Facies, depositional environments, and anatomy of the Subis build-up in Sarawak, Malaysia: implications on other Miocene isolated carbonate build-ups

Abstract

Isolated carbonate build-ups of Miocene age are important hydrocarbon reservoirs in SE Asia and globally. The architecture of the build-up rim and fore-reef talus is poorly understood. The Subis build-up is one of the cycle I–II carbonate outcrops exposed along the coastal Sarawak in an area of 5 × 6 km2, with an exposed thickness of 390 m. This provides an opportunity to document the composition and architecture of rim and talus deposits in an isolated build-up as an analogue for similar gas-bearing structures located offshore Central Luconia, in Malaysia, which were neither drilled nor cored so far. This paper documents sedimentological characteristics, lateral and vertical facies relationships, of the marginal sections of a build-up in a large quarry exposing the Subis Limestone. The Subis Limestone is composed of massive and branching corals, coralline red algae, benthic foraminifera, bivalves, gastropods, echinoids, and occasionally bryozoans and sponges. Eight microfacies types and four environments were defined to describe the Subis Limestone: outer talus, inner talus, reef rim, and lagoon. Nine genera of benthic foraminifera were interpreted, confirming an early Miocene age (cycle II) of the succession. Three major backstepping events were observed at the build-up, where they developed inward towards the center of the build-up.