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countable and uncountable, plural petroleums or petrolea
A flammable liquid ranging in color from clear to very dark brown and black, consisting mainly of hydrocarbons, occurring naturally in deposits under the Earth's surface. quotations examples
Coal and petroleum do not occur hap-hazard in nature, but the aggregate occurrence of each product in any region forms a definite, limited horizon, which, in its continuance, is independent of the bedding plane.
1940, William Plotts, Isogeotherm, Or Monist Theory of Stratified Mineral Occurrence and Origin, page 7