Abstract: Diagenesis and features of the 3rd member of Feixianguan Formation in the Jiannan Gas Field were studied taking well Jian43 as an example.Dolomitization,karstification and tectonic disruption were contributive to reservoir.Diagenesis sequence was illustrated as follows: algae agglutinate and micritization→first consolidation→atmospheric water erosion→secondary granular consolidation→neomorphism→compaction and pressolution→dolomitization→early tension fissure and filling→tectonic compression→buried erosion Ⅰ→petroleum entering→dolomite and fluorite filling as well as quartz filling and metasomatism→oil evolving to bitumen→buried erosion Ⅱ→late tension fissure→gas entering.There were 4 kinds of diagenesis environment including seabed (intertidal),atmospheric water,shallow bury and deep bury.Dolomite crystals,secondary granular consolidations,eroded fluorites,tensional fissures and pores without obvious deformation were the main identifications for buried erosions.