Extra heavy petroleum crude oil (50% of the mixture boils at >566 degreesC) has been analyzed directly, without prior fractionation, by a high-field (9.4 T) Fourier transform ion cyclotron resonance mass spectrometer coupled to an external micro-electrospray ion source. At an average mass resolving power, (m/Deltam(50%) approximate to 50 000), a single wideband (250-1250 Da) mass spectrum exhibited similar to 5000 resolved peaks with an average mass of 617 Da (e.g., up to 7-10 resolved peaks at each nominal mass). Their elemental compositions were positively identified by accurate mass measurement with an average deviation of less than 1 mDa from each assigned elemental composition. The number of elemental compositions at each nominal mass, the number of sulfur/ oxygen atoms in a molecule, and aromaticity each increase with increasing mass. On the basis of elemental composition alone, we resolve more than 3000 distinct chemical formulas (excluding C-13 isotopic species). Of the 3000 unique elemental compositions, we identify 12 major heteroatomic "classes"; (e.g., molecules containing N, NS, NS2, NO, NOS, etc.); for the various "classes", we identify more than 100 hydrocarbon "types" (e.g., molecules with the same number of rings plus double bonds); and for each "type", we determine the carbon number distribution (20-80 carbons) to reveal the number of alkyl carbons appended to aromatic rings. The present results represent the most complete chemical characterization ever achieved for such a complex mixture, based on a single experimental data set.