Sulphur-radical control on petroleum formation rates

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Abstract

Most petroleum is formed through the partial decomposition of kerogen (an insoluble sedimentary organic material) in response to thermal stress during subsurface burial in a sedimentary basin1,2. Knowing the mechanisms and kinetics of this process allows the determination of the extent and timing of petroleum formation, which, in turn, are critical for evaluating the potential for petroleum occurrences within a sedimentary basin. Kinetic models of petroleum generation are derived mainly from pyrolysis experiments1,2, in which it is usually assumed that formation rates are controlled by the strength of the bonds within the precursor compounds: this agrees with the observation that petroleum formation rates increase with increasing sulphur content of thermally immature kerogen2,3,4, C–S bonds being weaker than C–C bonds. However, this explanation fails to account for the overall composition of petroleum. Here I argue, on the basis of pyrolysis experiments, that it is the presence of sulphur radicals, rather than the relative weakness of C–S bonds, that controls petroleum formation rates. My findings suggest that the rate of petroleum formation depends critically on the concentration of sulphur radicals generated during the initial stages of thermal maturation. The proposed mechanism appears to provide a realistic explanation for both the overall composition of petroleum and the observed variation in formation rates.

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Figure 1: Plot of the activation energy for expelled oil generation from type-II kerogen against the sulphur mole fraction ((S/[S +C]) of the original thermally immature kerogen; New Albany Shale (), Woodford Shale (), Alum Shale (▪), Phosphoria Formation (♦), and Monterey Formation (•).
Figure 2: Plot of mol% of 1-phenyldodecane (PDD) degraded against diethyldisulfide (DEDS) mole fraction (DEDS/[DEDS + PDD]) used in the experiments at 350 °C for 72 h. Loss of PDD was determined by gas chromatography with a flame ionization detector calibrated with external standards.

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Acknowledgements

I gratefully acknowledge reviews by H. Helgeson and S. Larter, and helpful suggestions by Z. Aizenshtat, J. Clayton, R. Dias, D. King, P. Lillis, T. Ruble and K. Varnes.

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Correspondence to Michael D. Lewan.

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