Introduction

Kerogen is an important component in the organic-rich sedimentary rocks, and plays a key role in understanding hydrocarbon production. Three types of kerogen (Type I, Type II, and Type III) can be revealed from a Van-Krevelen type diagram depends on hydrogen/carbon (H/C) and oxygen/carbon (O/C) atomic ratios1,2. Therefore it has gained significant value since it is an important in conventional and unconventional exploration3. However, the oil and gas generated from kerogen are linked to the structure and thermodynamic properties of kerogen4.

The kerogen, without definition the molecular structure, is a mixture of organic compounds and can generate quantities of hydrocarbons. In this regard, several studies have applied several experimental technologies to investigate the molecular structure and properties of the kerogen5,6,7,8,9,10,11,12,13,14. Such techniques include X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS), fourier transform infrared (FTIR) and 13C nuclear magnetic resonance (13C NMR) spectroscopy. These studies have evaluated the structural features in the kerogens based on the disparity between the aliphatic carbon chains and numbers of carbon in aromatic component during the maturity, and thereby understanding the thermally decomposition of different types of kerogen6.

The key focus of the present study is the oil shales in Jordan, which is distributed in the north, central and south, with reserve reaching to 64.9 billion ton15,16. Despite this, oil shales condense in four well known sub-basins such as Lajjun, Yarmouk, Azraq-Hamza, and Jafr17. The oil shale deposits in these Jordan's sub-basins are promising unconventional oil resources contributing into the future energy prospect in Jordan. The Lajjun sub-basin in the Western Central Jordan has the largest and more condense oil shale succession in Jordan15. This sub-basin includes Attarat Umm Ghudran, Lajjun, Sultani, and Wadi Maghar areas (Fig. 1A).

Figure 1
figure 1

(A) Location map of Lajjun sub-basin including 8 major outcrops and related structures (after Ali Hussein18). (B) Lajjun-Dead Sea cross section showing the main outcrop of the oil shale succession between the two N–S faults (cross section has taken from Google Earth), (C) Geological map of the Lajjun sub-basin in Central Jordan, with the main Late Cretaceous Muwaqaar Chalk Marl Formation (modified after Shawabkeh19). The maps have drawn by Mohammad Alqudah using Coreldraw15 software.

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The oil shales in Jordan range from the Late Cretaceous to Eocene in age as assigned by Alqudah et al.17,20, and a description of those strata has been performed by Ali Hussein et al.21,22. However, many researchers have reported that the Mesozoic (Late Cretaceous) and Cenozoic (Paleocene) oil shales found within the Lajjun sub-basin in western Central Jordan, and have been investigated those oil shale successions in order to gain insights into their origin, depositional environment, quality, and commercial development15,17,21,23,24,25,26. These prior studies primarily focused on the geochemical characteristics of the whole rock of these oil shales, and indicate that the oil shales in the Lajjun sub-basin possess both economic and scientific importance for unconventional exploration.

This study aims to provide a perception of the molecular structure and properties of the kerogen components found in the organic-rich shale intervals situated within the Late Cretaceous Muwaqqar Chalk Marl Formation (MCM) at Lajjun outcrop, Western Central Jordan (refer to Fig. 1A). In this case, the study commences several approaches that are based on the effect of multi-atoms with organic sulphur included on the kerogen molecular structure (KMS) and fill the gap of knowledge dealt with the internal structure of sulphur-rich kerogen and the timing of oil generation. This comprehensive assessment is based on integrated findings from kerogen microscopic and unconventional geochemical analyses together with bulk kinetics modeling.

Geological setting

Jordan was covered with a shallow sea of southern Neo-Tethys Ocean during the Maastrichtian, where organic deposition was initiated in this time27,28,29. Highstand of sea level was influenced during this time, in regional scale with plate tectonics, which thus played a crucial role in formation of morphological features and resulted in a marine transgression over vast areas of the Arabian Plate17,30,31. However, the African-Arabian plate initiated a convergence movement toward Eurasia in the Cenomanian left platform deformed into many sub-basins and paleohighs30,32. Those sub-basins were characterized with a local restriction which favored for oil shale precipitation between the Middle Cretaceous to Paleogene33,34.

The biggest sub-basin in the Western Central Jordan is Lajjun sub-basin, which eight outcrops and several subsurface successions were found and described in literatures26,32,35. The Lajjun sub-basin encompasses of depression connected by two major faults (Fig. 1B), which is corresponded to the closure of the Neo-Tethys in the Late Cretaceous36. Accumulation of sediments in such setting was significantly affected in Lajjun, resulting in increasing in restriction and appearance of organic matter within the sediments of the MCM Formation22,37. The MCM Formation is the lithological unit exposed in the Lajjun sub-basin (Fig. 1C), which includes thick bedded marly limestones, marls, chalks, and locally bituminous marls22,37. The organic matter of the MCM Formation found in anoxic setting associated with restriction in the sub-basin22,26,32. However, those deposits of the MCM Formation seem disconnected and lenticular with a significant variation in thickness over short distances32, and crop out with few meters thick and around 80 m in the boreholes37.

In the Lajjun outcrop, the MCM Formation consists of siliceous phosphatic bearing marl at base with strikes of foraminifera (Fig. 2). Organic matter getting more at middle part of the section, which characterize with very dark color (Fig. 2). Limestones in the section are associated with bioturbated intervals. Faunas like gastropods and ostracods are scattered in the samples (Fig. 2).

Figure 2
figure 2

Sedimentological section of the Lajjun outcrop in the Lajjun Sub-basin, Western Central Jordan represents the lithology and the bio-contents in the oil shale samples of Late Cretaceous Muwaqaar Chalk Marl Formation. The section has drawn by Mohammad Alqudah using Coreldraw15 software.

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Samples and analytical techniques

A section, from the Lajjun exposure in the Lajjun Sub-basin, Western Central Jordan, includes ten organic-rich samples of the Late Cretaceous MCM Formation was investigated (Fig. 2). The weathered surfaces of the studied samples were avoided prior to perform the geochemical and petrological analyses. The Tmax values of the samples are lower than 430 °C according to previous published research26. Although Tmax can be influenced by many non-thermal factors38, the samples can be generally classified as immature.

The collected whole oil shale samples were milled into ≤ 2 mm, and then employed for used petrographic examination (i.e., organic matter facies). In addiation, the kerogen was isolated from the whole oil shale samples using a palynological procedure of extraction. In this case, the whole samples were crushed into 72 mesh sizes using a mortar and pestle, and then treated to remove the carbonate and silicate minerals using hydrochloric and hydrofluoric acids followed by washing of water to let the sample normalized. The kerogen residuals were sieved and mounted on slides using glycerine jellies. Oxidation and ultrasonic vibration were avoided to get more preserved organic matter. Later, the isolated kerogen was subjected to unconventional geochemical analyses and microscopic examination, including ultimate elemental (CHNS), FTIR analysis, Py–GC, and bulk kinetic modeling together with paleofacies studies. These analyses were performed at geochemical and organic petrological labs in the Department of Geology, University of Malaya, Malaysia.

Microscopic examinations

The whole oil shale samples underwent organic petrological examination and organic facies analysis using a standard polished block procedure39, as described in the following subsection.

The entire rock volume of the 10 samples was ground into small portions range from 2–3 mm, and then gathered with a SeriFix resin and hardener agent. Then, in order to let surface smoothed, silicon carbide paper and alumina powder were used following ASTM D2797-04. The polished blocks were then examined under white light with oil-immersion medium, cross-polarized and under UV light. LEICA DM4 microscope was used which prepared with fluorescence that allowed a good view of the organic matter (OM) input.

In addition, palynofacies analysis was conducted on the isolated kerogen using traditional thin section under a LEICA DM4 microscope. A total of 200 or more specimens for each sample including (i.e., amorphous organic matter, palynomorph and phytoclasts) were counted using transmitted cross-polarized and UV lights for palynofacies analyses (identifications and quantitative analysis).

Py–GC

Py–GC analysis was employed on the kerogen concentrate after removal of free hydrocarbon (S1) by Soxhlet extraction procedure and mineral matters by acids leaching procedure (HCL and HF). The Py–GC is useful to identify kerogen types and determine their oil generation potential. Kerogen was heated to 600 °C using a Frontier Lab Pyrolyzer (PY-2020iD), which was connected to an Agilent Technologies 5975 gas chromatograph fitted with an ultra-alloy capillary column (30 m × 0.32 mm I.D. × 0.25 µm) and flame ionisation detector (FID). The major compounds and components were recognized through matching the resultant retention times with standard published samples40,41.

CHNS

CHNS analysis was performed on the isolated kerogen fraction from 10 oil shale samples using Perkin Elmer (2400 Series II). The isolated kerogen samples were placed into aluminum vials and automatically injected following ASTM standard (P/N 0240–1289). The samples were heated at 1100 °C and then the composition of C, H, N and S were measured. Ascertain additional values, like oxygen content (O %), H/C and O/C atomics were estimated from the combination of elemental compositions (CHNS).

FTIR analysis

ATR-FTIR analysis, which is a commonly used to determine the distribution pattern of varies isolated kerogen groups in the studied organic-rich samples42,43. This analysis was performed on a PerkinElmer Spotlight 300 FT-IR microscope system. In this regard, 3 mg of the powdered samples were put on the Diamond/ZnSe plate of the PerkinElmer Universal ATR accessory unit. The infrared spectra were measured in both transmittance and absorbance modes at 4000–650 cm−1 wavelength frequency and the peaks were assigned based on published spectra42,43,44,45,46.

Bulk kinetics modeling

The precipitated kerogen fraction from four oil shale samples was subjected to the bulk kinetic study. These four samples were pyrolyzed using the unstable thermal settings in the Weatherford SRA pyrolyzer under incrementally heating rates starting with 1 at 300 °C and increases to 5, 10, 25 and finalize with 50 °C/min at 600 °C. The results were analyzed using Kinetics 2000 and KMOD software packages and applied to estimate the activation energies (Ea) and the frequency factor (A) following the works of Burnham et al.47,48 and Behar et al.49. Curves that represent heating rate were considered as essential information to confirm the aforementioned characteristics (Ea and A)50,51.

Results

Organic matter characteristics

Organofacies in the studied oil shale samples were identified by means of microscopic examination under UV radiation reflected light, as shown in Fig. 3. The organofacies are dominated by high amounts of liptinite, including structured and unstructured organic matter. The liptinites have fluorescence colours fluctuated from orange to yellow (Fig. 3). However, the hydrogen-rich liptinites display a high abundance of the alginate, and they were identified into telalginite and lamalginite based on their morphologies (Fig. 3).

Figure 3
figure 3

Photomicrographs of the studied oil shales samples of the Muwaqaar Chalk Marl Formation, under reflected UV light, with a field width of 0.2 mm, including different organic matter assemblages of liptinite such as alginite and bituminite.

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Telalginite arises from algae and appears in many shapes like fan-shaped, discs and lenses (Fig. 3d and e), whereas lamalginite occurs as thick lamellae (Fig. 3a–d).. Other unstructured organic matter such as bituminite also occurred in the studied samples (Fig. 3e and f). The bituminite have a greenish to yellowish fluorescence under UV light (Fig. 3e and f).. Foraminifera assemblages are dominant in the studied samples, which were infused with the mineral matrix and characterized by a greenish fluorescence when examined under the UV light (Fig. 3b,c and f).

Palynofacies and kerogen facies

Based on the palynofacies results, the most predominant component of the kerogen in the studied oil samples was fluoramorphinite amorphous organic matter (AOM) materials, with amount of herbaceous and algal organic matter being fairly low (Fig. 4). The structureless AOM appears to be light to dark brown, well aggregated with angular texture (Fig. 4A–D), while the fluorescence was orange to yellow under UV light (Fig. 4E–H).

Figure 4
figure 4

Photomicrographs of the isolated kerogen in the studied oil shales samples of the Muwaqaar Chalk Marl Formation, under transmitted normal (A–D), and UV (E–H) lights, with a field width of 0.2 mm.

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Algal organic matter was also found in lower quantities and represent tasmanite (Fig. 4B and F), prasinophyte (Fig. 4C and G), and pediastrum algae types (Fig. 4D and H) based on their morphologies. However, the existence of tasmanite algae pointed that that the studied kerogen fragments contain hydrogen-rich telalginite and lamalginite algae assemblages, as exposed to reflect UV light (Fig. 3). Moreover, the herbaceous organic matter––identifiable species of spore found as minor or absent (Fig. 4B,D,F and H).

Ultimate elemental (CHNS)

The ultimate elemental results show that the carbon contents are relatively high, ranging from 50.61 to 66.99 wt%, while the medium is 63.39 wt%. Hydrogen varies from 6.73 to 8.28 wt%, while nitrogen ranges from 1.25 to 1.93 wt% (Table 1). The sulphur content was also measured and found in high amounts, ranging from 11.18 to 14.19 wt% (Table 1). The oxygen values were estimated based on the CHNS contents and were found to be in the range of 9.05–30.23 wt% (Table 1). Accordingly, the atomic ratios H/C and O/C were obtained with values from 1.46 to 1.60 and from 0.10 to 0.45, respectively (Table 1).

Table 1 Ultimate elemental (CHN) and fourier transform infrared (FTIR) results of the isolated kerogen from ten oil shale samples of the Late Cretacouse Muwaqqar Chalk Marl Formation from the Lajjun outcrop in the Lajjun Sub-basin, Western Central Jordan.
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FTIR results

The spectra of representative samples show distinct bands in the aliphatic stretching, aromatic C–H stretching, and aromatic bending regions at 3000–2800 cm−1, 3100–3000 cm−1, and 1460–1350 cm−1, respectively (Fig. 5). Furthermore, methylene (CH2) groups were recognized based on the appearance of peaks near 2920 cm−1 and 2850 cm−1, while the peaks near 1600 cm−1 were given an aromatic C=C groups (Fig. 5). The spectra also display the asymmetric CH3 stretching anomaly at 2955 cm−1 and carboxy/carboyl group peak at 1710 cm−1. Based on the spectra of FTIR results, several parameters, including A-factor (AF), C-factor (CF), aliphaticity index (IAL), aromaticity index (IAR) are calculated (See Table 1). The AF is a proportion of the abundance of aliphatic over aromatic bands, while CF is a measure of the relative abundance of carboxyl over carboxyl and aromatic bands. The IAL and IAR, respectively, quantify the relative intensities of aliphatics and aromatics to saturate and aromatics. These FTIR parameters are useful for evaluating hydrocarbon generation potential and classifying kerogen portions, as argued in the coming sections.

Figure 5
figure 5

FTIR spectra of the isolated kerogen from representative oil shale samples from the Lajjun outcrop in the Lajjun Sub-basin.

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Py–GC

Py–GC results show that the isolated kerogen is characterized by unimodal chromatogram patterns of the n-alkene and n-alkane doublets, ranging from C1-5 to C30 (Fig. 6). The distribution of these n-alkene and n-alkane doublets is characterized by high abundance of low n-alkane molecules (C1–5–C14), with low quantities of waxy n-alkene and n-alkane (+ C15), as shown in Fig. 6. The dominant aromatic compounds observed in studied samples are toluene and o-xylene (Fig. 6). Higher amount of 2,3-dimethylthiophene is also observed laterally with aromatic and saturated hydrocarbons (Fig. 6).

Figure 6
figure 6

Py–GC pyrograms of the isolated kerogen in the studied oil shales samples of the Muwaqaar Chalk Marl Formation, showing n-alkene/alkane doublets and labelled peaks.

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However, the proportions of four components, namely carbon 8,2,3-dimethylthiophene, o-xylene and carbon 9:1 in the pyrolysates of the analyzed isolated kerogen in the studied samples (Fig. 6) were estimated as shown in Table 2. The pyrolysate distributions of these three components show a majorly 2,3-dimethylthiophene compound and C8 n-alkane, along with minor quantities of C9:1 n-alkane and o-xylene compound (Fig. 6), leading to a percentage 40.03–53.09%, 16.71–22.13%, 15.55–19.94%, and 14.51–18.30%, respectively (Table 2).

Table 2 Pyrolysis–gas chromatography (Py–GC) of the isolated kerogen from ten oil shale samples of the Late Cretacouse Muwaqqar Chalk Marl Formation from the Lajjun outcrop in the Lajjun Sub-basin, Western Central Jordan.
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The o-xylene related to C8 n-alkane, showing richness of C8 n-alkane over o-xylene, with values of the o-xylene/C8 and C8/o-xylene proportion from 0.75 to 0.87 and from 1.15 to 1.34, respectively (Table 2). Table 2 also illustrates the relative abundance of the C1-C5, C6-C14 and C+15 of the n-alkane and n-alkenes that measured from the Py–GC pyrograms of the resulted kerogens. Most of the samples show that the C6–C14 of the n-alkane and n-alkenes prevails (41.72–54.18%) over than the C1–C5 (32.15–48.27%) and C+15 (10.01–16.92%) of the n-alkane and n-alkenes.

Bulk Kinetic results

The bulk kinetic results of the isolated kerogen fraction from three studied samples are illustrated in Fig. 7. The bulk kinetic models (Fig. 7a) suggest that the Ea patterns are intensively fluctuated from 39 to 66 kcal/mol, while A is found to be between 2.1237E+13/1 and 5.0950E+13/1 s.

Figure 7
figure 7

(A) Bulk kinetic parameters in terms of activation energy distribution (Ea) and frequency factors (A) of the 4 selected kerogen-extracted oil shale samples. (B) Bulk kinetic results in terms of temperature (°C), transformation ratio (TR%) and Vitrinite reflectance (Ro%) of the 4 selected kerogen-extracted oil shale samples. Data were determined from open system pyrolysis at five heating rates (1, 5, 10, 25 and 50 °C/min).

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The kinetic models at a consistent rate at a 3.3 °C/My rate were also used to estimate the geological heating, thus paleo-temperatures and the therefore the vitrinite reflectance (%Ro) values and transformation ratio (TR) was accordingly predicted. In this case, the paleo-temperatures are plotted against the calculated %Ro and TR values and the results are presented in Fig. 7b, and show that the earlier stage of petroleum generation window (% 10 TR) takes place at 100–106 °C, which matches to a estimated %VRo is between 0.55 and 0.60, while the peak generation window, with corresponding Ro of 0.71–0.83% is expected to convert around 50% of kerogen during the geological temperatures between 122 and 138 °C (Fig. 7b), and thereby can generate significant amounts of oil.

Discussion

Kerogen composition and characteristics

Herein, we discuss the molecular structure of the isolated kerogen fraction together with the visual kerogen properties in order to get insight into the kerogen composition and characteristics from oil shales in the Late Cretacouse succession.

The visual kerogen of the studied oil shale samples has a mixture organic facies, with the dominance of marine origin organic matter assemblages (i.e., algae, bituminite and fluorescence AOM) and minor amounts of plant-like fossils (i.e., spores), as clearly indicated from microscopic investigation (Figs. 3 and 4). Most of the algal organic matter in the studied oil shale samples is presented by both telalginite and lamalginite types, which are assumed to have formed in mainly marine environment26,40,52,53. Other unstructured organic matter such as bituminite and fluorescence AOM are inferred as bituminite II following Pickel et al.54 and assigned the origin of the kerogen components to the marine environment55. However, these marine organic matter assemblages imply strongly hydrogen-rich kerogen categories. Further, the interpretation of the hydrogen-rich kerogen in the studied oil shale samples is corroborated by the pyrolysates of the kerogen degradation using Py–GC results, which can be used to assess the structural moieties of kerogen that related to alkyl-chain distribution, degree of aromaticity, and thereby gain knowledge on the kerogen types and their petroleum composition56,57,58,59,60,61,62. In this case, the high abundance of n-alkanes and n-alkanes of more than C10 indicates the presence of aliphatic-rich kerogen, while aliphatic-poor kerogen has n-alkanes and n-alkanes of less than C1062.

In the current study, the analyzed isolated kerogen displays a unimodal distribution pattern of n-alkane and n-alkane doublets, with the prevalence of n-alkane and n-alkane peaks of more than C10 and comparatively low aromatic components (Fig. 6). This pyrolysate distribution is further suggested that the analyzed isolated kerogen from the studied oil shale samples was mainly derived from aliphatic-rich kerogen.

Additionally, many literatures have shown that the length and type of aliphatic chains in FTIR spectra and its structure can be used to get information about the kerogen types42,43,44,63. In this regard, the FTIR spectra, particularly aliphatic and aromatic C–H stretching, shows that the analyzed kerogen fraction mainly composed of aliphatic stretching bending regions between 2800 and 3100 cm−1, with lesser amounts of aromatic bending regions at 1460–1350 cm−1 (Fig. 5), and confirms the strong aliphatic-rich kerogen. The FTIR spectra-based parameters such as AF and CF have also been used to evaluate the kerogen type44. Based on the AF and CF results, the analyzed isolated kerogen fraction has relatively high AF values of more than 0.60 and up to 0.68 (Table 1), suggesting the dominance of mainly kerogen type II, as clearly demonstrated from a modified correlation diagram of AF and CF parameters by provided Ganz and Kalkreuth44, as shown in Fig. 8a. Moreover, the ultimate elemental analysis of the isolated kerogens is in compatibility together with the molecular structure of the kerogen using FTIR spectra and used to corroborate the organic matter inputs and their kerogen types64,65. The higher H/C percentage suggested oil-prone Types I and II kerogen with higher hydrogen content, whereas higher ratios of O/C referred to Type III kerogen that fall within gas-prone52. Based on the results, the analyzed isolated kerogen fraction has high H/C values between 1.46 and 1.60 (Table 1), further indicating mainly Type II kerogen, as clearly confirmed from a Van-Krevelen type diagram based on applying the H/C and O/C atomic ratios (Fig. 8b).

Figure 8
figure 8

(A) Cross plot of the A-factor and C-factor of the FTIR specta for the isolated kerogen of the studied oil shale samples. (B) Cross plot of the H/C and O/C atomic ratios of the isolated kerogen of the studied oil shale samples, showing mainly Type II kerogen.

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The presence of the high S content of > 10 wt% with the molecular structure of analyzed kerogen from the studied Late Cretacouse oil shales usually indicates a high sulphur Type II kerogen26,66. This interpretation agrees with the good correlation between the S and H elements (Fig. 9). In addiation, the Type II kerogen enriched with sulphur also was confirmed from the pyrolysates of the kerogen degradation. In this case, the pyrolysate distributions show a majorly 2,3-dimethylthiophene organic sulphur compound, along with minor amounts of C9:1 n-alkane and n-alkane doublets and o-xylene compound (Fig. 6), further indicating Type II kerogen enriched in sulpher based on the plot illustrated by Eglinton et al.59, as shown in Fig. 10.

Figure 9
figure 9

Cross plot of the sulphur (S) content versus hydrogen (H) content of the isolated kerogen in the studied oil shale samples.

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Figure 10
figure 10

Ternary diagram based on n-alkyl chain length distribution of the pyrolysis products of the analyzed isolated kerogen of the studied oil shale samples (adapted after Eglinton et al.57).

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However, the Ea and A values also infer to the kerogen composition, as they are significantly influenced by the constancies in temperature and thermal conductivity of each kerogen group49,67,68,69,70. Types I and II oil-kerogen exhibit constricted value of Ea covered 47–56 kcal/mol69,71, the Type II-S kerogen in the other way display a more fixable range of Ea fluctuated between 44 and 66 kcal/mol26,49,72. In this regard, the broad activation energy distributions, with an average frequency factor of 3.56E+13/1 s observed in the analyzed isolated kerogen fraction from the studied oil shales are typical for Type II-S kerogen73.

Implication for unconventional oil shale exploration

In the current study, we considered a variety of parameters to get ideal for the efficacy of the oil production from kerogen within the oil samples of the Late Cretacouse MCM Formation from the Lajjun outcrop, Lajjun Sub-basin of western central Jordan.In this case, multi-geochemical results were discussed based on the molecular chemical structure and visual properties of kerogen and show that the kerogen of the MCM samples is rich in hydrogen and is considered as an oil-rich resource. This finding of the high concentration of hydrogen is evidenced by the ultimate elemental analysis, with dominantly Type II kerogen (Fig. 8b). The notably intensive fluorescent found in the liptinitic organic matter under UV light (Fig. 3 and Fig. 4e,f,g and f) serves as evidence that the studied shale rocks of the MCM succession have the extremely oil generative potential.

The higher quantities of aliphatic carbon chains than aromatic carbon number in the molecular structure of the kerogen is also confirmed by the applied Py–GC and FTIR results (Figs. 5 and 6). Accordingly, the kerogen in the studied MCM samples is mainly Type II kerogen (Fig. 8). The Type II kerogen in the studied samples would have a high sulphur-rich Type II kerogen, as indicated by the essential chemical properties of kerogen, with a high amount of sulphur content exceeds 11 wt% and up to 14.19 wt% (Table 1). II S. However, the highest organic sulphur, resulting in a mainly sulphur-rich Type II kerogen, as clearly demonstrated by the presence of the high concentrations of sulphur compound (i.e., 2,3-dimethylthiophene) associated with saturated and aromatic hydrocarbons (Fig. 10). In this case, the high sulphur oil can generate from the organic rich sediments of the Late Cretacouse MCM Formation during the early thermal retorting or pyrolysis process. This oil is classified as paraffinic with low wax products (Fig. 11) Besides, petroleum production stages and their geological temperatures of the studied.

Figure 11
figure 11

Ternary diagram based the relative percentages of the n-alkenes/n-alkane ranges, shown the oil types that can generate during decompacting or pyrolysis processes of the analyzed isolated kerogen in the studied oil shale samples (modified after Horsfield72 and Yang et al.41).

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In addiation, finding of sulphur in the molecular structure of the kerogen in the Muwaqqar Chalk Marl oil shales enhanced generation of oil over low maturity level stage72, corresponds to VRo values range between 0.4 and 0.8%74. This interpretation is further assessed using bulk kinetics modeling at heat rate of 3.3 °C/My. According to the kinetic models, the kerogen of most of the studied MCM oil shales found to be able to generate oil in the low temperatures (100–106 °C), which was associate with low VRo covered 0.55–0.60 Ro% (Fig. 7b). This is can be explained by existence of a weak bonds between carbon and sulfur (C–S) in the kerogen structure72, as works with the presence of Type II-S kerogen in the studied oil shale samples of the MCM Formation (Fig. 10). This is clearly demonstrated by the inversely proportional between the high S content and low geological temperature ranges and their equivalent VR values from the kinetic models (Fig. 12).

Figure 12
figure 12

(A) Cross plot of the sulphur (S) content versus hydrogen (H) content of the isolated kerogen of the studied oil shale samples. (B) Cross plot of the sulphur (S) content versus carbon (C) content of the isolated kerogen of the studied oil shale samples, showing the inverse proportional between the high S content and low geological temperature ranges and their equivalent VR values from the kinetic models.

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Furthermore, the kinetic model outcomes of this research works also indicate that the investigated oil shale intervals of the MCM Formation reached the high kerogen conversion ratio of up to 50% during the temperature ranges between 122 and 138 °C, matching to VR values of 0.71–0.83% (Fig. 7b). Therefore, commercial amounts of high sulphur oil can generate at shallow stratigraphic sections in the Lajjun sub-basin, Western Central Jordan. However, the same paleoenvironmental condition found throughout Lajjun sub-basin suggested equivalency of kerogen structure throughout the Lajjun sub-basin32,35,37. As one goes through the Lajjun sub-basin, the Muwaqqar Chalk Marl Formation reached few meters thick in the outcrop parts and burial depths between 117 and 150 m, with 25–60 m thick in the boreholes35. This indicates that the organic rich sedimentary rocks intervals of the MCM Formation in different portions likely achieved low level of maturity due to the shallow burial temperatures, and can be potential for future development in the retorting of such deposits to extract considerable quantities of sulfur-rich oil using different heating methods such as hydrothermal.

Conclusions

Comprehensive unconventional geochemical methods together with kerogen microscopy of the isolated kerogen from ten oil shale samples of the Late Cretaceous Muwaqqar (MCM) Formation taken from the Lajjun outcrop in the Lajjun Sub-basin, Western Central Jordan were conducted. The following significant inferences were drawn concerning the molecular structure, composition and characteristics of the kerogen and implications on its potential for unconventional oil exploration. The results are drawn the following major conclusion:

  1. 1-

    The studied oil shale samples contain high abundance of reactive liptinites, mainly algae and fluorescence AOM with minor amounts of spore, indicating kerogen enriched in hydrogen for strong oil generation potential.

  2. 2-

    According to ultimate elemental and FTIR results, the isolated kerogen from the studied oil shale samples is mainly Type II kerogen, with high hydrogen/carbon atomic ratio of more than 1.30 and higher A-factor of aliphatic stretching CH2 than C-factor of aromatic C–H stretching.

  3. 3-

    The pyrolysate distributions in Py–GC show that the molecular structure of isolated kerogen from the studied oil shales is characterized by a majorly organic sulphur (i.e., 2,3-dimethylthiophene) compound, along with minor amounts of C9 n-alkane and o-xylene compound, suggesting mainly high-sulphur Type II kerogen for potential of paraffinic oils with lower amount of wax.

  4. 4-

    Bulk kinetic analysis indicates that the kerogen Type II-S can be converted into high-sulphur oil over low geological temperatures range from 100 to 106 °C, agreeing to low Ro values in the range of 0.55–0.60%.

  5. 5-

    The highlighted results in this work suggest that substantial unconventional oil exploration operations from oil shale can be endorsed in the shallow deposits of the MCM Formation in the Lajjun Sub-basin, Western Central Jordan.