Abstract: The petrologic characteristics, diagenesis and pore types in Chang 4+5 reservoir of the Jiyuan oilfield in Ordos Basin are studied by varied methods, including examination of the normal physical properties, thin-section & casting thin-section, heavy minerals, X-ray diffraction and SEM analysis. The dominating pore type of Chang 4+5 reservoirs is intergranular pore and feldspar-dissolved pore, the subordinate is the dissolved intergranular pore. Assembly of the pore types is usually intergranular pore and dissolved pore-granular pore. The constructive diagenesis in Chang 4+5 reservoir is compaction, secondary enlargement of quartz and feldspar, cementation of carbonate mineral, etc. It is the compaction that reduces the primary pore space in Chang 4+5 reservoirs. The destructive diagenesis that keeps or enlarges pore space is dissolution, clay membrane of chlorite. It is insufficient for dissolution to improve the pore space.

Key words: Ordos Basin, clastic rocks, pore type, diagenesis, Chang 4+5 reservoir