It is generally considered that the lower limit of the effective reservoir depth of clastic reservoirs in the Nanpu Sag of the Bohai Bay Basin is 4 km. If deeper than 4 km, that the deep clastic reservoirs would be no longer have industrial oil and gas production capacity. However, the Well PG2 in the southern Nanpu Sag encountered high-quality clastic reservoirs in the 1st Member of Paleogene Shahejie Formation (Es1) to a depth of 4.2 km and obtained a high-production oil & gas flow. In order to reveal the characteristics and genesis of this reservoirs, and reduce the exploration risk of deep oil and gas layers, we discussed its characteristics and genetic mechanism.Methods:To investigate the characteristics and genetic mechanism of the Es1 reservoirs, thin sections and scanning electron microscopy (SEM) images were obtained from the Petro China Jidong Oilfield Company. Reservoir physical properties, including porosity and permeability, were evaluated from 126 core samples selected from 5 boreholes; the samples were tested with a RecCore-04 Nuclear Magnetic Resonance Rock Spectrometer at room temperature (20 ℃). The grain size, diagenetic features, and porosity were evaluated from 127 samples acquired from 3 boreholes, and thin sections were analysed under a polarizing microscope to highlight the reservoir spaces; the thin sections were dyed with blue epoxy. A total of 27 samples from 1 boreholes were selected, and the samples were tested using a QUANTA 200 scanning electron microscope with an accelerating voltage of 20 kV and a current of 50~100 pA Results:Research shows that the type of reservoirs deeper than 4 km in south of Nanpu Sag are low porosity and medium permeability, and reservoirs space incudes primary pores, secondary pores and micro-fractures. The sedimentary period of Es1 was in a strong hydrodynamic sedimentary environment. The braided river delta front divergent channel sand bodies were widely developed. The lithology is mainly coarse lithology such as medium-coarse sandstone, conglomerate sandstone and gravel. Rock type is mainly lithic feldspar sandstone. High content of rigid components such as quartz and magmatic rock cuttings. The reservoir still retains some of the original residual intergranular pores under strong compaction, and some rigid components form micro-fractures. There is regional unconformity contact between Es1 and Ed 3. The organic acid produced by the acidic fluid of atmospheric water in the early stage of diagenesis and after the source rock matures enters the reservoir through the oil source fracture and unconformity surface in the late diagenesis, resulting in feldspar and soluble debris dissolves to form secondary pores. The dissolution not only increases the pore space, but also enlarges the throat width.Conclusions: It is concluded that coarse lithology, high content of rigid components provide the material basis for the preservation of primary pores. Oil source faults and unconformities provide pathways for the entry of acidic fluids and promote the development of secondary pores. And the dissolution effect is better than the reservoir capacity for increasing the reservoir permeability. This study provides references for the evaluation of high-quality reservoirs and benefit exploration in the Bohai Bay Basin.