Abstract: The sedimentary environment of Eocene and Oligocene is braid river delta with similar rock constituents, but the reservoir characteristics vary a lot. According to the petrographic microscope examinations, well and geochemical data analysis, diagenesis and porosity evolution difference between Eocene and Oligocene in Shwebo Basin is discussed. Research shows that diagenesis evolution sequence especially the carbonate cementation time in sandstones, late structure compression effect on porosity, the match of tectonic uplift and acidic atmospheric water dissolution lead to the great difference between Eocene and Oligocene. From diagenesis evolution and structure compression effect, Oligocene had undergone large carbonate cementation before complete compaction, and even has large non-dissoluted carbonate cements till now, which made some grain basally cemented. Some pores were packed with early cementation, but the compaction was impeded greatly. Moreover, early cementation laid physical basis for the formation of secondary pores at late stage. Eocene was also compacted by late regional structure, besides gravitational compaction from overburden sediments, which destroyed primary porosity heavily. From structural uplift and acidic atmospheric water dissolution, the compressional movement of late Oligocene lifted the Oligocene strata up. Grain-to-grain calcite cement had been dissoluted by acidic atmospheric water which resulted in the formation of a lot of secondary pores. Besides, Paleogene reservoir diagenesis of Well A in Shwebo Basin is firstly researched. It is during the middle diagenetic A1 and A2 stages, which is good for protecting secondary pores.

Key words: Shwebo Basin, diagenesis, pore evolution, structure compression effect, secondary pore