Abiogenic petroleum origin is a term used to describe a number of different theories which propose that petroleum and natural gas are formed by inorganic means rather than by the decomposition of organisms. The two principal abiogenic petroleum theories, the deep gas theory of Thomas Gold and the deep abiotic petroleum theory, have been scientifically discredited and are obsolete.[1] Scientific opinion on the origin of oil and gas is that all natural oil and gas deposits on Earth are fossil fuels, and are therefore not abiogenic in origin. There are a few abiogenic petroleum theories which are still subject to ongoing research and which typically seek to explain the existence of smaller quantities of oil and gas.