The liquid products produced by thermolysis of natural organic substances of different origins at T greater than or equal to 400 degreesC were experimentally found to be adequate to natural paraffin-naphthenic crude oils in their X-ray and chromatographic characteristics. The main petroleum-generating component is the polynaphthene constituent of organic substances of biogenic or abiogenic origin. Thus, petroleum hydrocarbons are generated in two stages. In the first stage, the organic Substances of biogenic or abiogenic origin are polymerized to yield the polynaphthene component; at the second stage, the polynaphthene component is decomposed at a temperature of greater than or equal to 400 degreesC to give liquid petroleum hydrocarbons. in particular the naphthene hydrocarbons. The results of systemic studies showed that both hypotheses of petroleum generation-from organic and inorganic sources-are true. A similar two-stage process can also be realized on extraterrestrial objects under corresponding physicochemical conditions.