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.

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.

Gravity data along a north-south profile from Kohistan to the Punjab plain of Pakistan have been incorporated into recent interpretations of the gross structure of the foreland fold-and-thrust belt of the Himalaya. In northern Pakistan, large deviations from Airy Isostatic equilibrium are observed. An excess of mass characterizes the northern Kohistan arc, and a deficit of mass underlies a broad area extending from southern Kohistan to the Salt Range, while to the south a slight excess of mass seems to prevail in the region of the Sargodha high. This anomalous distribution of mass can be understood if the Indian elastic plate, which is assumed to overlie a buoyant “fluid,” is flexed down under the weight of both the overthrust mountains and the sediments eroded off the mountains and deposited in the foredeep basin. In many respects the intracontinental subduction of India beneath the Himalaya is similar to island arc formation, including the seismically active Sargodha high, a basement ridge analogous to the flexural bulge encountered seaward of oceanic trenches. Analysis of Bouguer gravity anomalies along a profile extending from the Sargodha high to the Main Mantle Thrust (MMT) shows that most of the negative-northward gravity gradient can be attributed to crustal thickening. In the Sargodha high area, an additional contribution of about 25 mgal appears to be due to excess of mass at lower crustal or upper mantle levels. The Moho discontinuity is interpreted to bulge up beneath the Sargodha high, then gradually increase in dip from 1° to 3° beneath the Salt Range and Potwar plateau (approximately equal to the change in dip of the basement surface). The Moho is interpreted to change from upwardly convex to upwardly concave beneath southern Kohistan. Finally, north of the Main Mantle Thrust it appears to bend down again, but at a steeper angle of about 15°. Shorter wavelength anomalies, superimposed on the regional Bouguer gradient, are modeled in terms of upper crustal density changes, including those due to: (1) offsets of the basement surface; (2) variable thickness of the Eocambrian evaporite sequence that forms the basal décollement; (3) thrusting and folding of relatively high-density, older parts of the stratigraphic section to higher structural levels, particularly in the Salt Range and northern Potwar plateau; and (4) thickening of the low-density Neogene molasse sequence into the axis of the Soan Syncline, a structural depression between the Salt Range and northern Potwar plateau. Subsurface densities of the overthrust wedge, as well as the definition of the shape of the top surface of the Indian plate interpreted from seismic reflection and drilling data, place bounds on the flexural rigidity of such a plate and the forces that deform it. In northern Pakistan, a steeper Bouguer gravity gradient suggests that the flexural rigidity of the elastic plate (D = 4.0 [± 2.0] × 10 23 Nm) is a factor of 10 smaller than the current values interpreted for the central and eastern Himalaya. Moreover, the maximum flexural stresses are probably concentrated within the crust, which may account for the seismic activity of the Sargodha high and southern Kohistan. At the end of the Indian elastic plate (arbitrarily chosen at the MMT), a large positive vertical shear stress, 9.2 × 10 12 N/m < S 0 < 1.6 × 10 13 N/m, is applied to account for the topographic load north of the MMT. In addition, to fit the gravity constraints it was necessary to apply a large negative bending moment, −1.4 × 10 18 N < M 0 < −0.85 × 10 18 N, at the end of the plate. The negative bending moment can be explained by the combined effect of the northward migration of the Indian plate and the southward differential compressional force generated by the crustal rocks stacked at mid-upper crustal levels beneath the northern Kohistan arc. In addition, buoyancy of the crustal rocks at deeper levels beneath the Kohistan arc may contribute to the negative bending moment. Consequently, in southern Kohistan the surface of the Indian plate is concave up; compressional stresses in the upper part of the plate are probably the primary source of the Hazara seismic zone, where incipient reverse faulting seems to take place. In contrast, the pronounced upward convexity developed along the flexural bulge can account for (1) tensional stress in the upper part of the Indian plate, which is large enough to produce basement normal faults interpreted beneath the Salt Range and Sargodha high; and (2) compressional stress in the lower portion of the crust, which causes the excess of mass and seismicity beneath the Sargodha high.