Coupling mechanism between tectonic transition and sedimentary migration of the Paleogene in Southwestern Huizhou, Pearl River Mouth Basin, and its exploration significance

Southwestern Huizhou in the Pearl River Mouth Basin is the core oil-rich area of the Huizhou Sag. The Paleogene tectonic transition exerts a key control on sedimentary filling and hydrocarbon accumulation conditions, but its coupling mechanism with sedimentary migration remains unclear. Based on 3D seismic, drilling, logging, and core data, this study systematically analyzes the characteristics of tectonic transition, sedimentary response laws, and coupling mechanism during the Wenchang-Enping period in Southwestern Huizhou. The results show that the tectonic transition presents fourfold evolutionary characteristics: the regional extensional direction rotates clockwise from the NWW direction to the SN-NNE direction, with the fault strike synchronously deflecting from the NE direction to the NW direction; the area of intense fault activity migrates from southeast to northwest; the subsidence center undergoes dual migration (cross-sag and along the fault strike); and the landform inverts from “south low and north high” to “north low and south high”. The sedimentary system migrates dynamically with the tectonic transition: during the Lower Wenchang period, the Xijiang 30 Transfer Zone serves as the core provenance channel, and the braided river delta is confined to the Xijiang 30 Sag, with medium-deep lacustrine source rocks developed in the eastern part, forming a "west sand and east mud" pattern; during the Upper Wenchang period, the sedimentary system expands northward across the sag to the Xijiang 23-Xijiang 24 Sag, and the Xijiang 30 Sag exhibits a “pass-through without deposition” feature; during the Enping period, the sag enters the depression stage, and multi-provenance supply from inside and outside the basin forms a large-scale braided river delta with a “sand-rich basin-wide” distribution. The coupling mechanism is manifested as follows: fault migration reconstructs the subsidence center, stress rotation drives landform transformation, and tectonic uplift converts provenance supply, with the three synergistically controlling the sedimentary distribution. This study identifies that the northern Xijiang 30 Sag-Xijiang 24 Sag is a favorable area for delta front reservoir development during the Upper Wenchang-Enping period, while the Lower Wenchang in the transfer zone of the southern sag can be used as a key deep exploration interval. The research provides crucial theoretical support for the Paleogene exploration in Southwestern Huizhou.