Abstract: In northern Dongying Sag, the reservoir formation and distribution of upper submember of the fourth member in the Shahejie Formation is the study focus in recent years. It is an important oil-bearing sequence. The solid-fluid interaction mechanism and its influence on physical properties of reservoirs could be studied by the observation of polarizing and fluorescence microscope and the analyses of SEM on thin sections in combination with regional tectonic evolution and thermal history of organic matters. Conclusions are drawn as follows. There are seven active fluid episodes in the variation of diagenetic environment, namely, alkaline, acidic, alkaline, acidic, alkaline, acidic and alternating alkaline and acidic setting, respectively. The interaction of acidic fluid and solid presents significant dissolution of carbonate minerals and feldspar, and quartz overgrowth to enhance the reservoirs; while the interaction of alkaline fluid and solid is characterized by quartz dissolution, feldspar overgrowth and the precipitation of ferrocalcite, ankerite and anhydrite, which largely infill pores and destroy the reservoirs. There are two hydrocarbon expulsion episodes, namely, the deposition period from the second member of Shahejie Formation to Dongying Formation, and the latest one of Guantao Formation. These two events are in correspondence with the development of two secondary pore zones.

Key words: Dongying Sag, solid-fluid interaction, diagenesis, reservoir characteristic, porosity evolution