Hydrocarbon generation potential evaluation via petrographic and geochemical analyses of El-Maghara coal in Sinai, Egypt

The energy demand increased dramatically owing to the evolution of industrial and domestic requirements and the associated decrease in oil and gas resources. This study aims to evaluate El-Maghara coal (with about 52 MT reserve) as a potential hydrocarbon source. The collect samples were subjected to petrographic, chemical analyses and Rock–Eval pyrolysis to investigate the detailed characteristics of this coal. Chemically, this coal is high volatile bituminous coal with high H and S content. The high H/C ratio indicates the high extraction yield of coal. The main maceral group in the studied samples is vitrinite (62.8%) followed by liptinite (31.3%) and inertinite (5.8%). The content of liptinite indicates the capability of this coal for petroleum production. Based on Rock–Eval Pyrolysis results and TOC content, the coal has excellent petroleum potential. The hydrogen index (HI) and H/C atomic ratio indicate the II kerogen type (oil prone) of this coal. This coal has Tmax and vitrinite reflectance values around 415.8 °C and 0.37%, respectively, indicating the immature stage of kerogen. The high reactive maceral content (94.2%), oil-yield (65.5%) and conversion from coal to oil (95.4%), indicated that this coal has a hydrocarbon generation potential for oil.


The study area
El-Maghara area is located approximately in the center of North Sinai, about 200 km northeast of Cairo, between longitudes 33°10' and 33°40' E, and latitudes 30°35' and 30°50' N. It is a rectangular sedimentary basin with an area of about 1300 km 2 (Fig. 1a).The region occupies great scientific and economic importance because it contains the ideal Jurassic section in Egypt as well as economic coal deposits.Therefore, it has been subjected to many geological studies (e.g.Refs. 31,32,34,35).Al-Far 31 divided the Jurassic sediments in the region into six formations (Fig. 1b) and described them in detail.Safa Formation has particular importance because it contains the economic coal layer, the geological reserve of coal is estimated at 52 million tons 32 .
El-Maghara coal mine has faced many problems since its establishment in 1964.The mine was closed due to the 1967 war for 15 years, which led to the destruction and collapse of the mine facilities.In 1982, mine rehabilitation operations began with the help of the British company, Babcock, but the mine was closed again in 2005 due to some technical and financial problems.One of the reasons that led to the closure of the mine is that coal can't be coked except by mixing it with a higher grade of coal, and that Egypt does not contain coal-fired power stations 37 .However, due to the problem of energy shortage facing many countries in the world, the use of El-Maghara coal to generate energy or liquefying it to obtain oil and gas using modern technologies should be reconsidered.

Physico-chemical analyses
The proximate and ultimate analyses were performed according to ASTM procedures in the Egyptian Mineral Resources Authority; Moisture 38 , volatile matter 39 , ash 40 , calorific value 41 , total sulfur 42 , CHN 43 , fixed carbon and oxygen were calculated by difference.The concentrations of measured proximate and ultimate analyses were calculated based on instructions in ASTM 44 and Suggate 45 .
Rock-Eval pyrolysis was used to measure free hydrocarbons (S1 = mg HC/g rock), residual hydrocarbon generating potential (S2 = mg HC/g rock), free CO 2 (S3 = mg HC/g rock) and temperature during maximum generation of hydrocarbons at S2 (T max °C) of the coal sample.LECO SC632 was used for TOC at Stratochem service lab, Egypt.These parameters were used to calculate the hydrogen index (HI), oxygen index (OI), potential yield (PY) and production index (PI) 46 .
The figures were processed with Adobe Illustrator CS5 software for re-drawing and enhancement 47 .In the present study, organic petrography is used to explore the maceral content and thermal maturity in 6 samples that were collected from the coal seam (Table 1).The analysis is conducted in the whole rock samples that were consolidated in epoxy resin and polished according to the procedures indicated in the ASTM 48 .The maceral composition is quantified by calculating the area percentages of each type from the photomicrographs by Image J software in incident light including white and fluorescence modes.The thermal maturity is measured using the procedures of ASTM 49 to calculate the vitrinite reflectance (R o %).The maceral content, especially reactive macerals (RM), can be used to determine oil yield and conversion of coal into hydrocarbon 50,51 using the following formulas 52 .

Proximate and ultimate analyses
The proximate analysis result of El-Maghara coal ( www.nature.com/scientificreports/during the sedimentation process due to geological factors such as the rate of sedimentation, changing water levels, and tectonic processes 53 .Edress et al. 19 pointed out the sea level fluctuations and associated water table changes during coal deposition in El-Maghara.Also, the microscopic composition of the studied coal assesses in the preservation of mineral mater because vitrain is characterized by cleats as well as micro-fractures and pores, which can trap mineral matter 54 . The results of the ultimate analysis, on a dry, ash-free basis, of El-Maghara coal are listed in Table (1).The studied coal samples have high C (68.82%), H (7.01%), total S (4.92%) and N (1%).The high S content of this coal is related to its depositional environment, where this coal was deposited in marine anoxic conditions 19 .This is confirmed by the recorded sulfur minerals; pyrite and copiapite 33 .The coal content of H controls the physical properties of OM; the transform of OM from solid to liquid to gas increases with increasing H content 23 .The H content has a positive correlation with Rock-Eval S1 (r = 0.68) and S2 (r = 0.83).The atomic ratio of H/C was about 1.22 and O/C was 0.07 in the El-Maghara coal (Table 1).This relatively high H/C ratio may be resulted from the abundance of H-rich macerals; derovitrinite and liptinite.The high H/C ratio is good indicative of the high extraction yield of coal 55 .These atomic ratios, H/C and O/C, were used by Van Krevelen 56 to determine kerogen type and hydrocarbon generation potential.The studied coal samples were plotted between Types I and II kerogens on the Van Krevelen diagram (Fig. 2a), indicating the richness of these samples with H (perhydrous coal).The H/C atomic ratio can be used for kerogen type discrimination; H/C > 1.4 indicates type I (oil-prone), 1 < H/C < 1.4 indicates type II (oil-prone), 0.4 < H/C < 1 indicates type III (natural gas-prone) and H/C < 0.4 indicates type IV (barren) 57 .Accordingly, the studied coal samples, with H/C = 1.22, are of type II kerogen and have the potential for petroleum production.Also, the atomic H/C-O/C plot (Fig. 2b) 58 can be applied to determine the studied coal rank; the studied coal samples have bituminous rank.
The coal content of liptinite is in forward proportion with the H%, HI and PY (Fig. 4), because liptinite macerals are H-rich and contain more aliphatic compounds than vitrinite and hence more oil-prone 60 .The presence of more than 12% liptinite is responsible for the recorded high HI (> 350 mg HC/g TOC) (Table 1) 61 .

Coal rank
Coal rank refers to the coal organic matter metamorphism (coalification), or coal maturity and measure of the degree of coal evolution from peat to meta-anthracite 62 .High rank coal has high R o , C and C/H ratio, and low VM, and vice versa 3 .So, coal rank can't be determined through a single parameter but by using many physical and chemical parameters; R o , moisture, calorific value, volatile matter and fixed carbon.The plotting of these parameters on the ECE-UN 63 and ASTM 44 systems (Fig. 5a,b) indicated medium to high volatile bituminous coal 44 which is correspondence to medium rank bituminous coal 63 .This is confirmed by the plot of calorific valuevolatile matter (Fig. 5c) of Suggate 45 .Accordingly, this coal is suitable for producing coal bed methane 53,[64][65][66] .The fuel ratio (FR) of the studied coals was around 0.99, placing the samples at the bituminous rank based on Frazer's 67 classification.These results are in agreement with Edress and Abdel-Fatah 68 .
The positive correlation of TOC (r = 0.69) with the fixed carbon (Fig. 6d) infers that both of them play an essential role in determining how well coal can be used as a potential source rock.As known, high rank coal has high fixed carbon and somewhat high TOC.On the contrary, the inverse correlation between TOC (r = − 0.68) and ash (Fig. 6d), indicates that the increase of mineral matter during the coal deposition process implicitly decreases the availability of organic carbon and thus negatively affects the ability of coal to produce hydrocarbons.

Thermal maturity
The thermal maturity of kerogen plays an essential role in determining the ability of source rocks to generate hydrocarbons as well as the types of hydrocarbons (gas or oil).The thermal maturity of source rocks is determined by several maturity indicators, such as T max values, production index (PI), and vitrinite reflectance (R o ) [71][72][73] .The T max values in El-Maghara coal samples ranged from 412 to 421 °C, which indicates the immaturity of kerogen, as identified from Fig. 7.This is confirmed by the low values of R o , which were less than 0.6, as well as the low production index values, which ranged between 0.02 and 0.03.

Hydrocarbon generation potentiality
The hydrocarbon generation ability of kerogen is assessed in this study from the petrographic composition, kerogen type, thermal maturity, genetic potential, HI values, and TOC content.The oil generation potential mainly depends on the liptinite content in the source rock; petroleum production requires > 15% liptinite 23,25,74 .The studied coal samples have liptinite content of around 31.3%, indicating the capability of this coal for petroleum production.Coal liquefaction potentiality is controlled by its content of reactive macerals (vitrinite and liptinite) 14,75 .Coal with Ro < 0.8, reactive macerals > 60% and volatile component (daf) > 35% is suitable for liquefaction and gasification 76,77 .The studied samples composed mainly of reactive macerals, varying from 90 to 98 wt % (Table 1), indicating their suitability for hydrocarbon generation.In addition, the calculated conversion (94.6-96.2%) to oil and oil yield (64.6-66.4%)(Table 1), illustrates the liquid hydrocarbon generation potential of this coal 78 .
The ternary plot of maceral composition was applied to deduce the kerogen and hydrocarbon type.The samples are mainly of Vitrinite (62.8%) followed by liptinite (31.3%), so the samples contain type III kerogen (Fig. 8a) and have excellent probability for hydrocarbon generation; oil and gas (Fig. 8b).
The hydrocarbon potential of coal depends mainly on the amount and type of organic matter and its thermal maturity 80,81 .The type of kerogen depends on the source of the organic matter, which largely controls the possibility of generating hydrocarbons and its type; Type I and II come from algae and are considered oil-prone, while type III comes from higher plants and is considered gas-prone 57 .Since the organic matter is mainly composed of C, H and O, the kerogen type in coal can be determined using the hydrogen index (HI) 82 .
El-Maghara coal recorded values ranging from 343.39 to 397.83 mg.HC/g.TOC For HI, which shows that the kerogen in those samples is type II, as illustrated in Fig. 8c.This is confirmed by the S2-TOC (Fig. 8d), which indicates that these samples can produce oil.The high content of coal from hydrogen-rich macerals (detrovitrinite and liptinite) also indicated the ability of this coal to produce oil 83 .
The PY values of El-Maghara coal samples were 211.9-273.2mg HC/g rock, indicating their excellent hydrocarbon generation efficiency based on Hunt's scale 84 , which is confirmed by the PY-TOC diagram (Fig. 8e).The relationship HI-TOC (Fig. 8f) also indicated that El-Maghara coal samples represent a source of gaseous hydrocarbons in addition to oil.The high H content of the studied coal (H > 5), H/C ratio > 0.9 and liptinite content > 15%, indicate its great capability to generate oil and gas 23 .The current results support the importance of using petrographic, proximate and ultimate analyses, inside Rock-Eval pyrolysis results, as concluded by Karayigit et al. 30 , for a more accurate interpretation of source rock hydrocarbon generation.

Comparison of the current results with others
The results of the studied coal samples were compared to the previous studies on El-Maghara coal (Table 2), and agreement was observed between the current and previous results, which indicates the consistency of the coal layer in the mine and the accuracy of the studies and analyses.It was noted that previous studies didn't use the Rock-Eval pyrolysis technique to determine the amount of organic matter and the extent of the possibility of producing hydrocarbons from this coal.

Figure 2 .
Figure 2. (a) Van Krevelen diagram showing data for the samples from El-Maghara, (b) Plot of H/C versus O/C for rank determination (after 58 ), blue dashed lines for oil yield after Saxby 59 .

Figure 3 .
Figure 3. (a) Sporinite in fluorescence light, sample 1 (b) Exsudatinite in fluorescence light, sample 2 (c,d) Resinite in fluorescence light, sample 3 (e-g) Cutinite of different origin and structure in Vitrinite matrix, (e) and (f) in fluorescence light, g in white light, sample 3 (h) Vitrinite in cell structure in whit light, sample 3.

Table 1 .
Coal measured and calculated data of petrography, proximate, ultimate and Rock-Eval.