The samples analyzed were those of petroleum source rocks from the Donga and Yogou formations of the Termit sedimentary basin (Figure 1). They were collected in collaboration with the Centre de Documentation et d’Archives Pétrolières du Niger (CDP). They include cuttings from the Koulélé-1D, Fana-1, Helit-1, Melek-1 and Ounissoui-E1 wells (Figure 1).

Sampling was carried out with a 10-meter sampling step.

Rock cuttings samples were washed to remove drilling mud. X-ray fluorescence (XRF) analysis and scanning electron microscopy coupled with energy dispersive spectrometry (SEM/EDS) were applied to determine the composition of major chemical elements in these samples.

XRF was performed using a portable Niton XL3t^® XRF spectrometer, coupled with a computer to ensure data transfer after analysis using Niton Data Transfer (NDT) software. Measurements in All-Geo mode were carried out on an accessory fitted with a Radio Frequency Identification (RFID) chip, enabling the analyzer to detect it automatically and convert it into a benchtop analyzer. Scanning electron microscopy coupled with energy dispersive spectrometry (SEM/EDS) were used to study the crystalline form of minerals using digital images, and to determine the major chemical elements and oxide composition of the samples analyzed.

The tool used is a D.C.A.R. Variable Pressure SEM/EDS (MEB FEG Supra 40 VP Zeiss), equipped with an X-ray detector (OXFORD Instruments X-Max 20) connected to an EDS microanalyzer platform (Inca Dry Cool, without liquid nitrogen).

The results of the distribution of major chemical elements in the Donga formation are shown in Table 1. They include silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), potassium (K), sulfur (S), titanium (Ti), magnesium (Mg), manganese (Mn) and barium (Ba).

Elements Mg, Mn and Ba have very low proportions, averaging no more than 0.5% (Table 1). The elements K, S and Ti have low average proportions of 3.99%, 2.65% and 2.07% respectively. Ca has a relatively low average value of 5.37%, with the highest value in sample PO.E2.Dg (13.82%).

Figure 2 shows the histogram of the different proportions of major chemical elements in the Donga formation. The Donga formation is relatively rich in silicon, aluminum and iron (Figure 2), with average proportions of 49.49%, 17.79% and 17.45% respectively. The highest proportions of these elements are found in samples PK.E22.Dg (56.77%), PF.E21.Dg (22.03%) and PF.E20.Dg (20.09%) respectively.

The major chemical elements present in the Yogou formation are listed in Table 2. They are composed of silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), potassium (K), sulfur (S), titanium (Ti), magnesium (Mg), manganese (Mn) and barium (Ba).

The elements Si, Al and Fe (Table 2) are the most abundant in the Yogou formation. Their average proportions are 57.77%, 16.17% and 15.57% respectively. The highest proportions of these chemical elements are found in samples PF.E11.Yg (63.29%), PK.E26.Yg (17.52%) and PK.E26.Yg (19.29%) respectively.

Elements Ca, K, S and Ti have low proportions, averaging 1.82%, 3.57%, 1.81% and 2.42% respectively. The element K has the highest value in sample PH.E42.Yg (5.02%). The other three chemical elements, Mg, Mn and Ba, are in very low proportions, averaging no more than 0.7% (Table 2).

Figure 3 shows the histogram of the different proportions of major elements in the Yogou formation. Figure 3 shows that silicon is the most abundant chemical element in the Yogou formation.

The Si/Al ratios presented in Table 1 and Table 2 range from 2.16 to 3.76, with an average of 2.80 for the Donga samples, and from 3.01 to 4.09, with an average

of 3.60 for the Yogou samples (Figure 4).

Figure 4 shows the histogram of Si/Al atomic ratios of major chemical elements for the Donga formation in green and the Yogou formation in orange.

Images and corresponding energy dispersive spectra of samples from the Donga and Yogou formations are shown in Figures 5-8.

The majority of element peaks in the EDS spectra, such as silicon (Si), aluminum (Al), magnesium (Mg), iron (Fe), sodium (Na), potassium (K), calcium (Ca), titanium (Ti) and sulfur (S), are common elements observed with XRF.

The elements gold (Au) and carbon (C) are taken from the sample support grid during analysis. The main chemical elements identified using energy dispersive spectrometry are shown in Table 4 and Figure 9, and the oxides in Figure 10.

The abundance of major elements such as silicon, aluminum and iron in the samples from Donga and Yogou formations revealed by XRF analysis is attributable to the presence of detrital minerals in these samples. Furthermore, the high proportion of Fe in these formations is either due to the presence of pyrite and goethite, or to substitutions in the octahedral and tetrahedral structures of Al³⁺ by Fe²⁺ and Si⁴⁺ by Al³⁺ or Fe³⁺ respectively in the clay material. These observations concerning the presence of non-silicates minerals iron-rich, notably pyrite and goethite, are in line with the work of [14] . Also, the presence of relatively high levels of the elements K and Ca is probably due to these substitutions. This could result in a slight charge deficit in the structure, which can be compensated for by these modifier cations (interfoliar) in the tetrahedral and octahedral sites as described by [16] . In addition, the uniform distribution of Fe and Mg elements on clay particles can be interpreted as the result of substitution of these elements in the clay crystal lattice and during the deposition of nanocrystalline grains of associated minerals such as pyrite, goethite as revealed by [14] .

Also, the average Ti, K and K₂O contents, with an average of 3.99% and 3.57% by weight for Ti, 2.07% and 2.42% by weight for K and 2.85% and 3.34% by weight for K₂O respectively for the Donga and Yogou formations, are relatively high. This suggests that the presence of titanium (Ti) in these formations is either linked to the crystalline networks of the clay minerals, or originates from detrital materials [16] [17] and confirmed by the work of [14] on the mineralogical characterization of the Donga and Yogou formations.

The works of [18] [19] has shown that Al₂O₃ and SiO₂ contents are respectively proportional to the presence of clays and silicate detrital materials such as quartz. In this study, SiO₂ is abundant in the Yogou formation, while Al₂O₃ is abundant in the Donga formation. This distribution characterizes a predominance of clay minerals in the Donga formation and is probably explained by the presence of sandy lenses in the Yogou formation, constituting Upper Cretaceous reservoirs in the Termit sedimentary basin, as described in the works of [20] [21] [22] [23] in the same Termit sedimentary basin.

Iron oxide (FeO) is also an important component in the Donga and Yogou formations, averaging 6.46% and 7.27% by weight respectively. This iron oxide (FeO) has known preferential associations with iron sulfides such as pyrite and marcasite. Mineralogical examination by [14] confirmed that the extent of iron concentration generally coincides with the presence of pyrite in these formations.

The Al/Si ratios, which vary from 0.27 to 0.46 with an average of 0.35 for the Donga formation and from 0.2 to 0.33 with an average of 0.28 for the Yogou formation, are respectively low (Table 2 and Table 3), suggesting according to [18] that Si in most sediments is associated with silicates and clay minerals. According to previous work [14] [24] , the SiO₂/Al₂O₃ ratio values in the Donga and Yogou formations belong to the range of SiO₂/Al₂O₃ ratio values for 2:1 type clays (2Si and 1Al which is 2 to 4) [25] [26] . These ratios are significantly higher than those of standard kaolinite (ranging from 1.73 to 1.8) [25] [26] . This may be due to the presence of siliceous minerals, particularly quartz, in the rocks of the Donga and Yogou formations, as shown by [14] .

The abundance of silicon and aluminum on the one hand, and the relatively high Si/Al ratios on the other, provided by X-ray fluorescence spectrometry, confirm a clay, quartz and muscovite enrichment of the rocks in these formations, as noted by [14] . The high iron content is linked to the presence of pyrite in samples from the Donga and Yogou formations. Then, the relatively high contents of chemical elements such as S, Ti, Mg, Mn, and also Fe, Ca, and K is due to the presence of accessory minerals such as garnet, siderite, goethite and anatase in the Donga formation and sericite, anatase and goethite in the Yogou formation and pyrite and muscovite in these two formations [14] .

In addition to the high Si/Al ratios, the K and Ca element contents are probably due to substitutions in the octahedral and tetrahedral structures of Al³⁺ by Fe²⁺ and Si⁴⁺ by Al³⁺ or Fe³⁺ respectively. The results of mineralogical analysis of clay fractions from the Donga and Yogou formations obtained by [14] reveal the presence of interstratified clay minerals such that illite, smectite and chlorite type. Si/Al ratio values between 1 and 3, obtained by EDS analysis, indicate the predominance of kaolinite, 2:1 clays such as smectite and illite and siliceous minerals as described by [14] .