Source-to-sink system during rifting-depression transition period and its exploration significance: A case study of the Upper Enping Formation at southeastern margin of Huizhou 26 sub-sag, Pearl River Mouth Basin

Rifting-depression transition period is a significant stage of transition for a rifted basin from extension to subsidence, during which the tectonic activities get stable along with declining extension, affecting the sedimentary system and favorable reservoir distribution in the late rifting period. With the guidance of the “source-to-sink” coupling ideology and based on the analysis of petrological characteristics, logging and seismic facies, we propose the idea of “tectonic activity-controlling facies, and valley/fault-controlling reservoir” to analyze the origin and distribution pattern of favorable reservoirs according to the differential activities of boundary faults during the rifting-depression transition period of the Upper Enping Formation at southeastern margin of Huizhou 26 sub-sag. The major conclusions can be drawn as follows. First, the Mesozoic intrusive rocks of the Dongsha uplift serve as the provenance to the upper member of the Enping Formation in the study area, with five major catchment units developed during the rifting-depression transition period. The supply capacity can be characterized by the quantitative analysis of the area and head of these catchment units. Second, the sediment-transport pathways are composed of the valleys and boundary faults. The morphology and scale of valleys within provenance affect the water kinetic energy of each drainage, and the planar combination pattern of boundary faults (i. e. the concave corner type, the straight and flat slope type, and the straight and flat fault type) and their vertical throws serve to influence the way of sediments converging in lake, as well as the distribution of sand bodies. Third, the “source-to-sink” coupling with four elements including source, valley, boundary fault and sedimentary system helps establish a sedimentary framework of “fan deltas in the south and braided deltas in the north” resulting from the differential activities of boundary faults at the tectonically quiet stage of the rifting-depression transition period. Sedimentary characteristics are finely depicted to disclose favorable facies zone and quality reservoir distribution, that is, the coarse-grained deposits of channel sub-facies, braided river delta plain facies, that is formed by sediments transported along the “valley-boundary faults” to lake, and characterized by well-developed primary pores and high permeability. These new understandings serve to promote the exploration progress of the Enping Formation and manifest exploration direction of the Paleogene deep strata at the southwestern area of Huizhou Sag.