Comparative petrographic and geochemical study of the Puertollano oil shale kerogens

The bituminous series in the Puertollano oil shale field (central Spain) consists of three oil shale beds named PA, PB and PC from top to bottom. The kerogens from the three oil shales have been studied by microscopy, thermogravimetric analysis, Rock-Eval pyrolysis and Fourier transform infrared (FT-ir) spectroscopy. The shale oils have also been studied by FT-ir, nuclear magnetic resonance spectroscopy and gas chromatography and the results interpreted in terms of variations in palaeoenvironmental conditions and maturity. The three oil shales are dominated by autochthonous algal material (lamalginite and telalginite) in different proportions. The elemental composition of the kerogens and their thermal behaviour are, however, very similar, indicating that both telalginite and lamalginite may represent different growing states of the same algae, Botryococcus. The shale oils show that the kerogens are made up of highly aliphatic, weakly branched macromolecules typical of rocks rich in Botryococcus. Some differences in the chemical composition of shale oils are attributable to the absorptive and catalytic effect of the mineral matter. Some other differences such as the higher carbonyl/carboxyl to aliphatic ratio and the higher degree of substitution of aromatic structures found in the upper oil shale would indicate a slightly lower degree of thermal evolution of this oil shale. This is confirmed by the fluorescence parameters of lamalginite which showed a greenish-yellow hue in the lamalginite from the upper band and a yellow hue in the lamalginite from the two lower beds.