Genetic Type and Distribution of the Oils in the Nanpu Depression,Bohai Bay Basin

A great quantity of oils were discovered recently in both the foreshore and onshore of the Nanpu Depression,Bohai Bay Basin.Totally 67 oil and oil sand samples were collected to perform detail conventional geochemical analysis to unravel differential characteristics and classify genetic types for the oils.Four groups of the oils in the depression are recognized,including(1)onshore oils produced from Es3 interval,characterized by high abundance of 4-methyl steranes but low level of oleanane and low methylphenanthrene indices.Also the oils have a relatively higher degree of thermal maturity;(2)onshore oils produced from Es1 to Ed and Ng to Nm.They have converse features of the group-I with widely secondary alteration by water washing and biodegradation for most of the oils from Ng to Nm;(3)offshore oils from Ed interval,which are differentiated from onshore oils with relatively high ratios of gammacerane to C30hopane and steranes to hopanes;(4) oils from Ordovician burial hill,which have apparent low contents of steranes and terpanes but high content of pregnane,tricyclic terpane and diasteranes,indicate relatively high thermal evolution of the oils.It is expected that there are several favored source rocks developed in both onshore and offshore regions of the Nanpu Depression,and there is a better quantity of Es1 to Ed source rocks offshore than to onshore.Great petroleum potential in the depression was suggested after our study.