Abstract: Six oil sand samples were collected from the sandstone reservoirs of Yanchang Formation in the northwestern areas of the Ordos Basin,Northwest China.Oil sand extracts were analyzed by GC-MS.A series of alkylated fluorenones have been detected.They are products of oxidation-reduction reaction of fluorene and benzo-fluorene in reservoir.This process can be considered as the result of that during the time of subaerial exposure in the later diagenetic stage A,the fluorene and benzo-fluorene in oil sand are oxidized to fouow the permeating of meteoric water with bacteria and oxygen under the alkali-diagenetic enviornment and participation of the sialicate in Yanchang Formation.