Biodegradation of petroleum is an important alteration process with major negative economic consequences for oil production and refining operations. Most of the world's crude oil in reservoirs is biodegraded. Although it has been suggested in recent years that biodegradation of petroleum in reservoirs proceeds anaerobically, little is still known about the processes and the controlling factors involved in subsurface hydrocarbon and non-hydrocarbon degradation. As the supply of hydrocarbons is unlikely to be limiting in oil reservoirs, essential nutrients such as nitrogen or phosphorus might control the rate or degree of biological degradation. Here we examine the possibility of crude oil itself providing nitrogen as a nutrient in the form of nitrogen compounds for microbial use. We use excellent natural biodegradation sequences, which cover the whole range from low to severe degrees of biodegradation. We suggest that small amounts of petroleum nitrogen can be used during biodegradation at high degrees of biodegradation [> level 4 of Peters and Moldowan ( The Biomarker Guide; Prentice Hall: Englewood Cliffs, NJ, 1993), PM level > 4], but at lower degrees of biodegradation, nitrogen as an essential nutrient has to be supplied from other sources, most likely from ammonium ion, which is abundant in oilfield waters, sourced from clay and other mineral buffers.