Syn-rift structural style, basin fill and sequence stratigraphy and their control on development of organic facies in the Beibu Gulf basin, South China Sea

Beibu Gulf basin is one of the four Cenozoic rift basins on the northern continental shelf of the South China Sea. The structural style and sedimentary fill of the basin were studied by structural interpretation, seismic stratigraphy, well log correlation, and core examination from a database including over 5000 km of multi-channel seismic lines, multiple well data, and cores from six wells. Nature, abundance and variation of organic matter in each depositional sequence are examined and documented by Rock-Eval pyrolysis analysis and biomarker analysis.; Two major faults systems, ENE trending normal faults and NW trending transfer faults, are recognized. The basin underwent two phases of faulting. In the first phase (late Paleocene-late Eocene), ENE trending graben-bounding faults were most active and resulted in large simple grabens and half grabens. In the second phase (Oligocene), the older faults were reactivated and newly generated E-W trending secondary faults cut the early syn-rift sedimentary sequence and resulted in complex syn-rift structural configuration and southward shifting of depo-centers. Backstripping analysis indicates two phases of rapid subsidence during middle Eocene and early Oligocene on the down-thrown hanging wall blocks in major sub-basins. The basin has been dominated by post-rift thermal subsidence since early Miocene. A structural inversion affected the western part of the basin during late Pliocene and Quaternary.; The Cenozoic rift fill in the basin can be divided into syn- and post-rift megasequences separated by a basinwide unconformity. The syn-rift sequences are characterized by two major cycles of basin filling in response to variations in structural subsidence, climatic conditions, and sediment supply. The first cycle (lower Paleocene-Eocene) was characterized by rapid subsidence, deep lacustrine settings, and limited sediment supply, resulting in deposition of organic-carbon-rich, oil-prone lacustrine sediments in the transgressive and early highstand systems tracts. The second cycle (Oligocene) was characterized by renewed structural subsidence, fluvial-deltaic dominated depositional setting, and well developed regional drainage systems with abundant sediment supply. As a result, the second syn-rift sequence contains modest amounts of gas-prone organic matter derived mainly from higher plants.