The main diagenetic types of sandstone reservoirs in the Yanchang Formation of Zichang area

the Ordos Basin

include compaction

cementation

disolution and fracturing. The diagenetic authigenic minerals are dominated by chlorite

authigenetic quartz and calcite. Two different phases of hydrocarbon inclusions have been identified according to the formation time series of the host diagenetic minerals. The hydrocarbon inclusions formed in the first phase mainly occur in the dissolution pores of quartz and feldspar or along earlyhealed fractures

and are coated by quartz or feldspar overgrowth. The hydrocarbon inclusions formed in the second phase are distributed along the lateformed fractures or in sparry calcite cement. The late fractures cut the early fractures or the quartz overgrowth

and go beyond the grain contact. The homogenization temperature of the paragenetic brine inclusions of hydrocarbons has two peaks at 90-105℃ and 105-120℃ respectively. Analysis of salinity and density of the two generations of fluid inclusions reveal that the oil/gas reservoirs in the Yanchang Formation were formed continuously in one stage. The test data of fluid inclusions and illitic K-Ar isotope dating and the thermal history show that late Early Cretaceous (100-120Ma) is the main hydrocarbon accumulation period in Zichang area

the Ordos Basin.