The Paleogene Enping Formation in the Huizhou Sag, Pearl River Mouth Basin, has been identified as a key target for deep oil and gas exploration. However, research on the diagenesis of these deep reservoirs still remains limited. This study evaluated the role played by diagenetic processes on the reservoir quality of the Paleogene Enping Formation in the Huizhou Sag, Pearl River Mouth Basin, from braided river deltas to meandering river deltas. A core observation, thin section examination, cathode luminescence analysis, scanning electron microscopy, mercury penetration, porosity–permeability test, and other analytical methods were performed to analyze the diagenesis and its impact on the physical properties of the deep, low-permeability sandstone reservoirs in the Enping Formation within the study area. It was shown that the reservoir composition maturity of the Paleogene Enping Formation in Huizhou Sag is relatively high, and the reservoir space is dominated by dissolved pores, accounting for more than 48.2%. The deep and ultra-deep clastic reservoirs are typically characterized by “low porosity, low permeability, and strong heterogeneity”. In particular, the reservoir space of the deep, low-permeability reservoir of the Enping Formation is significantly affected by diagenesis in which mechanical compaction notably altered the porosity of the Enping Formation reservoir, with a reduction in pore volume ranging from 12.5 to 27.2% (average 18.9%); cementation usually enhances pore reduction by between 2.1 and 28.7% (average 11.7%), while dissolution has resulted in an increase in pore volume ranging from 1.4 to 25.6% (average 10.1%). A further analysis revealed that the deep reservoir type in this region is characterized by “densification”, as evidenced by the correlation between reservoir porosity–permeability evolution and hydrocarbon accumulation.
