Belait Formation Research Articles

The Impact of Clay Minerals on the Porosity Distribution of Clastic Reservoirs: A Case Study from the Labuan Island, Malaysia

Clay mineral content and diagenetic processes are vital factors that affect reservoir quality, especially in tight clastic reservoirs, which are crucial for industrial and scientific purposes. The presence of clay minerals poses one of the most significant challenges in evaluating sandstone reservoirs’ quality. Even though wireline logs may provide a good indication of the reservoir quality, there have been cases where they have failed. This work focuses on the clay minerals’ impact on the porosity and permeability of a clastic reservoir. Typical outcrops from Labuan Island, Brunei–Sabah Basin, were chosen as a case study to investigate the petrophysical and petrographic parameters together with clay mineralogy profiling. The effects of the clays on the petrophysical parameters of the sandstone reservoir were evaluated through air permeability, spectral gamma ray measurements, a petrographic thin section analysis, a visual porosity estimation, and a grain size analysis. Field and petrographic studies revealed that Belait and Temburong formations contain massive, interbedded, laminated, and cross-bedded sandstones. Using an image analysis of the thin sections, porosity values ranged from 7.3% to 23.5%, with different macro and micro porosity distributions. According to the spectral gamma-ray investigation and air permeability, permeability reduction is strongly associated with clay minerals. The microporosity and permeability of the analyzed samples showed a unique pattern influenced by the grain size distribution. It was found that the textures dominated by mud grain size had a more significant impact on the air permeability and visual porosity, with coefficient of determination values of 0.83 and 0.70, respectively. The Belait Formation displayed a higher porosity and permeability compared to the Temburong Formation. This research provides new insight into the potential reservoir of Stage IV (the Belait Formation offshore equivalent) compared to Stage II (the Temburong Formation offshore equivalent).

Geology, resource potential and organic petrography of the Neogene coals in the Miri area of Northwest Sarawak, Malaysia

There are three coal-bearing geological formations in the Miri area, viz. the Belait Formation, the Lambir Formation, and the Liang Formation. Of the three coal-bearing geological formations, only the sub-bituminous B Tutoh coals from the Belait Formation, with an estimated resource of 203 million tonnes, are believed to be of economic potential. The Tutoh coals are dominated by vitrinite group, especially desmocollinite. The sub-bituminous B from the Lambir Formation and the lignite from Liang Formation are of low economic potential due to the limited area of the hosting formations.

Strontium isotope stratigraphy of late Cenozoic fossiliferous marine deposits in North Borneo (Brunei, and Sarawak, Malaysia)

• Sr-isotope stratigraphy of fossiliferous marine deposits in northern Borneo. • Burdigalian to Tortanian ages revealed for thirteen fossil-rich shallow marine deposits. • Origin of reworked specimens could be traced by the obtained SIS ages. • Trace element and stable isotope chemistry of the well-preserved shells can be linked to habitat-conditions. • Altered and recrystallized specimens helped tracing diagenetic pathways. Neogene marine deposits of North Borneo are locally very rich in fossils that provide glimpses into the past biodiversity. However, dating these onshore sediments with biostratigraphy is often hampered by the lack and/or the poor preservation state of index fossils. Therefore, the fossiliferous sites were targeted with strontium isotope stratigraphy (SIS) to obtain higher precision relative dating. Well-preserved macrofossils were screened using a multidisciplinary approach, and 87 Sr/ 86 Sr ratios of the most pristine remains were used to date the embedding sediments. Most of the measured ages fall in the expected chronostratigraphic framework established by large scale studies for the region. The oldest, Burdigalian (early Miocene) ages were measured for the Sibuti Formation in Sarawak (17.71 ± 0.2My and 16.7 ± 0.2My) followed by a Serravallian (middle Miocene) age within the Belait Fm in Brunei (12.1 + 1.4/-1.2My). Eight localities from the younger units, the Miri and Seria formations in Brunei, gave a range in age from 10.5 ± 1 to 7.0 + 0.9/-0.5My (Tortonian-Messinian). Reworked fossil assemblages from Tutong beach were also investigated and the SIS ages of Late Miocene support an origin from the younger part of the Seria Fm. One locality, in Lumapas where limestone crops out in Brunei, gave an unexpected younger age (Tortonian, late Miocene, 10.6 ± 1My) compared to estimates projected for its assumed stratigraphic position in the lower Belait Formation (late Burdigalian). These challenging data require more research, yet if the young age is accepted, the stratigraphic situation of the limestone needs further revision.

The significance of ophiomorpha trace fossils as key sedimentological parameters for paleoenvironment assessment – case example in Klias and Kudat Peninsulas, Sabah

Trace fossils amongst other sedimentological features are one of the key sedimentological parameters commonly used to assess paleoenvironment. This paper discusses on the occurrence of trace fossil of ichnogenera Ophiomorpha observed in outcrops of Temburong and Belait formations around Klias and Kudat peninsulas. The burrows which are commonly observed at the outcrop sites belong to four ichnospecies: O. rudis, O. annulata, O. recta and O. nodosa. They are variably oriented from vertical to sub-vertical and horizontal, branched to unbranched. In Temburong Formation, O. rudis, O. annulata, O. recta ichnospecies were observed quite commonly which are associated with deep marine environment. Meanwhile, Ophiomorpha nodosa that typically related with shallow-marine trace fossils only observed in Belait Formations. Based on the trace fossil occurrence in conjunction with other sedimentological observations, the paleoenvironment assessment was further improved.

Insights on deepwater turbidite sedimentary features from outcrops in onshore Northwest Sabah, Malaysia

This paper discusses findings of sedimentary features observed from deepwater outcrops in Northwest Sabah. The observations were based on 7 outcrop localities of Lower Oligocene – Upper Oligocene sections around Klias and Kudat Peninsulas, and Kota Kinabalu, covering Temburong, Belait, West Crocker, and Kudat formations. Temburong Formation (Lower Oligocene-Lower Miocene) was observed at Menumbok (Stop 1) and Tanjung Aru (Stop 2). It comprises interbedded shale and sandstone turbidite sequence representing deposition in deepwater setting. At Batu Luang (Stop 3) the deepwater turbidite Temburong Formation is observed overlain unconformably by stacked fluvial channel deposits of Belait Formation (Middle to Upper Miocene). Here, the overlying fluvial deposits comprise predominantly conglomerates which are believed to represent basal Belait Formation lies over regional unconformable surface correlatatable to Deep Regional Unconformity (DRU). Deepwater West Crocker Formation (Lower Oligocene-Lower Miocene) was observed at King Fisher-Sulaman, Kota Kinabalu (Stop 4), composed of predominantly thick sandstone interbedded with shale. Meanwhile, Sikuati Member of the Kudat Formation (Lower Miocene) was observed at Pantai Sikuati (Stop 5) and Bukit ‘V’ (Stop 6) outcrops. It consists of interbedded sandstone and shale of turbidite successions. At Tanjung Simpang Mengayau (Stop 7) thick sandstone interbedded with thin shales of Kudat Formation was observed. In general, the Temburong Formation at Stops 1 and 2 show Bouma sequence of Ta-Tb-Te successions, but relatively thicker Te at Stop 1 suggesting a prolonged mud suspension deposit compared to Stop 2. At Stop 3, the deepwater Temburong Formation was subjected to a substantial tectonic uplift and massive erosion associated with DRU. Further northwards at Stop 4 West Crocker Formation exhibits thicker Ta successions which implies deposition within a slightly higher energy regime as compared to that of Temburong Formation at Stop 3. Meanwhile, at Stops 5 and 6 different depositional energy levels are shown in the lower and upper stratigraphic intervals. In the lower interval deposition took place within a higher energy condition as shown by thick Ta. It was followed by gradual decreasing depositional energy regime in the upper interval as shown by predominance Tc-Td-Te. Meanwhile, at Stop 7 a thick massive graded sandstone overlain by thin mudstone bed was observed which implies a prolonged Ta succession followed by short-term Te of mud suspension deposits.

Organic matter characteristics and hydrocarbon generating potential of the Miocene Belait Formation, Brunei-Muara district, Brunei Darussalam

The characteristics of the source rocks in the early to late Miocene Belait Formation of Brunei Darussalam have been evaluated through the application of Rock-Eval and biomarker analyses, as well as by organic petrographical examination. Analyses have been performed on coal, coaly shale/carbonaceous claystones and shale samples from the Formation, retrieved from outcrops across the Brunei-Muara district, and covering a wide lateral extension of the Formation. The geochemical results indicate that the coal and some carbonaceous samples (with TOC from 14.1% to 67.7%) from the Belait Formation have “good to excellent” generating potential, quantity and quality, due to their high S2 and TOC values, which are consistent with the intermediate hydrogen index (HI) values ranging from 144 to 258 mg HC/g TOC. Organic material of “fair to good” quantity, quality and hydrocarbon generating potential is recorded for the coaly shale samples in the Belait Formation. Non-coal samples generally have the lowest potential as source rocks, due to their low TOC, HI and S2 values. Within the Formation, the coal layers together with a number of coaly shale and shale layers contain type III (gas prone) and type II-III (mixed oil and gas) kerogens. This is proven by the dominance of huminite, often of fluorescent varieties, which is confirmed though maceral analysis, as well as a trend of n-alkanes skewed towards heavier compounds. Evidence of the possible generation of liquid hydrocarbon is offered by the fact that liptinite is the second most abundant maceral group in most of the samples, as well as by the perhydrous nature of part of the huminite. Furthermore, kerogen type IV (dry gas) has been identified in a majority of the coaly shale samples. Facies analysis of the Belait Formation through the use of maceral indices and biomarkers data indicates that the coals were deposited in a mangrove setting in a lower delta plain, where mainly herbaceous and arboreal plants were growing. The depositional conditions were predominantly oxic, with a possible marine influence, and hence brackish conditions. All samples are found to be thermally immature to early mature, with Tmax values ranging from 356 °C to 441 °C, huminite reflectance values of up to 0.46% and high carbon preference index (CPI) values.

PhaseKinetics for assessing the compositional evolution of petroleum generated from the early to Late Miocene source rock, Belait Formation, Brunei-Muara district, Brunei Darussalam

The Early to Late Miocene Belait Formation in Brunei-Muara district has been investigated using the PhaseKinetic approach for compositional petroleum generation kinetics. The source rock characteristics of the selected coal and coaly shale samples indicate kerogen type III-II and type III with moderate HI recorded in the range of 156–258 mg HC/g TOC. The Rock Eval pyrolysis data also show immature organic matter with maximum recorded Tmax values of 415 °C. Pyrolysis gas chromatography (PyGC) reflects that the source rocks generated petroleum grading from paraffinic-naphthenic-aromatic (P–N-A) high wax to gas and condensate, which can be interpreted as heterogeneous kerogen derived from terrigenous higher land plant material. The main activation energies range of two selected coal samples (S24 and S27) are located at 49–52 kcal/mol accounting each for only 15% of the total bulk reaction. Assuming a 3 °C/My geologic heating rate, calculated onset temperatures for the generated hydrocarbon at transformation ration, TR = 10% were 105–120 °C for the two samples. The end of hydrocarbon generation at approximately TR = 95% is expected to occur at temperatures around 170 °C and above. MSSV pyrolysis gas chromatography affirms the existence of high wax paraffinic kerogen type–III–II with some marine input for one end member sample (S24). The 4-component compositional kinetic model indicates that the maximum generation of different petroleum components of sample S24 is predicted at lower values of temperature and vitrinite reflectance if compared with sample S27. The other end member of sample (S27) is more typically humic coal-like with type-III kerogen generating mostly gas and condensate.

Causes and consequences of Oligocene-Miocene relative sea level change in Asia: An example from the Labuan Island (Sabah), Malaysia

Belait Formation: (A) Trough cross-bedded sandstone (fluvial) (B) Interbedded sandstone-mudstone (sandstone dominated) facies overlying mudstone facies (marine). • The Miocene Belait Formation in the Labuan Island formed due to forced regression. • The upper part of the Belait Formation indicates transgression and marine flooding. • The depositional environment changed during the Oligocene-Miocene in Sabah and Sarawak. • This was linked to regional tectonics and/or global relative sea level. • No record of paleoclimate shift like central Asia is inferred in this study. Labuan Island represents the emergent part of a north-eastward plunging anticline. The stratigraphic succession at Labuan is made up of four formations: the Crocker Formation, the Temburong Formation, the Setap shale Formation and the Belait Formation. The sedimentological aspects of the Labuan stratigraphic succession have been studied by several authors but the stratigraphic relationships between the formations is still a matter of conjecture. In this paper we have undertaken an outcrop based (1) sedimentary facies analysis of the Middle Miocene Belait Formation and (2) inferred the mode of stratigraphic sequence building of the Oligocene-Miocene successions. In significant contrast to the tide-storm interactive shallow-marine Belait Formation of Brunei and Sarawak, the Belait Formation of the Labuan island is made up of a lower fluvial and an upper shallow marginal marine to shelf facies association. The sedimentary succession from the Crocker, Temburong, Setap Shale and Belait formations represents a marine regression either due to a relative sea-level fall (forced regression) or to sediment supply exceeding the rate of sea-level rise (normal regression). The ~17 million years unconformity separating the fluvial facies association of the Miocene Belait Formation from the underlying shelfal sedimentary units (Oligocene Temburang and/or Setap shale formations) is a subaerial unconformity that is a consequence of rapid uplift and associated forced regression, and thus indicates the influence of regional tectonics on the mode of stratigraphic sequence building The change in paleocurrent pattern recorded by earlier researchers across the Oligocene-Miocene boundary also indicates tectonic tilt, and a change in sediment dispersal direction because of upliftment. The upper marginal marine facies association of the Belait Formation indicates transgression and the thick shelfal mudstone unit that overlies the marginal marine deposit indicates subsequent maximum flooding. Our sedimentological and stratigraphic analyses reveal that the shift in depositional environment during the Oligocene-Miocene transition is linked to regional tectonics and consequent sea level change and unrelated to paleoclimatic shift.

Coupled ‘storm‐flood’ depositional model: Application to the Miocene–Modern Baram Delta Province, north‐west Borneo

AbstractThe Miocene to Modern Baram Delta Province is a highly efficient source to sink system that has accumulated 9 to 12 km of coastal–deltaic to shelf sediments over the past 15 Myr. Facies analysis based on ca 1 km of total vertical outcrop stratigraphy, combined with subsurface geology and sedimentary processes in the present‐day Baram Delta Province, suggests a ‘storm‐flood’ depositional model comprising two distinct periods: (i) fair‐weather periods are dominated by alongshore sediment reworking and coastal sand accumulation; and (ii) monsoon‐driven storm periods are characterized by increased wave‐energy and offshore‐directed downwelling storm flow that occur simultaneously with peak fluvial discharge caused by storm precipitation (‘storm‐floods’). The modern equivalent environment has the following characteristics: (i) humid‐tropical monsoonal climate; (ii) narrow (ca <100 km) and steep (ca 1°), densely vegetated, coastal plain; (iii) deep tropical weathering of a mudstone‐dominated hinterland; (iv) multiple independent, small to moderate‐sized (102 to 105 km2) drainage basins; (v) predominance of river‐mouth bypassing; and (vi) supply‐dominated shelf. The ancient, proximal part of this system (the onshore Belait Formation) is dominated by strongly cyclical sandier‐upward successions (metre to decametre‐scale) comprising (from bottom to top): (i) finely laminated mudstone with millimetre‐scale silty laminae; (ii) heterolithic sandstone–mudstone alternations (centimetre to metre‐scale); and (iii) sharp‐based, swaley cross‐stratified sandstone beds and bedsets (metre to decimetre‐scale). Gutter casts (decimetre to metre‐scale) are widespread, they are filled with swaley cross‐stratified sandstone and their long axes are oriented perpendicular to the palaeo‐shoreline. The gutter casts and other associated waning‐flow event beds suggest that erosion and deposition was controlled by high‐energy, offshore‐directed, oscillatory‐dominated, sediment‐laden combined flows within a shoreface to delta front setting. The presence of multiple river mouths and exceptionally high rates of accommodation creation (characteristic of the Neogene to Recent Baram Delta Province; up to 3000 m Ma−1), in a ‘storm‐flood’‐dominated environment, resulted in a highly efficient and effective offshore‐directed sediment transport system.

Hydrocarbon source potential of Eocene-Miocene sequence of Western Sabah, Malaysia

An evaluation of the petroleum generating potential of onshore Eocene-Miocene sequences of Western Sabah, Malaysia was performed based on organic petrological and geochemical methods. The sequences analysed are the Belait, Meligan, Temburong and West Crocker formations of western Sabah. The Belait Formation which is Stage IV equivalent in the offshore represents one of the major source rock/reservoirs of the petroleum-bearing Sabah Basin. The Eocene-Early Miocene West Crocker and Temburong formations are deepwater turbidites whilst the Miocene Meligan and Belait formations are shallow marine fluvio-deltaic deposits. The vitrinite reflectance and pyrolysis Tmax values show that the Belait samples are generally immature for hydrocarbon generation, whereas the Meligan, Temburong and West Crocker samples are in the mature to late maturity stage of hydrocarbon generation. The overall bulk source rock properties of the Belait and Meligan show fair to good petroleum source rock potential with TOC more than 1 wt %, hydrocarbon yield in the range of 400–1300 ppm and moderately high HI for many of the samples. Most of the samples representing the Temburong and West Crocker formations have TOC less than 1 wt% and have no to fair hydrocarbon generating potential. Interestingly, the samples collected in the West Crocker Formation characterized by slump deposits (MTD) have TOC>2 and possess good to excellent hydrocarbon generating potential. The organic matter present in all of the studied formations is mainly of terrigenous origin based on the abundance of woody plant materials observed under the microscope. Consequently, the analysed sequences are predominantly gas prone, dominated by Type III and Type III-IV kerogen except for minor occurrence of mixed oil-gas prone Type II-III kerogen in the Belait Formation and in the slump mass transport deposits (MTD) of the West Crocker Formation.

Quartz surface morphology of Tertiary rocks from North East Sarawak, Malaysia: Implications for paleo-depositional environment and reservoir rock quality predictions

Abstract The relationship between quartz surface textural defects (derived from weathering and diagenesis), palaeo-depositional environment and reservoir quality were studied using Tertiary outcrop rock samples obtained from the Belait and Lambir formations of the Sarawak Basin, Malaysia. Thin sections were used for mineral identification and to make observations regarding grain size and texture. Morphological characterization of the samples was performed using scanning electron microscopy (SEM) attached with energy-dispersive X-ray spectrometry (EDX) system, to show variations in quartz surface texture. The SEM images of Belait conglomerates reveal euhedral quartz crystals characterized with prominent mechanical weathering defects (such as straight and conchoidal fractures and striations). Conversely, the analysis of the Lambir sandstones identified chemical weathering features (such as chemical etchings, pitting, solution pits and notches). On the basis of petrology, SEM and CT scan images, evaluation results of reservoir quality indicate that the Lambir Formation in this study area is high-energy coast deposit, with apparent tide-dominated features; while Belait Formation is neritic-delta deposit, with obvious wave-dominated features; reservoir quality of the Belait Formation and Lambir Formation are poor, but the porosity of the Belait Formation is relatively higher than that of the Lambir Formation.

Ichnology of shallow marine clastic facies in the Belait Formation, Brunei Darussalam

Trace fossil assemblages from a variety of outcropping tidal and shoreface facies in the Belait Formation of Brunei Darussalam were characterized to determine their value in distinguishing shallow marine sedimentary environments on a low energy margin. Most trace fossil assemblages in tide-dominated environments have a proximal Cruziana ichnofacies, although higher energy tidal channels have a distal Skolithos ichnofacies. Tidal sand flats tend to have more diverse and abundant traces than tidal channels. Trace fossils are more abundant in tidal mud flat facies than in tidal channels and sand flats, and whilst mud flat assemblages are more diverse than tidal channel assemblages, they are less diverse than sand flat assemblages. Generally, these assemblages are similar to those in equivalent depositional environments in higher tidal range settings. W ave-dominated environments also have Cruziana assemblages, except for the upper shoreface that has a distal Skolithos assemblage. Lower shoreface assemblages are more abundant and diverse than upper shoreface assemblages and belong to the proximal Cruziana ichnofacies. Trace fossils are more abundant and diverse in offshore transition facies than in either the upper or lower shoreface and belong to the distal Cruziana ichnofacies; assemblages in interbedded storm sands often include a minor component of the Skolithos ichnofacies. These assemblages are comparable to those from wave-dominated deposits from higher energy settings, except for the upper shoreface environment which contains a lower energy assemblage than in equivalent facies elsewhere. T race fossils tend to be less diverse, less abundant and individual traces generally are smaller in tide-dominated depositional environments when compared to wave-dominated depositional environments. Also, Ophiomorpha form complex burrow systems in wave-dominated environments in contrast to simple burrows in tide-dominated environments.

Low energy, low latitude wave-dominated shallow marine depositional systems: examples from northern Borneo

The depositional environments of the wave-dominant successions in the middle to late Miocene Belait and Sandakan Formations in northwestern and northern Borneo, respectively, were determined based on grain size distributions, sedimentary structures and facies successions, as well as trace and microfossil assemblages. Generally, progradational shoreface successions in the Belait Formation were deposited in very low wave energy environments where longshore currents were too weak to generate trough cross-bedding. Shoreface sands are laterally continuous for several km and follow the basin contours, suggesting attached beaches similar to the modern Brunei coastline. In contrast, trough cross-bedding is common in the coarser Sandakan Formation and back-barrier mangrove swamp deposits cap the progradational succession as on the modern northern Dent Peninsula coastline, indicating barrier development and higher wave energy conditions than in the Belait Formation. The Borneo examples indicate that barrier systems that include significant tidal facies form under higher wave energy conditions than attached beaches with virtually no tidal facies. Also, Borneo’s low latitude climate promotes back-barrier mangrove which reduces tidal exchange and reduces tidal influence relative to comparable temperate climate systems. The results of the study indicate that depositional systems on low energy, wave-dominated coasts are highly variable, as are the sand bodies and facies associations they generate.

Depositional Environment of Shallow-Marine Sandstones from Outcrop Gamma-Ray Logs, Belait Formation, Meragang Beach, Brunei Darussalam

The outcrop of shallow marine sandstone, Belait formation at Meragang Beach, Brunei Darussalam have long been regarded as tide-, wave- and storm-dominated environment that reveals the reservoir heterogeneities and variation in sand body geometry. Intensive outcrop study methods were available from previously conducted work on Belait Formation. But for this study the priority was to acquire gamma-ray profile at 5-10 cm scale by using Digital Total GR Count Detector and re-logged of Sedimentological log (in cm) of 311.8 m thick outcrop to describe the depositional environment and sedimentary facies architecture. The sediments deposited in a tide-dominated environment mostly interpreted in all cycles which were characterized by sub-depositional environment which includes; tidal channel, muddy tidal flat and mixed tidal flat sub-environment with sedimentary facies of planner cross-bedding, trough cross-bedding, herringbone cross-stratification, wave ripples and thin parallel lamination with coal fragmented bed and carbonaceous material. The gamma-ray profile for tide dominated environment shows different shapes; block left-boxcar, bell shape, funnel shape and irregular trend. In wave- and storm-dominated are wave ripples, thin parallel lamination with mud drapes, large planner cross-bedding, trough cross-bedding, highly bioturbated (skalithos) sandstone and hummocky cross-stratification. Whereas, the gamma-ray profile shows different shapes; left-boxcar, right-boxcar, funnel shape and irregular trend and in storm-dominated it show only left-boxcar with no sharp edges on top and bottom. This approach combines excellent data quality to interpretate different depositional environment and its impact on sandstone quality which implies that the outcrop gamma-ray log can use to characterize the rock deposited in different depositional environment in different facies. It also reveals that the changes in gamma-ray profile with in the sandstone depend upon the environment in which the sandbody deposited that reveal in reservoir heterogeneities in subsurface well correlation.

Coal-bearing strata of Labuan: Mode of occurrences, organic petrographic characteristics and stratigraphic associations

The current study describes the mode of occurrences of Labuan Cenozoic coals as observed in the field and identifies the coal macerals based on their organic petrographic characteristics as observed under reflected white light and blue light excitation. In this study some sedimentological aspects such as the depositional environment were correlated with the organic petrological characteristics of the coals. Based on the organic petrographic features and the thermal maturity as determined by vitrinite reflectance some new stratigraphic associations have been identified. Similarities in sedimentological and organic petrological characteristics suggest that the shaly strata associated with carbonaceous sandstones exposed on the north-western part of Labuan Island and the outcrop exposed in the south within the east Kiam Sam Peninsula belong to the same sequence within the Setap Shale Formation. Based on petrographic characteristics described and vitrinite reflectance values obtained from this study, at least four distinct units that are associated with coal-bearing sediments can be recognized. Strata with vitrinite reflectance (VR, %Ro) in the range of approximately 0.4–0.5% is the youngest and belongs to the Belait Formation which include the Tg. Layang-Layangan unit. The oldest sequence outcropping at Tg. Punei near the Labuan Crude Oil Terminal (Shell Terminal) belongs to the West Crocker Formation, possesses vitrinite reflectance in the range of 0.8–0.9%, whereas the intermediate unit is the Setap Shale Formation which possesses VR of 0.55–0.6% (including the East Kiam Sam sandstone unit) and Temburong Formation which possesses VR in the range of 0.65–0.75%.It ought to be noted that the differences in the thermal maturity variations within the Cenozoic sediments of Labuan reported here does not form the fundamental basis of the stratigraphic subdivision of Labuan, but an attempt to associate it with what appear to be the still unresolved stratigraphic issues of Labuan. These VR values suggest the Belait Formation is immature, Setap Shale Formation is early mature, while the Temburong and West Crocker Formations are at peak maturity for liquid hydrocarbon generation. The analysed coals are predominantly mangrove-derived and considered to be oil-prone as suggested by the common occurrences of oil haze, suberinite, bituminite, exsudatinite and perhydrous vitrinite.

An assessment of paleodepositional environment and maturity of organic matter in sediments of the Setap Shale and Belait formations in West Sabah, East Malaysia by organic geochemical methods

The black shale samples collected from two Neogene formations in the Klias Peninsula area, West Sabah, have been assessed and characterized in details by gas chromatography, gas chromatography-mass spectrometry and a variety of organic geochemical parameters. The aims of this study are to describe the characteristics of organic matter of these sediments in terms of source/type of the organic matter, assess its thermal maturity and paleoenvironment of deposition, based primarily on biomarker distributions. The results of both formations do not reveal significant differences within the rock extracts. The gas chromatograms of the saturated hydrocarbon fractions of the Setap Shale and the Belait formations displayed monomodal n-alkane distributions and nearly identical regular sterane compositions with a predominance of C27 regular steranes. These are consistent with open marine depositional environments dominated by marine biological matter. Another related feature of these rock extracts is the presence of a high relative abundance of gammacerane, indicating anoxic marine hypersaline source depositional environment. The relatively high abundance of common land plant-derived biomarkers, such as bicadinanes and oleananes, is a clear indication of a major terrigenous input to the source of the extractable organic matter. The predominance of oleanane biomarkers in both formations is indicative of angiospermis input and Tertiary source rocks. The high C29/C30 hopane ratios, moderate development of C33–C35 hopanes, high abundance of tricyclic terpanes and a slight predominance of C27 regular sterane over C28 and C29 steranes are characteristic features tending to suggest a significant marine influence on these source rocks, thereby suggesting a mixed source input. The 22S/(22S+22R)C32 hopane ratio has reached equilibrium, and this is supported by the high maturity level as indicated by the 22S/22SC31–33 extended hopane ratios and 20S/(20S+20R)C29 regular steranes ratios.

An organic geochemical investigation on organic rich sediments from two Neogene formations in the Klias Peninsula area, West Sabah, Malaysia

Twenty organic rich outcrop samples from the Belait and Setap Shale formations in the Klias Peninsula area, West Sabah, were analysed by means of organic petrology and geochemical techniques. The aims of this study are to assess the type of organic matter, thermal maturity and established source rock characterization based primarily on Rock-Eval pyrolysis data. The shales of the Setap Shale Formation have TOC values varying from 0.6 wt%–1.54 wt% with a mean hydrogen index (HI) of 60.1 mg/g, whereas the shales of the Belait Formation have TOC values ranging from 0.36 wt%–0.61 wt% with a mean (HI) of 38.2 mg/g, indicating that both formations have TOC>0.5 wt% the minimum limit for hydrocarbon generation. The samples do not contain sufficient hydrogen-rich organic matter (types I, II and III) to be considered good quality source rocks. The kerogen of type II is predominantly of marine origin. This organic matter was deposited in clastic algal environments. Geochemical data (Rock-Eval, Tmax values ranging from 377–451°C for shales of the Setap Shale Formation and ranging from 466–498°C for shales of the Belait Formation, typical of high maturity), indicate variable maturation histories in both formations. The maturation level varied from the beginning to the peak stage of oil generation in the Setap Shale Formation and it has reached the over-mature level in the samples of the Belait Formation.

Relationship between structural style, overpressures, and modern stress, Baram Delta Province, northwest Borneo

The Baram Delta differs from most large (passive margin) deltas because of its location on an active postcollisional margin. Structural style and evolution is influenced both by gravity‐driven deformation and regional stress. Key structural features of the delta include (1) late development of the toe fold and thrust belt (latest Miocene‐Recent) with respect to 17 Ma recent development of growth faults on the shelf; (2) a narrow active region of extension on the outer shelf upper slope; (3) absence of well‐developed shale diapirs; and (4) inner shelf onshore folding, thrusting, and growth fault inversion. Overpressure magnitude, type, distribution, and principal stress orientation and magnitude for the shelfal area were determined from well data. The inner shelf onshore area of inversion displays decreasing vertical stress gradient offshore and pronounced transfer of disequilibrium compaction overpressure from the Setap (shale) Formation into the overlying Belait Formation. This loss of overpressure permitted partial recoupling of once detached deltaic sediments with basement. Evidence for partial recoupling includes decreasing minimum horizontal stress gradient from NE (area of maximum inversion) to SW across the shelf and coast‐perpendicular inner shelf Shmax directions appropriate for inversion, but with stress magnitudes insufficient to cause deformation. These observations constrain models to explain why toe fold‐thrust belt activity occurred relatively late, while inner shelf folding developed early and is inactive today. During delta progradation disequilibrium compaction overpressures developed in the outer shelf slope area and helped decouple the section from basement late in the delta history, hence compressional deformation shifted further offshore. Conversely onshore inner shelf inversion caused late overpressure loss, resulting in partial recoupling of the deltaic section with basement.

Characteristics of repeated, detached, Miocene–Pliocene tectonic inversion events, in a large delta province on an active margin, Brunei Darussalam, Borneo

The Baram Delta province evolved during the Middle Miocene to present day from a foreland basin to a shelf margin. Episodic folding events affected the region, causing uplift of the hinterland, delta progradation, and inversion of gravity-related faults. Existing models place the N–S- and NE–SW-trending folds in a strike-slip transpressional setting. New geological mapping and re-interpretation of existing data suggest the region is better understood as the development of a west-verging thrust belt in a Middle Miocene foreland basin (filled by sandstones and shales of the Belait and Setap Formations), with key major folds (Jerudong and Belait anticlines, Belait syncline) forming during the Middle Miocene as fault bend and fault propagation folds. The deformation style is complicated by the predominantly shaley Middle Miocene–Pliocene Setap Formation becoming thicker and more overpressured from south to north. Onshore where the Setap Formation is thin or absent, the Belait formation is attached to the underlying Lower Miocene and older sequences. Offshore and in a narrow onshore strip the overpressured Setap Formation causes deformation in the Beltait Formation to be detached from the underlying Cretaceous–Tertiary accretionary prism ‘basement’. The detached style exhibits considerable structural complexity, including lift-off folds, growth faults, shale diapirs and pipes. Onshore thrust and inversion features are dominantly N–S-trending and began activity in the Middle Miocene; deformation is probably associated with an E–W maximum horizontal stress direction. In the Late Miocene (around 7.5 Ma; Watters et al., 1999) NE–SW striking inversion folds developed, located mostly over early counter-regional faults and associated reactive diapirs. Folds verge towards the NW when underlain by counter regional faults, and towards the S or SE when underlain by regional faults. Folds and diapirs along N–S trends were also reactivated. Continuation of this deformation into the Pliocene is largely confined to the offshore area. Onshore the N–S structures (no detachment) were not reactivated during the Pliocene.

Depositional Environment and Sequence Stratigraphy of Miocene Deltaic Cycles Exposed along the Jerudong Anticline, Brunei Darussalam

ABSTRACT Kilometer-scale prograding clinoforms associated with deltas are rarely seen in outcrop; however, one such example is found in a Miocene sand-shale sequence exposed along the Jerudong anticline in Brunei Darussalam. Regional sequence stratigraphic interpretation shows that large clinoforms at the base of the Miocene Belait delta represent a succession of at least three major sand-shale sequences. The stratigraphically highest and best exposed sequence exhibits large slumps and sharp-based detached sand bodies at its base. Accumulation of these units most likely occurred during a relative sea-level lowstand. An overlying 1-1.5 km thick shale unit is interpreted to have developed during subsequent transgressive and early highstand conditions. Rapid progradation of thick sand-dominated shoreface deposits characterizes the late highstand systems tract. The clinoforms below show similar depositional geometries: slumps and thin blankets of shallow-marine sandstones mark the individual bases, shales and mudstones succeed, and progradational shoreface and tidal deposits form the top of each clinoform. New sedimentological and micropaleontological data document that all sediments (regardless of whether sand- or shale-dominated) formed in a shoreface to shelfal setting in front of a mud-rich delta. This differs from previous studies interpreting a continental-slope to deep-marine depositional environment for all shale-dominated units, and FIG. 1. A) Landsat satellite composite image of NE Brunei Darussalam (after Sandal 1996) and B) geological map (after Wilford 1961). The Belait Formation is dominated by shallow-marine shoreface and tidal sandstones. The Setap Shale Formation (Setap Shales) is mainly composed of shelfal mudstones and shales. Note pronounced progradational geometries at the base of the Belait Formation on the eastern margin of the Belait Syncline. Black boxes indicate locations of Figures 2 and 4. End_Page 913------------------------ indicates that kilometer-scale clinoforms can develop entirely on the continental shelf in water depths less than 200 m.