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.

The Pearl River Mouth Basin is the largest petroliferous basin in the northern South China Sea, where hydrocarbon accumulation is strongly controlled by fault activities. This study performed the quantitative evaluation of the effects of faults on hydrocarbon migration and accumulation in the basin. The results indicate that the critical values of vertical migration of middle-shallow hydrocarbon, including the active strength of faults and the ratio of fault throw to shale caprock thickness, were up to 10 m/Ma and 5, respectively. The lateral hydrocarbon migration efficiency of the unbreached relay zone was higher than that of the barely breached and strongly breached types. The lower critical value of shale gouge ratio for the clay sealing efficiency was 0.32. Additionally, the zones with the EW-trending transtensional faults were found to have unique dual functions of migration and stress sealing, suggesting that the linking fault positions play important roles in the lateral migration of hydrocarbons. Finally, seven hydrocarbon accumulation models controlled by faults in different tectonic settings were constructed to clarify the effects of faults on the vertical and lateral migrations of hydrocarbon. These models suggested that fine hydrocarbon exploration should be undertaken in the northeastern Baiyun Sag, and that middle-deep hydrocarbon exploration should be enhanced in the Enping, Huizhou, and southwestern Baiyun Sags.