Sedimentological and biofacies analyses of the Late Oligocene- Early Miocene transition of the Gomantong Limestone, Central Sabah Sub-basin, Malaysia

The carbonate production which was widespread throughout Southeast Asia in the Cenozoic, contains significant hydrocarbon reserves. In Malaysia, 40% of the total gas reserves were sourced from the middle Miocene carbonate... [ view full abstract ]

The carbonate production which was widespread throughout Southeast Asia in the Cenozoic, contains significant hydrocarbon reserves. In Malaysia, 40% of the total gas reserves were sourced from the middle Miocene carbonate build-ups in the Central Luconia Province, Sarawak. However, with traditional exploration, plays in many of the productive basins of Malaysia have matured and the availability of hydrocarbons is in decline. Thus attention has increasingly switched to focus on older and deeper plays within the existing fields. These new hydrocarbon plays involve carbonate build ups which were deposited in the Late Oligocene to Early Miocene (Cycle I and II). Onshore Sabah, the Gomantong Limestone, deposited during this time interval, was studied as a subsurface analogue for this region. Combined field and petrographic studies, with foraminiferal identification, provide new insights into the significant changes in carbonate production which are reflected in the variation of skeletal components, the architecture of the build ups and their lateral-vertical distributions in this area. Based on the identification of foraminifera, the Gomantong Limestone has now been dated from Chattian (Te 1-4) to Burdigalian (Te 5). The Gomantong Limestone was initiated (Late Oligocene, Te 1-4) on the muddy substrates of the Labang palaeo-high. The dominant component during this phase was the flat-shaped foraminifera (Lepidocyclina sp.) forming thinly bedded (0.5 m) biostromal deposits. The flat-shaped foraminifera indicate mesophotic conditions, where the energy levels were relatively low. As the carbonate system stabilized, with less siliciclastic input, platy corals (Agariciidae) became prominent in the later part of Late Oligocene (Te 1-4). The platy coral framestone facies is composed of thinly bedded biostromal sheets. A further decrease in siliciclastic input in the early Miocene (Te 5) allowed the colonisation of brain corals (Cyphastrea sp.), forming a more thickly bedded coral framestone facies. The corals are 2-4.5 m, with a carbonate mound morphology. An increase in siliciclastic input during the later part of Early Miocene (Te5) resulted in colonisation by opportunistic sponge communities with a higher tolerance to the siliciclastic rich conditions. The spongia dominated facies are thickly bedded (2-3 m) forming a spongia mounded shape. However, the continued siliciclastic influx into the carbonate system led to its demise by the end of Early Miocene (Te5). This demonstrates the interplay between factors such as sea level fluctuation and sedimentary influx, influenced by rifting of the Sulu Sea Basin in the Early Miocene, which may have facilitated the demise of the carbonate system.