A rock from which oil or natural gas is generated from organic materials. Source rocks are geologically rare. 90% of the world’s petroleum is generated in 6 discrete intervals, representing about 30% of post Cambrian geological time. Most petroleum is geologically recent – 67% is generated and trapped in the last 80 million years, whilst some older petroleum accumulations would have been destroyed through geological time by tectonic processes. 25% of the world oil has been generated during the last 5 million years