Response of Neogene Deltaic Depositional Systems Migration to Sea Level and Sediment Supply Changes, Northern Shelf of the South China Sea

Abstract

Neogene delta provides significant reservoir sandbodies of oil and gas in the Zhujiangkou basin. Paleogeographic, isopach, and deltaic lithofacies mapping of fourteen depositional sequences establish a 18 Ma high resolution (>0.5 Ma) record of Neogene deltaic sedimentation. Based on the analysis of lithologic, biostratigraphic, and geochemical stratigraphy from cores, the sequences boundaries are recognized and facies variations are revealed using hundreds wells and seismic data. Deltaic depocenters migrate and change from Miocene (ca. 23.5 Ma) to pliocene (ca. 5.5 Ma) and documents three primary phases of paleodeltaic evolution in response to long-term sea level changes, variations in sediment supply, and thermoflexural basement subsidence. (1) the earliest Miocene(ca. 23.5 – 21 Ma): deltaic depocenter in Huizhou sag shows northeastward migration. Fluvial-dominated delta significantly progradated seaward with high sediment rate and low sea level, forming upward-corasing lithofacies associations, and oblique progradation configuration in seismic reflection; (2) early Miocene (ca. 21–14.7 Ma): high sea level, low sediment supply, and high accommodation rate resulted in tidal-influenced delta regressive landward, and southwestward migration of deltaic depocenter. Lithofacies successions are upward-fining and composed of tidal-flat, tidal bar, supratidal muds, and shelf tidal sand ridges in delta front. (3) middle - late Miocene and Pliocene (ca. 13.8 – 5.5 Ma): higher sediment supply, fluctuating sea level, and intense thermoflexural basement subsidence of Baiyun sag resulted in deltaic depocenter further migrating westward to Xijiang-Enping sag, with outer-shelf delta, and shelf margin delta developing and wave reworking, and feeding sediment to deeper-water slope and basin-plain of the South China Sea (SCS). This study illustrates the widely known variability of mix-influenced deltaic depositional systems. Results from Neogene of the SCS further gives an example that although sea level changes provides the template for the whole basinal sequences, regional tectonics (rates of subsidence and accommodation), changes in sediment supply, and flexural subsidence determine the regional to local preservation of sequences and facies successions.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015