Abstract: Studies of the upper 4th member of Shahejie Formation of Paleogene in Well RD1 in the Dongying Sag of the Bohai Bay Basin have indicated that,oil-and-gas from the same source rock in different stratum has different characteristics.Some are heavy oil;the others are high-gravity oil with pure color.Gas reservoirs locate deep in the upper 4th member of Shahejie Formation of Paleogene,where dry bitumen are usually found.This phenomenon shows that,oil-and-gas storage conditions are different due to time changes and different underground environments in different stratum.Further studies prove that,oil-and-gas storage time length is determined by underground diagenetic process.Analysis of the upper 4th member of Shahejie Formation of Paleogene in Well RD1 shows that,the specific diagene-tic environment is characterized by feeble acid(pH=5.7-6.8) and oxygen-free(Eh-10 mV) conditions.Diagenetic minerals are also distinct,for example,crowds of authigenic kaolinite and automorphic pyrite as well as blue fluorescence of organic phase under microscope.Oil-and-gas may store for a long time in reservoirs under the above-mentioned diagenetic environment.Over the belt,oil is heavy.Under the belt,oil cracks to gas.Studies of the specific diagenetic environment have great value for petroleum accumulation potential in new and old formations and storage availability of liquid hydrocarbon.