Abstract:

Damintun Sag is situated in the northeast corner of Bohai Bay Basin.The middle and lower parts of the Sha-3 member and buried hill of the Tertiary System in the north part of the sag preserve large amount of high wax crude oil;the central part of the sag contains mixed oil and the south part contains normal oil.Sha-3～Sha-4 members are fine source rocks with 2.16% average organic carbon contents.Abundant organic matter,including higher plant and lower hydrobionts in the source rocks,increased the wax contents.The ratio between dibenzothiophen and phenanthrine is less than 1.0,suggesting that the whole Shahejie Formation was formed in weak oxidation environment of fresh water.Besides,the north part had been long in the low maturation stage.This weak oxidation-low maturation environment is the key factor for the high wax oil to be preserved.The high contents of drimane in the oil indicate that microbial action made some contribution for the formation of the high wax oil.

Key words: high wax oil, terrestrial organic matter, microbial reworking, oxidation-reduction environment, maturation, Damintun sag