Abstract: We analyzed Shengli and Kuito crude oils directly without prior fractionation using a high-field (9.4 T) fourier transform ion cyclotron resonance mass spectrometer coupled to an external microelectrospray ion(ESI) source with an average mass resolving power of 400 000 (m/△m 50%) over mass range of 200-1 200 Da. The elemental compositions of the obtained peaks in positive mode spectra were positively identified using accurate mass measurement with an average deviation of less than 1 mDa. On the basis of elemental composition alone, we resolved more than 4 200 distinct chemical formula of basic compounds in the ESI positive mode. The basic compounds, as we find, belong to 10 major heteroatomic “classes”, e.g., molecules containing N, NS, NO, NOS, etc. For each individual “class”, we identify more than 300 hydrocarbon “types” (e.g., molecules with the same number of rings plus double bonds) and for each “type”. We determine the carbon number distribution (15-70 carbons) to reveal the number of alkyl carbons appended to aromatic rings. These results represent the most complete chemical characterization ever achieved for such a complex mixture.