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Influence of Basin Physiography on Coastal-Shelf Sedimentary Processes: Integrating Ocean Modelling and Sedimentary Facies Analysis in the Mio-Pliocene, NW Borneo, South China Sea


Coastal sedimentary dynamics reflect the complex interaction of tidal, fluvial and wave processes. Palaeotidal modelling allows understanding of the sensitivity of tidal sedimentary dynamics to inferred control parameters in predictive process-based models for coastal sedimentary deposition. Integrating facies analysis and palaeotidal modelling in turn permits semi-quantification of tidal forcing along ancient coastlines, and deconvolution of parameters that control sedimentary architecture at an outcrop-reservoir scale. We use Fluidity-ICOM to model tides and associated bed shear stress of the South China Sea during the Oligo-Miocene. Palaeobathymetric uncertainty has been evaluated through a suite of sensitivity tests. Results show that diurnal tides dominate and the predicted tidal range along palaeocoastlines of the developing South China Sea was higher (meso-macrotidal) relative to the present day (micro-mesotidal). A wider Luzon Strait and lack of through-flow across the Sunda Platform facilitated a larger transfer and storage of tidal energy from the Pacific Ocean. The higher ambient tidal potential, coupled with tectonically-controlled changes in shelf width and bathymetry, set up local funnelling and shoaling effects resulting in elevated bed shear stress offshore northwest Borneo, south and east Vietnam, the Beibu Gulf and Gulf of Thailand. The Miocene Belait Formation (Berakas Syncline, Brunei) represents the onshore correlative to reservoir units in the prolific, hydrocarbon-bearing Baram Delta Province. Facies associations are attributable to a complex interaction of depositional processes in a shoreface-delta front to embayed coastal setting. On an inter-sand body to parasequence scale, exposures show discrete arrangements of broadly wave- or tide-dominated facies, each typically preserving a subordinate mixed-process signature. On an intra-sand body scale, a clear textural control on bedform type is consistent with combined-flow phase diagrams, justifiable through integrating facies analysis with numerical modelling results. The perceived absence of ‘typical’ upper shoreface facies in offshore Champion Field cores can in turn be ascribed to a limitation in the shoreface facies model employed. Palaeotidal modelling therefore provides important indicative insight for predictive models of sedimentary processes and facies architecture for coastal-shelf deposition, in a given tectonic and oceanographic setting.