Abstract: At the steep slope of the Shijiutuo uplift (Bohai bay basin), there is thick mixed-deposition of lacustrine carbonate and clastic in the first member of the Shahejie Formation (Es₁, 38.0－32.8 Ma), which is superimposed on the sandbody of fan delta. The cumulative thickness of the total reservoir is over 150 m, with tested productivity over 10×10³ m³ and proven reservoir over 1×10⁸ m³. Based on the analysis of thin section, electron probe, cathode luminescence, it is found that the freshwater diagenesis environment has played a decisive role in the reservoir formation and evolution. The dissolved pores in grains formed in the beach bar depositing lay a good foundation for later dissolution of gravels and lithology. Early exposure forms the atmosphere water seepage belt and undercurrent belt. There are vertical leaching seams, micro emulsion cementations and large amounts of fabric nonselective dissolution pores found in the seepage belt. In the upper part of the belt of undercurrent, fabric selective dissolution pores form a large number of oolitic mold and bioclastic mold, as well as the grain rim cements. The pores formed at the early leaching stage is important reservoir space，and also play a decisive role on dissolved pores formed by the later stage acid geofluids. The micro stalactitic cements, the grain rim cements, and the micrite envelope which formed in the freshwater diagenetic environment play a grid supported role to the preservation of the early formed pores.