Wings, Mushrooms and Christmas Trees: Insights from Carbonate Seismic Geomorphology into the Tectono-Stratigraphic Evolution of Central Luconia; Miocene-Present, Offshore Sarawak, NW Borneo, Malaysia

Kosa, Eduard
Exploration, Sarawak Shell Bhd, Miri, Malaysia.

Central Luconia is a gas province located offshore Sarawak, NW Borneo. More than 200 Miocene to Recent carbonate build-ups are known to exist in the province, many of them hydrocarbon-bearing. Despite more than 50 years of E&P activity, little is known about the carbonate geomorphology. Since the proliferation of interpretation-workstation technology, a simplistic interpretation of the ‘Top Carbonate' seismic reflection has been accepted as a definite representation of the carbonate province. Owing to limitations of seismic-interpretation techniques and technology, build-ups are depicted as smooth, cylindrical or conical structures linked together by ‘Basal Carbonate'. Miocene to Recent, deltaic sediments overlie the province. The prevalent model of evolution of Central Luconia infers (i) a ‘maximum transgression' initiating the carbonate growth in the Middle Miocene, followed by (ii) progressive burial of the province under Borneo-sourced clastic deltas. The model invokes a hiatus between the demise through ‘drowning' of the build-ups and the deposition of deltaic ‘megaforesets'. Deep-water sediments are implied to surround the ‘drowned' carbonates, which consequently appear to form enormous, sealed tanks ready to contain hydrocarbons. Drilling results do not support this, however. hydrocarbon columns in Central Luconia tend to be short and terminate at intersections of the carbonate edifices with clastic sequence boundaries. Owing to the perceived temporal disparity between carbonate and clastic deposition, overburden stratigraphy is also deemed unusable for correlation between carbonate-reservoir layers. Recently, an alternative model of the clastic stratigraphy has been proposed, interpreting it as a succession of stacked delta-lobes punctuated by exposure and/or flooding surfaces and evolving contemporaneously with carbonates. In this study, carbonate-seismic geomorphology is used to unravel the history of carbonate growth and thus to tie it to the clastic stratigraphy. Clinoforms, back-steps, karst, erosion, and carbonate-clastic intercalations are used to demonstrate the temporal relationships between carbonate and clastic strata. Clastic stratigraphy is shown to provide a template for zone-correlation between isolated carbonate build-ups. The result is a coherent model of the tectono-stratigraphic evolution of Central Luconia, which can serve the purposes of future exploration as well as improved understanding and management of current fields.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012