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It is the Hydrogen Sulfide content in the crude which decides whether it is sour or sweet. The cut off level is 10 PPM. If the level is more than 10 PPM it is rated as sour crude. Such crudes are dangerous to handle as they give off dangerous H2S vapours. Examples of famous sour crude - CABINDA CRUDE, MAYA CRUDE,AL RAYYAN CRUDE ETC.. Sometimes these crudes needs to be spiked with certain chemical so as to make them more stable..