What is crude oil and what are its main components?

Crude oil is a naturally occurring mixture of hydrocarbons found in underground reservoirs. Main components: Paraffins (alkanes) - straight or branched chain saturated hydrocarbons (CnH2n+2), typically 15-60% of crude. Naphthenes (cycloalkanes) - ring-structured saturated hydrocarbons, 30-60%. Aromatics - ring structures with conjugated double bonds (benzene derivatives), 3-30%. Also contains: sulfur compounds (mercaptans, sulfides, thiophenes), nitrogen compounds, oxygen compounds, and metals (V, Ni, Fe). Composition varies by source - light crudes have more paraffins, heavy crudes more aromatics and metals. API gravity classifies crude: light (>31 API), medium (22-31), heavy (<22).