Characterization of kerogen from Youssoufia phosphate (Morocco) using rock-eval, on-line and preparative pyrolysis

Abstract

The organic matter represents 3 to 4 in wt% in the black phosphates of Youssoufia (Morocco). Few research papers have been published on this phosphate until now, and they are essentially oriented toward the determination of its compounds structures. The present study focusses on the chemical structure and the genesis of this rock phosphate. The organic matter of the Youssoufia phosphate is subdivided in lipids, soluble organic matter (4.2 mg/g of the dry phosphate), humic substances (1.7 mg/g of the dry phosphate) and kerogen (34 mg/g of the dry phosphate). Kerogen represents the main form of the trapped organic matter (85 wt%) in Youssoufia rock phosphate. It was investigated using a combination of spectroscopy (FTIR, solid state ¹³C NMR) and pyrolytic methods (Rock-Eval, on-line and preparative pyrolysis). This kerogen exhibits H/C, O/C and S/C atomic ratios of 1.21, 0.17 and 0.07 respectively, and can be classified as an II-S type kerogen. FTIR and solid state ¹³C NMR spectra indicate a highly aliphatic nature with a low contribution of oxygen-containing functions. The Rock-Eval pyrolysis indicated a hydrogen and oxygen index of 318 and 53 respectively. Globally, the results confirmed that the sediment did not reach the thermal maturity. Preparative pyrolysis was carried out at 600 °C and resulted in large weight losses (52 wt%). One-line pyrolysis and preparative pyrolysis give complementary results. The matrix is formed by cross-linked aliphatic chains. The obtained results from both pyrolitic methods showed the presence in the kerogen of hydrocarbons, esters, and carboxylic acids.