Mineralogical Properties of Moroccan Rif Bituminous Rocks

Abstract

Bituminous rocks, including oil shales, are sedimentary rocks. They include a group of rocks like shales, marls and carbonates. Containing 60–90% mineral matter and less organic matter. Five techniques namely, XRD, FRX, ICP/AES, MEB-EDX and FTIR analysis were used to characterize the the mineral material of Moroccan Rif bituminous rock. The results obtained by these methods showed that the dominant mineral phase in all samples is quartz (70.04–84.46 wt%). The TA sample contains high amounts of calcium oxide (CaO) and magnesium oxide (MgO), as well as a task amount of carbonates compared to the other samples. These quantities were confirmed by ICP/AES, MEB-EDX and ETIR. According the XRD analysis, the illite and the chlorite (clay minerals) and other mineral are existed in various proportions of low weight.

1. Introduction

Morocco is almost entirely dependent on oil and gas imports to cover its energy needs. However, large oil shale deposits are available in many parts of the country but remain untapped. According to the latest study recently published by the World Council of Energy-on-energy reserves in the world, Morocco was ranked in sixth position, with respect to the world’s reserves of oil shale. The Rif deposit (North of Morocco) is one of three major Moroccan deposits which come up to ground level. It was discovered during the thirties in the last century and has been the least studied in comparison with the other deposits of Morocco (Al-Tamahdt and Tarfiyah), and the reason for this is due to the low amount of organic matter [1, 2].

Bituminous rocks, including oil shales, are sedimentary rocks. They include a group of rocks like shales, marls and carbonates. Containing 60–90% mineral matter and less organic matter [3]. Knowledge of minerals and the distribution of elements in this type of rock is very important for developing various exploitation plans.

This work focuses on the Moroccan Rif deposit, which is the first deposit in Morocco of bituminous rocks. Which was discovered by chance by extracting construction materials [4, 5, 6]. According to the literature, this region has not been studied in detail. The inorganic composition was investigated using several techniques: FTIR analysis, X-ray diffraction analysis (XRD), X-ray fluorescence spectrometry (XRF), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and scanning electron microscopy (SEM), to characterize the Moroccan Rif bituminous rock samples.

2. Geological setting of Moroccan Rif

The Moroccan Rif is a western end of the Maghreb chains which is a segment of a larger whole i.e.; the Alps [7]. The Moroccan Rif includes three major geological domains or nappe complexes: i) The Internal Zone, or Alboran Domain, ii) The Flysch nappes and iii) The External Zone (Figure 1) [7].

Figure 1.

Geological map of the Moroccan Rif.

The External Zone mainly results to the separation in the Miocene of the Mesozoic sedimentary cover and of Paleogene deposited on northern margin of the plate in Africa [8, 9, 10]. It consists by three types of units which are divided from NE to SW and top to bottom, into the Intrarif, Mesorif and Prerif units [8].

The Flysch nappes generally overlie the External Rif with the exception of some klippes that are located on the Internal Zone as a result of a complex evolution [11]. These Flysch units are of Cretaceous-Early Miocene age. Their basement does not crop out at present and may be considered as the deposits of a swell located on ocean or transitional crust in between the External and Internal Zones [8, 9, 12].

The Internal Zone consists of old socle of Paleozoic nappes and of limestone chain. These elements are derived from the dilaceration of the Alboran plate. The internal zone outcrops along the Mediterranean coast (Alboran Sea) between Ceuta and Jebha where it disappear under the sea thanks to a big transfer fault [13] to reappear later in the west of Al Hoseima (Figure 1).

3. Sampling and stratigraphy of study areas

The studied samples of Moroccan Rif bituminous rocks are located in four sites called: Tangier, Tetouan, Bab Taza and Arba Ayacha (Figure 2). A sample of 20 kg has been collected from each station (20–80 cm depth) and only one sample has been collected from each site.

Figure 2.

Study area showing the bituminous rocks sampling location.

TA sample of Tangier site is from Upper Cretaceous – bituminous marls intercalated with limestone in the unity of the Melloussa-Chouamat (from Massylienne nappe of Flyschs nappes) (Figures 3A and 4a) [16].

Figure 3.

Stratigraphic columns of upper cretaceous age of the unity of Melloussa-Chouamat (Flyschs nappes) and the Intrarif of the unity of Tangier (external zone), showing the investigated sample position (A: [14] (B: [14]) (C: [15]).

Figure 4.

Photographs for the four sites of the samples.

TE, BT samples of Tetouan and Bab Taza sites respectively are from Upper Cretaceous bituminous Phtanites include in schistose marls in the unity of Internal Tangier and unity of Ketama – External Tangier successively (from Intrarif of External Zone) (Figure 3B,C and 4b,c) [14, 15].

AA sample of Arba Ayacha site is from Upper Cretaceous oil shale intercalated with limestone and white marls in the unity of the Melloussa-Chouamat (from Massylienne nappe of Flyschs nappes) (Figures 3A and 4d) [16, 17].

4. Materials and methods

The four samples (TA, TE, BT, AA) were subsequently dried, crushed, homogenized and sieved to <180 μm in order to undertake the following analytical operations (Figure 5).

Figure 5.

Diffractogram of main mineral constituents of crude samples.

4.1 X-ray diffraction analysis

The crude Samples were analyzed by X-ray diffraction to determine the main mineral constituents. The analyses were carried out on a Brucker D8 Advance diffractometer equipped with a copper anticathode (λCuKα = 1.541838 Å), the analytical conditions were 40 kV, 40 mA, Ni filter, angular range 2θ° [2.5 to 65].

4.2 X-ray fluorescence spectrometry (XRF)

The XRS analytical technique are used to measure and identify the inorganic elements concentration in the sedimentary rocks. The rock raw samples were analyzed using an FX AXIOS PW4400 X-ray fluorescence (XRF) spectro-meter system.

4.3 Inductively coupled plasma-atomic emission spectrometry (ICP-AES)

The ICP-AES analytical technique are used to determination of mass concentrations of inorganic elements in the sedimentary rocks [18, 19]. The rock raw samples were analyzed using a Jobin Yvon Ultima 2 m spectrometer after digestion in HF acid.

4.4 Scanning electron microscopy (SEM)

The SEM is an important qualitative or semi-quantitative technique for knowing particle composition and morphology. As well as for analyzing surface elements, SEM is usually used to obtain topographical images of mineral grains and to determine their distribution.

Each sample of the whole rock was analyzed by a Cambridge Stereoscan 120 instrument coupled with an elemental analysis (EDX) unit. The apparatus was adjusted at 7–10 mm WD, 30 kV HV and 10,000X magnification.

4.5 FTIR analysis of crude sample

FTIR spectra were recorded on KBr discs. Each crude sample (~0.5 mg) was added to 100 mg of dry KBr powder, uniformly mixed and reground. The entire sample was transferred to a die and pressed under vacuum in the standard way. Spectra were recorded between 400 and 4000 cm⁻¹ using a Vertex 70 Model spectrometer.

5. Results and discussion

5.1 X-ray diffraction analysis

The powder diffractograms of the crude samples (Figure 5) show that the major constituents of these samples are: quartz, calcite and plagioclase with predominance of quartz, with some traces of hematite only in TA sample. The plagioclase is present in all samples except the AA sample [20].

Clay minerals are rare or at least less visible on untreated material (no cementite dissolution). Semi-quantification of major minerals using their major reflections is provided in Table 1 [20].

Sample	Quartz	Calcite	Plagioclase	Hematite	Clay
TA	++++	+++	tr	tr	++
TE	++++		++		+
BT	++++		++		++
AA	+++++				+

Table 1.

Semi-quantification of major minerals in crude samples.

The mineral composition of the fraction smaller than 2 μm of the samples studied is qualitatively shown in Table 2. After decomposing cements (carbonate and organic matter), it appears that the most abundant clay minerals are not chlorite or illite, but phases of a swelling type smectite or interstratified illite/smectite. Smectite dominates in the sample TA while for the samples BT and TE it is interstratified. Furthermore, it is noticed that the AA material is virtually devoid of clay fraction, except some traces of chlorite and illite on the threshold of detection limit.

Sample	Chlorite	Chlorite	Chlorite	Chlorite	Chlorite
TA	+	+++	+	+
TE	++		+++	+	+
BT	++		+++	+	+
AA	tr			tr

Table 2.

The mineral composition of the <2 μm fraction of the samples.

5.2 X-ray fluorescence spectrometry (XRF)

The XRF analysis of the samples (Table 3) shows very high of their content of quartz (70.04–84.46 wt%). The mass concentrations of calcium oxide (CaO) and magnesium oxide (MgO) are respectively 0.013–0.84 wt% and 0.47–1.32 wt%, indicating very low content of dolomite and calcite (carbonates) in the samples. The mass concentration of aluminum oxide (Al₂O₃) ranges between 3.90 and 12.34 wt%, ferric oxide (Fe₂O₃) between 0.74 and 3.56 wt% and sulfur trioxide (SO₃) from 0.08 to 0.74 wt%; the two last proportion ranges indicate the low content of pyrite (FeS₂).

	TA	TE	BT	AA
SiO2	80.22	70.04	73.78	84.46
Al2O3	7.929	12.34	11.87	3.908
Fe2O3	2.172	3.569	1.808	0.644
CaO	0.84	0.36	0.191	0.013
MgO	1.216	1.31	1.323	0.48
TiO2	0.2	0.3	0.31	0.09
SO3	0.1	0.11	0.08	0.74
K2O	0.64	0.73	0.92	0.23
ZnO	0.14	0.05	0.05	0.05
Na2O	0.31	1.815	1.246	0.18
MnO2	0.034	0.09	0.04	0.02
CuO	0.03	0.02	0.04	0.01
P2O5	0.05	0.08	0.1	0.02
V2O5	0.024	0.1	0.06	0.1
Cr2O3	0.02	—	0.04	0.02
BaO	0.02	0.02	0.02	—
NiO	0.01	0.02	0.02	0.01
SrO	0.01	0.031	0.004	0.01
Rb	0.01	0.004	0.01	—
CoO	—	0.04	0.01	0.01
As2O3	—	0.031	—	0.004
PbO	—	—	0.01	—
Y2O3	—	—	—	0.001
L.O.I	6.01	9.013	8.08	8.983
Total	99.991	99.995	100.012	99.99

Table 3.

Major, minor and trace element XRF analyses of 4 samples, wt%.

5.3 ICP-AES analyses

Table 4 shows the mass concentrations of inorganic elements. Silica is between 32.2–39.3% by weight, followed by aluminum between 2.44–5.88% by weight. The calcium mass concentration is much higher (5.6 wt%) in the TA sample than the other samples. Mass concentrations of the rest of the inorganic elements are equal to or less than 1% by weight. This was confirmed by X-ray spectrometry.

Sample, wt%	Al	Ca	Fe	K	Mg	Na	P	S	Si	Ti
AA	2.44	0.032	0.228	0.563	0.229	0.170	0.012	0.304	39.300	0.031
BT	5.88	0.159	0.596	0.996	0.350	1.080	0.013	0.0209	34.100	0.148
TE	5.34	0.251	1.208	0.888	0.350	0.860	0.035	0.0209	32.900	0.076
TA	3.31	5.607	1.045	0.640	0.498	0.203	0.053	0.04	32.200	0.074

Table 4.

Mass concentrations of inorganic elements in the bituminous rock samples of 4 samples by ICP-AES analysis.

5.4 Scanning electron microscopy (SEM)

According to SEM observations, there are some differences in particle shape between samples. Figure 6 shows the SEM–EDX analysis of the studied rock samples. It is seen that the surface of the TA sample is mainly composed of small particles on the order of 1 μm. While the surface of the TE sample appears compact and is dominated by particle agglomerations. For sample BT, the microstructure consists of large holes and heterogeneous particles of different sizes, while the surface of sample AA consists of homogeneously dispersed particles.

Figure 6.

SEMs of raw bituminous rocks samples: A) for TA sample, B) for TE sample, C) for BT sample, D) for AA sample. Taken from [20].

The EDX analysis shows that the surface of the samples is dominated by quartz and aluminum oxide species, especially for the TE and BT samples. The carbon peaks observed in the EDX spectrum (Figure 6) indicate the total carbon content, which includes organic carbon (as organic matter) and metallic carbon (as carbonate). These results are in good agreement with the results obtained with the previously mentioned analytical techniques.

5.5 FTIR analysis of crude sample

FTIR spectra of the crude samples are shown in (Figure 7). Based on the known bituminous rocks data [21], the absorption bands observed in the spectra of the four studied samples were attributed as follow:

Figure 7.

FTIR spectra of the crude samples.

Characteristic bands of organic matter: The bands within the vibration range 400–875 cm⁻¹ belong to aromatic C-H structure, alkyl-Si and sulfides … The absorptions between 1030 and 1170 cm⁻¹ corresponding to CC, CO and CN stretching and HCC, CCC bending bands. The vibration bands range 1500–1900 cm⁻¹ belong to aromatic CH=CH structure and COOˉ, CO of naphthalene groups. The bands appear between 2300 and 2500 cm⁻¹ belong to C=C and C=N of unsaturated liaisons. The bands between 2800 and 3000 cm⁻¹ correspond to stretching of aliphatic chains. The band within the vibration range 3000–3700 cm⁻¹ belongs to NH and OH stretching of (alcoholic, phenolic and carboxylic …) groups and also of aromatic C-H structure.

Characteristic bands of mineral matter: In spectra of crude rocks, the absorption bands of inorganic compounds are the most intense and often mask those of fundamental bands of organic compounds. The Fe₂O₃ phase may be characterized by absorption bands at 520, 460 and 470 cm⁻¹ which are difficult to distinguish from those of the organic matter bands. The 1433 cm⁻¹ absorption band visible only in the spectrum of TA sample belongs to the bending vibration of carbonate (CaCO₃) and confirms our findings of XRD results. The bands characterizing clays correspond to frequencies between 3000 and 3700 cm⁻¹ in the OH region (H₂O).

The bands due to vibration of Si-O-M structure fall into three series. The first set is displayed with very intense frequencies to 1035, 1094, 1096, 1099, 1089 and 1165 cm⁻¹. The second set is less intense and the bands are located at frequencies between 600 and 800 cm⁻¹. As for the last of medium intensity, it is between the frequencies 460 and 520 cm⁻¹. The predominance of quartz and element of silica (Si) in the mineralogical composition of the four examined samples have already been proved by XRD analysis.

6. Conclusion

The study of bituminous rocks of Moroccan Rif provides important information about representative samples in this deposit. Mineralogical composition of the crude samples is dominated by the quartz with the presence of calcite (carbonate) only in the TA sample (bituminous marls) and also confirmed by FTIR analysis. The clay minerals and other mineralogical components are present as different low weight proportions.

The mineralogical components that were found by X-ray fluorescence spectrometry (XRF) and X-ray diffraction (WRD), show that the samples of bituminous rocks studied in the Moroccan Rif, characterized by a high content of silica (quartz), and lower contents of dolomite, calcite (Carbonate), and Pyrite. That was confirmed by other analyzes: ICP/AES, MEB-EDX.

Acknowledgments

The author is grateful for the support for this work from the University of Mohammed V - Rabat, Morocco.