The crude oil precipitate recovered from the bottom of oil tanks in the Ratawi oil field is analyzed by positive mode atmospheric pressure photoionization ultrahigh-resolution mass spectrometry (APPI UHR-MS). The precipitate is fractionated into three fractions and according to the APPI UHR-MS data, the mass spectrum of the top fraction is similar to that of the original oil while the middle fraction shows the lowest M n and m/z distribution. The data obtained with APPI technique show that hydrocarbon molecules with aromatic rings and long alkyl chains are more abundant in the bottom fraction. In addition, vanadium is also found to be abundant in the bottom fraction. This study shows that aromatic compounds with long alkyl chains can be responsible for the solid deposits that are potentially harmful to oil processing equipment. Moreover, these results provide a basis for understanding the chemistry of the oil sludge formation.