PETROLEUM FORMATION Petroleum, also called crude oil, is a naturally occurring hydrocarbon mixture formed from the remains of ancient plants and microscopic marine organisms. Its formation is a geological process that takes millions of years, involving biological, chemical, and physical changes under the Earth’s surface. STAGES OF PETROLEUM FORMATION 1. ACCUMULATION OF ORGANIC MATTER Petroleum originates mainly from plankton (tiny marine organisms) and plant remains. These organisms die and sett