A significant change in the composition of sediments of the northern South China Sea took place during the terminal Oligocene (23.8 Ma BP), accompanied by an abrupt change in geochemical composition, and meanwhile there also occurred a depositional hiatus and the formation of olistostromes in abyssal sediments. These sedimentary events coincided in time with the jump in direction of the seafloor spreading axis of the South China from E-W to NE-SW, representing the most important tectonic event in the tectonic evolution of the South China Sea since the Oligocene. This tectonic event caused the sedimentary environment in the Baiyun subbasin to be transformed from a neritic continental shelf in the late Oligocene to a continental slope since the early Miocene, and the provenance of sediments in the northern South China changed from mainly proximal areas in the Oligocene to mainly distal areas in the Miocene. Furthermore, the region has generally exhibited features of sea transgression since the early Miocene. This abrupt change had far-reaching effects on the formation of petroleum accumulations in the northern South China Sea.