Abstract: This quantitative study is conducted by means of cast thin-section observation and petrophysical property analysis of the reservoir. It is shown that the Chang 4+5 and Chang 6 of Yanchang formation in Jiyuan area is dominated by relatively tense and low permeable reservoirs with high feldspar, deep buried and complicated pore structure, which are important factors causing the reservoir quality to become poor. By reformations of compaction, cementation, dissolution and metasomatism, these reservoirs are now in diagenesis periods of late A and early B, of which the compaction could be result in overwhelming loss of reservoir porosity as the most important reason of the reservoirs becoming poor.

Key words: super-low permeability reservoir, diagenesis, pore evolution, quantitative characterization, Ordos basin, Jiyuan area