Constraints upon the use of 24-ethylcholestane diastereomer ratios as indicators of the maturity of petroleum

Abstract

We have measured the ratio 20 S/(20 S + 20 R), the relative abundances of the 24-ethylcholestane diastereomers, in nine pairs of closely adjacent samples of coals and shales which represent a range of geological ages and different sedimentary basins. Within some sample pairs, the values were similar, in others they were very different. The differences between the values for coal and shale members of pairs are attributed to effects of sediment matrix type upon the processes which cause the changes in the ratio. The kinetic parameters ( E and ΔS) for the changes in the 20 S/(20 S + 20 R) ratio have been estimated using previously published measurements for the Hod #1 well (shale) and Jupiter #1 well (coal), and these values were found to be very different. The process in the coal sequence has a much lower activation energy, and is therefore less responsive to differences in heating rate regimes than is the case for the shale sequence. Values of the 20 S/(20 S + 20 R) ratio in two other sequences are shown to be consistent with this explanation, and the implications of these observations for the practice of using sterane isomer ratios for assessment of maturity of sediments and crude oils are discussed.