Sequence Stratigraphic, Sedimentologic and Petrographic Insights of the Miocene (Stage IVA) Outcrops of the Klias Peninsula and Labuan Island, Sabah, Malaysia, Borneo
Lukie, Terrence; Balaguru, Allagu
Exploration, Talisman Malaysia Ltd., Kuala Lumpur, Malaysia.
The Miocene Stage IV reservoirs are the primary target for present-day exploration in offshore Sabah, Borneo. The Stage IV ranges in thickness from 3 to 5 kms and comprises interstratified sandstones and shales deposited within marginal marine to shoreface environments. Exposed on the Klias Peninsula and Labuan Island, the Stage IV sits on the Middle Miocene Unconformity (MMU), also known as the DRU (Deep Regional Unconformity) and marks the onset of the next major phase of sediment accumulation after the Early Miocene Stage III in NW Sabah.
A nearly complete sequence within the Stage IVA is observed in the outcrops at Bethune Head, Ganggarak Quarry and Layang Layangan Beach. The unconformity (sequence boundary) is marked by fluvial deposits of the IVA cutting into the delta front/prodelta and offshore shales of the Miocene Stage III. The lowstand package (LST) comprises fluvial sandstones and conglomerates, which are capped by tidal flats or a highly bioturbated sandstone with burrows extending up to ½ metre into the underlying dark grey shale (Glossifungites), marking the onset of transgression
The transgressive package (TST) is dominated by wave-dominated delta to shoreface sandstones and shales, which are capped by a thick offshore shale (Bethune Head) to prodelta (Batu Luang) section interpreted as the maximum flooding surface (MFS). This represents the end of transgression and the beginning of the relative sea-level highstand.
The highstand package (HST) is made up of coarsening-up sandstone packages deposited in a wave-dominated delta system containing tidal flats, distributary channels, delta front sands and prodelta sediments. These sand packages are contained within the clinoforms of the delta as it prograded into the basin during the highstand.
The sandstones in the IVA typically fall into the quartz-dominated, sub-lithic arenite classification and have a pore-system dominated by primary pores with subordinate secondary pores formed by the dissolution of unstable grains (feldspars and unstable lithics). Overall reservoir quality varies with facies type; the fluvial and deltaic sandstones can have similar porosity measurements, but the higher degree of sorting in the deltaic sands is an indicator of better permeability in this facies. Although the lowstand sands have relatively good reservoir quality, the transgressive and highstand shelf sediments have a wider distribution making them more targetable.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012