The Rivers of Luconia: The Effects of Sea-Level Lowstands on the Stratigraphy of a Mixed Carbonate/Clastic Province; Miocene-Present, Offshore Sarawak, NW Borneo

Central Luconia is a gas-producing carbonate province offshore Sarawak, NW Borneo, with some 250 carbonate build-ups, now mostly covered by clastics. Historically, these clastics have been interpreted as open-marine, pro-delta sediments deposited over drowned carbonates. Sequence-stratigraphic frameworks have been based on subdivision of the stratigraphy into regressive cycles punctuated by flooding surfaces. Recent studies have lead to re-interpretation of these strata as stacked delta lobes separated by sequence boundaries as well as by flooding surfaces, with palaeo-coastlines extending basinward of most contemporaneous build-ups. Carbonate build-ups are interpreted to have been periodically engulfed by clastics and land-locked during sea-level lowstands. Palaeo-rivers would consequently be expected to flow over the exposed shelves during forced regressions, producing erosional geomorphology at sequence boundaries. Shallow-marine, paralic and fluvial sediments would be expected in the uppermost parts of the stacked shelf units, which are indeed characterised by distinct upward-cleaning trends. This study documents evidence for fluvial and other erosional geomorphology and links this to hydrocarbon habitats, carbonate-clastic interactions and reservoir/seal distribution. Fluvial processes are shown to have exerted major control on carbonate inception in the Middle Miocene, as well as on hydrocarbon-retention capacity of the clastic overburden following later carbonate demise and burial.