Analysis of the IR spectrum of the sub-layer R₃ from the Tarfaya deposit, presented in Figure 2(a), allows us to propose an allocation of absorption bands based on the work done by other authors (Table 2). [27] [35] [36] [37]

The major result of the IR analysis of the sub-layer R₃ is, as expected, the presence of a large amount of mineral material. Indeed, all the bands relating to this material are broad and very intense. In contrast, the organic material is present in the form of fine bands of low intensity.

In the spectrum of the sub-layer R₃ freed of carbonates RH, Figure 2(b), we note the complete disappearance of the bands characterizing the carbonates and an increase in the intensity of the bands relating to the organic matter.

In addition to our previous observations, we have identified the appearance of bands at 1379 and 1457 cm⁻¹, which can be attributed to the symmetrical and asymmetrical deformation of C-H bonds found in the CH₂ and CH₃ groups. However, these bands are unfortunately masked by the presence of mineral matter in the raw shale.

Regarding the spectrum of sample K₃ (sub-layer R₃ freed of carbonates and silicates, Figure 2(c)), we note the absence of any band characteristic of mineral matter and the appearance of the following bands, relating to organic matter.

1050 cm⁻¹ and 1163 cm⁻¹: these bands may correspond to the Csp²-O or Csp³-O vibrations of the ether, ester or alcohol functions.

1705 cm⁻¹: groupe C=O.

880 cm⁻¹: pentasubstituted aromatic ring.

840 cm⁻¹: tetra-substituted aromatic ring with two adjacent hydrogens.

820 cm⁻¹: trisubstituted aromatic ring with three adjacent hydrogens.

We conducted X-ray diffraction analysis on the samples from the Tarfaya deposit to identify the mineral elements present in the oil shale (Figure 3). The analysis of the X-ray diffraction patterns revealed that the sublayer R₃ primarily consists of carbonates such as calcite (CaCO₃) and dolomite (CaMg(CO₃)₂), as well as silica in the form of quartz (SiO₂) and hematite (Fe₂O₃).

The X-ray diffraction analysis of the RH sample (Figure 3(b)) indicates the presence of several phases, including quartz in the form of SiO₂, pyrite (FeS₂), and clay.

The X-ray diffraction pattern obtained for sample K₃, which consists mostly of organic matter, shows a significant reduction in the lines characteristic of the mineral phases present in the raw shale and carbonate-free shale (Figure 3(c)). However, the characteristic lines of pyrite are still observable, indicating that the treatment of the raw shale with hydrochloric and hydrofluoric acid does not affect the pyrite content. This result has already been reported by other authors [36] [38] .

X-ray photoelectron spectroscopy, also known as XPS or ESCA, is a valuable technique used in research, development, and industrial manufacturing for surface characterization. This technique enables the identification of chemical elements present on a material’s surface up to a depth of approximately 5 nm and provides information about its composition. Additionally, XPS provides valuable insight into the chemical bonds existing between these elements. We applied this technique to characterize the elements present in each sample. The results obtained are given in Table 3 and Figure 4.

Upon comparison of the results obtained from the three samples (Table 3), it can be observed that the RH sample is distinguished by the absence of calcium (Ca) and magnesium (Mg) elements, indicating that the rock was effectively treated with hydrochloric acid, leading to the complete dissolution of carbonates. Furthermore, the hydrofluoric acid treatment of silicates and clays in sample K₃ was also effective. These findings were corroborated by X-ray microprobe analysis (Table 4 and Figure 5). In addition, the latter method revealed the presence of other elements that exist at greater depths.

It is worth noting that the presence of fluorine peaks in the ESCA spectrum of sample K₃ can be attributed entirely to the trace amounts of fluorine resulting from the hydrofluoric acid treatment of the RH sample. This fluorine was not removed during the washing process.

EPMA analysis results are given in Table 4 and Figure 5. From these results, it can be seen that during the demineralization stages, the mineral elements present in the sample R₃, corresponding to carbonates, silicates and clays, such as Ca, Mg and Si, completely disappeared. We also note the appearance of other elements that could not be detected by XPS. This proves that the X-ray microprobe is a very sensitive technique compared to XPS, but the two methods remain indispensable and complementary for characterizing the surfaces of materials.

This study was conducted using five distinct solvents (toluene, water, phenol, quinoline, and shale oil), all under the same conditions outlined in the experimental section. The yields and compositions of the resulting oils were recorded and are presented in Table 8. The data indicate that the oils extracted with quinoline and phenol solvents yielded significantly higher volumes compared to those produced by the other solvent.

The increased yield and maltene content found in the oil produced by phenol suggests that this solvent not only acts as an effective extraction agent, but also interacts with kerogen molecules. In fact, previous studies conducted by Abourriche et al. [40] have shown that phenol reacts with the double and triple bonds of the decomposition products, resulting in the production of alcohols and aldehydes.

In order to investigate how the quantity of solvent impacts the recovery yield at 390˚C, a series of experiments were conducted using a 10 g mass of RH and gradually increasing amounts of three distinct solvents (toluene, water, and phenol), with all other parameters held constant. The resulting data is presented in Figure 7 and Figure 8.

Figure 7 displays the relationship between the amount of phenol used and the mass of chloroform insoluble. The observed decrease in mass of the insoluble substance with added phenol can be attributed to the fact that phenol tends to react with the kerogen molecules, resulting in the breakdown of larger molecules and the formation of smaller ones. It should be pointed out, as demonstrated by Leach [41] , that transalkylation reactions can occur between 350˚C - 550˚C. Additionally, the transformation of phenol to phenolate in the presence of traces of water or bases gives it nucleophilic properties, making it capable of attacking various carbonyl groups. In a previous study conducted by Koel et al. [42] which explored the use of neoteric solvents in oil shale research, it was found that these solvents attack the kerogen, leading to chemical modification or degradation of the kerogen.

During the process of extracting organic matter from oil shales, the pressure measured is consistently below 1.2 MPa. This pressure is significantly lower than the critical point of phenol (T = 419˚C, P = 6.1 MPa) [43] [44] [45] , indicating that the conditions for recovering organic matter from oil shales using phenol can be considered subcritical extraction. Furthermore, upon completion of each manipulation (i.e. once the autoclave was opened), a release of gas with a distinct sulfur odor was detected. This release was more pronounced with increased amounts of phenol. The elimination of sulfur through the action of phenol was confirmed by elemental analysis (as described in the following paragraph).

In regards to the extraction process using either toluene or water (as depicted in Figure 8), two distinct extraction zones were observed. Within the first zone, the mass of insoluble matter increased as the volume of toluene or water was increased. Similar results were found in a study conducted by Zhuang [46] , which investigated the supercritical extraction of petroleum pitch using toluene. This increase in the mass of insoluble matter can be explained by repulsion forces between the oil molecules present in solution. It’s worth noting that the shape of the variation curve of the CHCl₃ insoluble material as the volume of toluene increases is similar to that observed with water. In the second extraction zone, we noticed that as the volume of toluene and water increased, the mass of CHCl₃ insoluble material decreased. At a temperature of 390˚C, the pressure in the autoclave reached 4.5 MPa and 28.5 MPa for a volume of 60 ml of toluene and water, respectively. Interestingly, toluene and water exceeded their critical point in this scenario, which enhanced their solvent power. As a result, we observed a noticeable increase in extraction yield. Canel and Missal [47] conducted a study on the supercritical extraction of oil shale from Göynük (North-West Turkey) using water, and their findings were consistent with our own. Their research demonstrated that the extraction efficiency increased in tandem with the increase in pressure.

1) Analysis of the obtained oils

a) Elemental analysis of the obtained oils are presented in Table 9.

According to Table 9, the results of the elemental analysis indicate that the oil produced using phenol extraction has lower sulphur content and a greater aromaticity factor (H/C low) when compared to the other oils. Additionally, the ¹³C and ¹H NMR analysis of the maltenes extracted from phenol-derived oil supports the increase in aromaticity factor mentioned previously (see next paragraph).

b) Analysis of the obtained oils by SEC

Table 10 groups the characteristics of oil shale extracts obtained by various solvents, analyzed using SEC. The results of the analysis (Table 2) reveal that water tends to cause kerogen degradation, resulting in a lower average molecular weight. Furthermore, the molecular distribution of the extract is relatively dispersed, albeit less significant than those of extracts obtained via maturation with toluene or without any solvent. When phenol is present as a solvent, the resulting oil shale extract exhibits different characteristics. Despite having a relatively

O is determined by difference.

high average molecular weight, the mixture dispersion is considerably low. The properties of this extract are more promising compared to those obtained using water as a solvent, which yields an extract with a very low average molecular weight upon extracting organic matter from oil shale.

c) Analysis of maltenes by ¹H and ¹³C NMR spectroscopy

To demonstrate the effect of phenol on increasing the aromaticity factor of the produced oils, the maltenes were characterized through ¹H and ¹³C NMR spectroscopy after the removal of phenol. Figure 9 and Figure 10 present the obtained spectra. As shown in Figure 9, it is evident that the maltene obtained using phenol as the solvent contains a higher concentration of aromatic compounds. The ratio of aromatic C-H to aliphatic C-H in the oil was found to be higher, as observed in the ¹³C NMR spectrum of the maltenes (Figure 10). The spectrum shows that the maltene obtained from the oil using phenol as the solvent contains a greater number of aromatic carbons (located between 110 and 140 ppm). This further confirms that the presence of phenol plays a role in increasing the aromaticity of the extracted oils. Apart from the aromatic carbons, the ¹³C NMR spectrum also displays signals between 0 and 50 ppm, corresponding to aliphatic carbons, and signals between 50 and 110 ppm, which can be attributed to C-O ethers and C=C double bonds.

^aWeight average molecular weight; ^bNumber average molecular weight; Ip: polydispersity index = M ¯ w / M ¯ n .