Abstract: In order to find out the sedimentary patterns and development rules of Wenchang Formation and Enping Formation strata in the gentle slope conversion zone in the south of Xijiang main depression in Pearl River Mouth Basin, the characteristics of the sedimentary systems in the two gentle slope conversion zones are summarized from the aspects of lithology, bedding structure, structural characteristics and geophysical characteristics through drilling, logging, earthquake, core analysis and laboratory tests, and the fracture characteristics in the study area are analyzed. The study shows that during Wenchang period (T$_{80}$-T$_{\rm g}$), NE and NW trending depression-controlling faults are developed and distributed at the basin edge, and the fault combination forms are parallel, stepped and oblique. During the Enping period (T$_{70}$-T$_{80}$), NE, EW and SE faults were developed and distributed in the center of the basin. The fault combinations were in the same direction, reverse direction and oblique crossing. Although it belongs to gentle slope zone, Wenchang Formation has steeper topography, deeper lake basin and braided river delta than Enping Formation. The Enping Formation has a broad and gentle topography, with shallow water deltas, the activities of central faults in the lake basin and the occurrence of seasonal floods, making the underwater branch channels of the shallow water deltas extremely developed. The two different types of deltas have different lithologic combination characteristics: the braided river delta sandstone of Wenchang Formation has coarse and thick grain size and thin mudstone layer, and the shallow water delta sandstone of Enping Formation has coarse and thin grain size, thick mudstone layer and coal bearing, which are all favorable areas for oil and gas exploration.

Key words: gentle slope transition zone, Paleogene, rift period, Xijiang Main Depression, Pearl River Mouth Basin