Influence of the conditions of deposition on the chemistry and the reflectance variations of the Brent coals

Specular reflectance infrared (i.r.) microspectroscopy was performed in order to account for reflectance anomalies in the Brent coal series (bituminous coals of the Upper Jurassic) of the North Sea. Main bands of i.r. spectra recorded on a Nicolet System 800 coupled with a NIC PLAN microscope were integrated. These data allowed us to relate two distinct reflectance trends (Top and Basal Brent coals) to depositional conditions. Top Brent coals may originate from reducing environments where organic matter was well preserved. For this series, defined as a reference series, good correlations were obtained between the i.r. data based on OH, aromatic and aliphatic C---H as well as C=O band intensity and maturity parameters such as vitrinite reflectance (VRo) and relative depth. Basal Brent coals associated with coastal and upper delta plain environments may originate from more oxidizing depositional settings where organic matter was altered. Spectra exhibited strong oxygen-bearing bands (OH and C=O) and weak aliphatic C---H bands. Because of such variations in the chemistry of vitrinites, the reflectance measurements were strongly biased. Aromatic ν C---H (3000–3100 cm−1) i.r. bands were also accurately studied. The results suggest that aromatic C---H concentration is rank controlled, and independent of depositional setting. Large intra-Brent VRo variations observed in a single well (0.2% in 4 m depth) were possibly related to a diagenetic oxidation process, providing that all the samples were extracted from oxidizing environments. Those variations may also strongly correlate with an increasing degree of oxidation of the upper and coastal delta plain depositional settings. These data suggest that vitrinite reflectance may be strongly influenced by oxidation and must be carefully handled when extrapolating VRo data to the reconstruction of the thermal history of sedimentary basins. Infrared microspectroscopy is demonstrated to be particularly adapted to establish the oxidation of vitrinite that may lead to overestimation of VRo values.