Geochemistry, petrography and spectroscopy of organic matter of clay-associated kerogen of ypresian series: Gafsa-Metlaoui phosphatic basin, Tunisia

Abstract

This work presents geochemical characterization of isolated kerogen out of clay fraction using petrography studies, infrared absorption and solid state 13C nuclear magnetic resonance (NMR) spectroscopy, with N-alkane distributions of saturated hydrocarbon. Mineralogical study of clay mineral associations was carried out using X-ray diffraction (XRD), on Ypresian phosphatic series from Gafsa-Metlaoui basin, Tunisia. The XRD data indicate that smectite, palygorskite and sepiolite are the prevalent clay minerals in the selected samples. In this clay mineral association, the N-alkane (m/ z=57) distribution indicates that the marine organic matter is plankton and bacterial in origin. The kerogens observed on transmitted light microscopy, however, appear to be totally amorphous organic matter, without any appearance of biological form. The orange gel-like amorphous organic matter with distinct edges and homogenous texture is consistent with a high degree of aliphaticity. This material has relatively intense CH2 and CH3 infrared bands in 13C NMR peaks. This aliphatic character is related to bacterial origin. Brown amorphous organic matter with diffuse edges has a lower aliphatic character than the previous kerogen, deduced from relatively low CH2 and CH3 infrared and 13C NMR band intensities. © 2008 The Authors Journal compilation © 2008 The Society of Resource Geology.