The Mg'contents of protomagma in Nanpu Sag are mainly 0.65～0.75.The incompatible elements of rare earth and trace elements are rich and compatible elements are deficient.This suggests that the protomagma was controlled by the fusion of upper mantle source rock.The structural sequences of stretch rift basin is usually composed of coarse grain sequence in the bottom

volcanic rock bearing sequence in the lower part

deep water sequence in the mid part and filling sequence in the upper part.Nanpu Sag could be divided into 6 sequences accordingly.The third and fourth sequences controlled the distribution of the two main petroleum systems of the sag.The formation mechanism of the sag was related of the upwelling of the mantle

and was controlled by the evolution model of active rift.