Infrared spectra confirm that, when heated to 200 °C, Bakken kerogens form anhydrides from
carboxylic acids. The anhydrides in turn thermally decompose at temperatures below 250 °C.
The thermal production of CO, the absence of CO2, and the low temperature of the anhydride
decomposition are all consistent with a radical chain mechanism initiated by a kerogen radical.
Anhydride decomposition occurs by addition of a radical to the anhydride carbonyl to form an
ester and to liberate an acyl radical that rapidly loses CO to form an alkyl radical to continue
the chain. This pathway is one of several by which kerogens thermally decarboxylate.
By treatment with CrCl2, the radical populations of Kimmeridge and Bakken kerogens were significantly
decreased. When both kerogens are mildly heated, anhydride formation at low temperatures can be detected
by infrared spectroscopy only when radicals have been removed by CrCl2 treatment. The “native” kerogen
radicals react with thermally formed carboxylic acid anhydrides to form esters and CO. Some of the observable
kerogen radicals are persistent and reactive. The Acholla and Orr method for pyrite removal also reduces the
radical population of these kerogens.
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