The compositional variation of oil-soluble gaseous hydrocarbons in high-pressure in-place oil from more than 40 oil wells in the Northern Jiangsu oil field, China, was studied. Samples in which the effects of the factors of pressure and biodegradation had been got rid of were chosen as the representatives that could really reflect the original composition of hydrocarbon gases. Such samples were compared compositionally with their corresponding hydrocarbons formed by decarboxylation of volatile low-molecular fatty acids. Thus highly correlative regularities were found to exist between them. The numerical values of both are graphically expressed. The two curves on the diagram are very similar. These two sets of values have a simple correlation coefficient of 0.9935, which presents their genetic relation.Based on the study of the formation of gaseous hydrocarbons from decarboxylation of volatile fatty acids, the following regularities of the isomers are revealed:1) n-butyric acid + isobutyric acid = propane:2) 1 2 isopentanoic acid(2-methyl-butyric acid) + n-pentanoic acid = n-butane; and3) 1 2 isopentanoic acid(3-methyl-butyric acid) = isobutane.The thermodynamics of decarboxylation was analysed and the enthalpy change △H, entropy change △S and Gibbs free energy change △G in the following reaction were calculated:CH_3COOH(1)--CH_4(g) + CO_2(g).Thus the author considers that the reaction for the formation of hydrocarbons due to the decarboxylation of fatty acids is an exothermic one with increased entropy and decreased free energy as well as a spontaneous one. According to the CRC rule, the resultant Gibbs free energy change △G is approximately-41.84 kJ/mol, which is well within the range of the energy levels of biophysiological changes.According to the changes in odd-carbon preference in n-alkanes of petroleum and sediments and the analysis of fatty acids and amino acids, it is considered that hydrocarbon gases do not originate from pyrolysis or thermal degradation but from decarboxylation of organisms. But this biological reaction is not ordinary biochemical reaction in the artificial fermentation for generating marsh gas. On the basis of the change tendency of the fatty acid content in the fermentation liquor for marsh gas generation and the change tendency of the carbon isotopes in methane and carbon dioxide in the marsh gas, it can be judged that the biochemical reaction for forming hydrocarbon gases is a special biological one occurring after the fermentation.