petro- Also petri-.
[Greek petros, stone, petra, rock.]
To petrify something is literally to turn it into stone, though it is more frequently used figuratively to refer to somebody so frightened that they are unable to move or think; a petroglyph (Greek glyphē, carving) is a rock carving, especially a prehistoric one; petrography is the systematic description of the composition and properties of rocks based on observation; petrology is the study of rocks, including their origins, structure, and composition
Petroleum derives from Latin oleum, oil. A number of terms in petro- refer to petroleum, for example petrochemicals, substances obtained by the refining and processing of petroleum or natural gas, and petrodollar, a notional unit of currency earned by a country from the export of petroleum.