DRIFT spectroscopic study of diagenetic organic–clay interactions in argillaceous source rocks

• Diagenetic organic–clay interactions are investigated with thermo-DRIFT spectroscopy.
• Part of the OM stable at 250–550 °C is bonded to clay via H2O bridges.
• The desorption of combined OM increased with depth.
• Smectite illitization drived the desorption of combined OM.
• DRIFT spectroscopy is useful in diagenetic studies.

Thermo diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to study the diagenetic organic–clay interactions in argillaceous source rocks from the Dongying Depression, Bohai Bay Basin, China. The results indicated that aliphatic organic matter (OM) represented the most prominent organic functional groups in the spectra, with two CH2 stretching vibrations at approximately 2926 cm−1 and 2852 cm−1. The peak areas of these vibrations correlated well with the amount of total organic carbon (TOC), indicating that the OM evolution may be represented by the variation in CH2 with depth. Infrared spectra obtained from samples that were heated to 105 °C, 250 °C and 550 °C suggest that the aliphatic OM consists of two fractions: combined OM and free OM. The former was more stable between 250 °C and 550 °C. This phenomenon was correlated with the H2O stretching vibration near 3300 cm−1, indicating that this OM was bonded to the clay via H2O bridges. The location of the broad H2O stretching band gradually shifted with depth from 3298 cm−1 to a higher wavenumber of 3305 cm−1, whereas the corresponding bending band shifted rapidly from 1640 cm−1 to 1605 cm−1, indicating a weakening of the hydrogen bond and a decrease in the combined OM fraction. The correlation between the diagenetic smectite illitization and the decrease in the amount of combined OM leads to the conclusion that the smectite illitization may be a driving force for the OM desorption. This study demonstrates the usefulness of the thermo-DRIFT approach for exploring diagenetic OM–clay interactions in argillaceous source rocks.

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