded together in response to the closure of the Palaeo-Tethyan Ocean. However, the precise location of the sutures and timing of the collisions are poorly constrained, especially the southward continuation from Peninsular Malaysia into west Indonesia. This study presents new zircon U–Pb geochronological, petrological, elemental and Sr–Nd–Pb–Hf–O isotopic data in order to investigate the origin and tectonic setting of the granitoids from the Bangka and Belitung islands, west Indonesia. Although possessing different mineral compositions, the two groups of samples yield similar crystallization ages of c. 230–220 Ma and enriched Nd–Hf isotopic compositions (whole-rock ε Nd ( t ) = −9.6 to −2.4 and zircon ε Hf ( t ) = −12.1 to −0.1), and probably originated from melting of a mixed source of metagreywacke and metapelite with a subordinate meta-igneous component. Both granitic groups formed during post-collisional processes, and when this interpretation is combined with regional investigations on the granitic magmatism, the Bangka–Belitung granitoids may represent the southward continuation of the Main Range granitoids. If correct, the distribution of granitoids with distinct petrogenesis across both islands implies a location for the Palaeo-Tethyan suture zone between Bangka–Belitung and West Kalimantan.

Abstract This paper describes the Ordovician stratigraphy of the Sibumasu Block, which formed part of equatorial peri-Gondwana during the early Paleozoic, and summarizes the palaeoenvironmental conditions that prevailed during that time. Upper Cambrian sequences are commonly dominated by coarse-grained clastic rocks of shallow-marine origin. The lithology of the Lower Ordovician varies from region to region, with fine-grained clastic rocks in the Shan Plateau, shallow-marine clastic rocks in northern to western Thailand and shallow-marine carbonate rocks in southern Thailand to northwestern Malaysia. Middle Ordovician strata consist entirely of shallow- to deeper-water carbonate rocks, with the exception of turbidites in midwestern Malaysia. Upper Ordovician rocks show an overall deeper environment, and the uppermost Ordovician consists of pelagic and deeper, fine-grained clastic rocks. On the basis of palaeobiogeographical data, the Sibumasu Block was adjacent to Western Australia throughout the early Paleozoic. Upper Cambrian rhyolitic rocks in the Shan Plateau and Lower–Middle Ordovician volcanics in midwestern Malaysia may have formed as a result of tectonomagmatism associated with the subduction of the Proto-Tethys Ocean plate into the northern Gondwanan margin. The close similarity of Late Ordovician fossil assemblages from Sibumasu and South China may be due to a sea-level rise during this time.

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.

Isolated carbonate platforms are abundant and widespread in Cenozoic strata and in the present-day oceans of Southeast Asia. The purposes of this article are (1) to describe the basic oceanographic setting of present-day Southeast Asia oceans; (2) to synthesize, compare, and contrast observations of the character of extant platforms in the context of fundamental oceanographic controls; and (3) to leverage these insights to develop a more complete understanding of older isolated platforms, especially the Miocene systems of Central Luconia. The data, presented to mimic an offshore-to-nearshore transect, illustrate Holocene platforms with a spectrum of sizes, depositional relief, facies abundances, and water depths. Although the first-order patterns of relief, size, and orientation are controlled by the geologic setting and Pleistocene history, the results demonstrate the influences of physical processes (waves, tides, currents), siliciclastic sediment, and chemical oceanography (nutrients, salinity, temperature) on the sedimentologic and geomorphic character of these platforms. Careful and critical application of these concepts to Central Luconia reservoirs in isolated carbonate platforms provides actualistic examples and process-response analogs. Although these perspectives offer understanding into controls on horizontal and vertical reservoir heterogeneities, they also emphasize that any one modern system can only be a partial analog for an ancient reservoir in an isolated carbonate platform.

Remote-sensing analysis of high-resolution satellite imagery of modern carbonate platforms in the Celebes Sea, east of Sabah, Borneo, Malaysia, was used to map geomorphology and sediment. Unsupervised classification of satellite images was interpreted in the context of environmental facies of seven isolated carbonate platforms and calibrated using analyses of surface sediments. In total, 140 sediment samples were collected and analyzed for grain-size and sorting. The grain-size analysis showed that sediment varied among the geomorphic elements, which included island, island/volcano, reef complex, carbonate sand shoal, grass-covered sand shoal, shallow lagoon, and deep lagoon. To generate carbonate sediment texture maps, the proportion of mud and the grain-size attributes (mean grain size and sorting) of each sediment sample provided a basis to classify samples into rock-equivalent textures. Integration of remote-sensing, field, and sedimentological data provided a means to characterize texture distribution maps and depositional facies maps. These maps suggest that mudstone to wackestone occurs mainly in the deep lagoon; wackestone to packstone is dominant in the shallow lagoon; the carbonate sand shoal is characterized by packstone to grainstone; and the reef complex is made up of boundstone to rudstone. These results facilitate estimates of the proportions of potential reservoirs on these platforms and the heterogeneity in facies distribution, based on the size of various recent carbonate platforms. Diagenesis notwithstanding, ancient analogs indicate the Selakan and Maiga platforms could be potential reservoirs, whereas the Selakan and Gaya platforms display more facies classes and represent poor potential reservoirs.

The Subis Platform is considered one of the very few outcrops in Malaysia which records remarkable changes in the growth history of a carbonate system. The Subis Platform is located near Batu Niah, Sarawak. Stratigraphically, the Subis Platform is named the Subis Limestone, a member of the Tangap Formation. This article discusses the older succession of the Subis Limestone at the Subis-2 well and the Hollystone Quarry. Both well and outcrop indicate a slightly older succession based on the occurrence of larger benthic foraminifera and calcareous nannofossils. The age of the Subis Limestone ranges from Oligocene to Miocene, based on the occurrence of the larger benthic foraminifera Miogypsinoides sp. (late Oligocene, Te4) and Miogypsina sp. (early Miocene, Te5), as well as on the calcareous nannofossils Sphenolithus capricornutus and Sphenolithus conicus (Te4). The boundary between the late Oligocene and the early Miocene coincides with a sharp change from foraminifera-dominated facies to coral-dominated facies, shown at the Hollystone Quarry. The Subis Limestone records a transgression event from mixed siliciclastic–carbonate (Subis-2 well) to clean biohermal carbonates as shown in the outcrops of the Subis quarries. Our findings on the Oligo–Miocene boundary were then compared with those from other carbonates around Southeast Asia. It is clear that coral reefs existed in Southeast Asia earlier than was first thought, by Oligocene times. The role of localized tectonic events, siliciclastic influx, oceanic mineralization, and Indonesian Throughflow are the main controls to determine the biota changes from foraminifera to coral-dominated facies.

Shallow marine mixed siliciclastic–carbonate shoals, a carbonate platform, and the subsequent development of a reefal buildup occur in sequence from the late Chattian to the Aquitanian in the Niah area of Sarawak. They document the transition from larger foraminifera-dominated, calcitic environments to scleractinian coral–dominated, aragonitic environments in SE Asia, which correspond to a significant increase in biodiversity. A late Chattian to early Aquitanian phase of carbonate sedimentation was initiated by larger foraminifera on shallow marine argillaceous shoals raising from the seabed at about 60 m in water depth occasionally up to near sea level. Carbonate production is almost entirely the result of the accumulation of larger foraminiferal shells dominated by Eulepidina dilatata , a species that could thrive thanks to its photosymbiosis with microalgae. Such mixed carbonate–clastic shoals formed repeatedly on a muddy shelf during a period stretching from about 23.5 Ma to 22.3 Ma. Following a period of siliciclastic deposition, a roughly circular carbonate platform with an area of some 25 km 2 was formed at around 21.2 Ma in stratigraphic continuity with the underlying shallow marine sandstones of the Nyalau Formation. Known as the Subis Limestone, it consists at first of bedded carbonates characterized by the presence of red algae, a high diversity of free benthic and sessile endosymbiotic sessile foraminifera, and a variety of organisms typical for reefal environments, including colonial corals. A reefal buildup started forming on the carbonate platform as early as 21.1 Ma. This phase of growth was likely initiated by low-relief patch reefs, 150 to 200 m in diameter and 60 to 80 m in height, such as those exposed in a southern quarry. Analogous with same-age reefal development models from the Java Sea, it is proposed that the patch reefs coalesced through time to form a larger isolated carbonate buildup that grew up at least until the end of the Aquitanian at 20.4 Ma. This Subis buildup reached an area of 16 km 2 ; it has a preserved thickness of 260 to 280 m and had a paleo-relief of about 100 m above the surrounding sea floor. It is asymmetrical, with a reef wall forming high, west-facing cliffs and another reef wall likely extending on the NE edge of the buildup, beyond the Niah Great Cave. A further increase in faunal and floral diversity occurred during this phase, concomitant with the diversification of ecological niches within the buildup. Reefal and peri-reefal environments are dominated by red algae; solitary and colonial corals (domal, branching, and platy), with subordinate foraminifera (large and small benthic); and associations of foraminifera and algae forming laminar foralgal binding tissues, sponges, hydrozoans, bryozoan, bivalves, echinoderms, and serpulids. The reef rim consists of coral framestone and algal-foraminiferal bindstone. The backreef facies is characterized by rudstones and floatstone with coral debris, and the lagoon facies includes microbial crusts, green algae, articulated and nonarticulated red algae, benthic foraminifera (Miliolids), ostracods, gastropods, and up to 4-m-high platy corals pillars. Forereef deposits include grain- and mud-supported reef debris; a debris apron present some 2.5 km away from the western edge of the buildup consists of debris flows and calciturbidites embedded in outer neritic shales. The upper part of the buildup is missing as a result of recent subaerial erosion. Three successive steps in the development of carbonate ecosystems are identified, which are linked to a series of innovative symbiotic relationships established during the late Chattian and the Aquitanian. During an early phase (23.5–22.3 Ma), monospecific populations of endosymbiotic larger benthic foraminifera thrived on shallow marine muddy shoals. At around 21.2 to 21.1 Ma, new species of endosymbiotic larger benthic foraminifera, sessile-encrusting foraminifera, and coralline algae colonized shallow marine grounds and created a carbonate platform. From about 21.1 to at least 20.4 Ma endosymbiotic scleractinian corals, red algae, and a diverse association of organisms created patch reefs and a buildup.

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.