Abstract: Based on the analysis of diagenesis and formation sequence of the authigenic diagenetic minerals of Yanchang 4+5 sandstone reservoir in Jiyuan area in Ordos basin, the characteristic of fluid inclusions in diagenetic minerals and the representing hydrocarbon accumulation phases are studied. The results show that the main diageneses include compaction, pressolution, cementation, recrystallization, dissolution and fissuring process. The diagenetic minerals are authigenetic quartz, albites, dolomite, calcite and clay mineral, etc. Three different phases of hydrocarbon inclusions have been identified according to the formation time series of the host diagenetic minerals. The first phase of fluid inclusions is early-forming organic fluid inclusions, which have nothing to do with the event of hydrocarbon accumulation; the second phase of fluid inclusions is non-fluorescent asphalt inclusions, which represent the residues after damage of early oil-gas reservoirs, recording one event of hydrocarbon-accumulation reconstruction; the third phase of fluid inclusions has fluorescence characteristics of hydrocarbons absorbed in pores, distributed in albites, calcareous and siliceous cements and other late-generating cements, showing that late diagenesis occurs at the same time with charging of large amount of oil and gas, which is the record of present petroleum reservoir forming process.

Key words: Ordos basin, Yanchang formation, hydrocarbon accumulation time, fluid inclusion