Hydrocarbon accumulation dating by fluid inclusion characteristics in Chang7 tight sandstone reservoirs of Yanchang Formation in Ordos Basin

There are various fluid inclusions in the Chang 7 reservoir of the Yanchang Formation in the Ordos Basin.According to the diagenetic stages and homogenization temperatures of host minerals for fluid inclusions,two different phases of hydrocarbon inclusions have been identified.The hydrocarbon inclusions of the first phase locate in the interiors and fissures of quartz overgrowth area and the homogenization temperature of brine inclusions is from 120℃ to 140℃.The hydrocarbon inclusions of the second phase locate in the late fissures which cut across quartz and(or)feldspars and the homogenization temperature of brine inclusions is mainly from 90℃ to 110℃.Combined with the thermal and burial histories of the Chang 7 Formation,the tight reservoirs has experienced two stages of fluid filling.The main accumulation took place during the late period of Early Cretaceous.