Molecular weight distribution of olefinic compounds in Precambrian and Palaeozoic oils

Three Russian Precambrian and Palaeozoic crude oils from cold reservoirs (< 25°C) of various basins and deposits were analysed for molecular weight (MW) distribution of olefinic compounds content. Olefin contents within narrow distillate fractions (DFs) measured by 1H NMR were found to increase as a linear function with molecular weight. There was also an increase in the correlation between weight content of olefins and saturates within DFs and superheavy oil fractions > C50, determined by isolation procedures. The linear character of MW distributions of olefinic compounds observed can be explained only from the position that the olefin molecules were generated within an oil from the saturated hydrocarbons, both with the same carbon number in the molecule. The data obtained support the radiolytic concept for the origin of olefins in oils.