Younes Jalilian, Mohammad Hossein Saberi

Faculty of New Science and Technology, Department of Petroleum Engineering, Semnan University, Semnan 35131-19111, Iran

Keywords:

Kinetic parameters

Rock-Eval pyrolysis

Hydrocarbon potential

Source rock

Sarchahan Formation

Faraghun Formation

Zagros Basin

Persian Gulf

Iran

A B S T R A C T

Source rock assessment is a key step in any petroleum exploration activity. The results of Rock-Eval analysis showed that Sarchahan Formation was in the late oil window, while the Faraghun and Zakeen Formations were just in the early stages of the oil window. Furthermore, Sarchahan, Zakeen and Faraghun Formations exhibited different kerogen types (types-Ⅱ, types-Ⅲ and type-Ⅲ, respectively). Refining the kinetic parameters using the OPTKIN software, the error function returned error values below 0.1,indicating accurate optimization of the kinetic parameters. Based on the obtained values of activation energy, it was clear that Sarchahan Formation contained type-Ⅱ kerogen with an activation energy of 48-52 kcal/mol, while Zakeen and Faraghun Formations contained type-III kerogen with activation energies of 70-80 kcal/mol and 44-56 kcal/mol, respectively. The geographical distribution of the samples studied in this work, it was found that the organic matter (OM) quantity and quality increased as one moved toward the Coastal Fars in Sarchahan Formation. The same trend was observed as one moved from the southern coasts of Iran toward the shaly and coaly portions of Faraghun Formation in the center of the Persian Gulf.

1. Introduction

Identification and evaluation of source rock are very important, particularly in early exploration stages. This can be considered when several types of source rock (rather than one) are identified within the area in question. Petroleum geochemistry enhances the exploration efficiency by determining a number of parameters controlling the oil accumulations. These include the source rock quality, organic matter (OM) richness, thermal maturity, and histories of hydrocarbon (HC) generation, migration, and accumulation in the reservoir formation. Some three-fourths of the oil and two-thirds of the gas in the world have been already explored.In the past decades, Paleozoic deposits in the Arabian Plate and the Persian Gulf have been greatly regarded by petroleum geologists, because the majority of the oil and gas formed within this region has been sourced from the Paleozoic deposits. According to investigations performed in the Persian Gulf and adjacent areas, it has been suggested that, in terms of maturity, the Paleozoic rocks exhibit an increasing trend as one moves toward Iran, even though the statement is still associated with some uncertainty due to the deficiency of data on the Paleozoic deposits in the southern and southwestern Iran and the Persian Gulf Basin.

The presence of massive bodies of mature OM-rich rocks in the Persian Gulf has brought about numerous producing reservoirs in the surrounding countries. The Persian Gulf Basin is one of the world’s most fruitful oil zones hosting the majority of the world’s oil reserves (some 715×109barrels,i.e. 57%, as of writing this article) along with almost half of the world’s proven natural gas reserves (462.2 TCF, i.e. 45%).The use of Rock-Eval device in the evaluation of source rock has a long history. This device continues to be used as a reliable tool in the discussion of the source rock. Pierre C et al. (2020) used Rock-Eval as an online system to investigate the hydrocarbon generation potential of oil shales. Fadiya S et al. (2020) examined the potential of the Miocene age source rock in the Niger Delta using the total carbon content (TOC)indexFadiya S et al., 2020. Chen JQ et al. (2021) evaluated the hydrocarbon generation potential of carbonate formations of the Ordovician period in the Tarim basins by using Rock-Eval parameters. Feng YW et al. (2020) in the North Carnarvon sedimentary basin which is one of the gas exploration areas, used the Rock-Eval data to study the source rocks. The present study is aimed at determining kinetic parameters, OM maturity, HC generation potential, and generated amount of HC for probable source rocks across the Coastal Fars and the Persian Gulf Areas.

Saberi MH et al. (2016) studied paleo-logs of the wells drilled into Paleozoic sediments and showed that Sarchahan Formation is the most probable source rock across the Paleozoic deposits. In addition, the presence of some coal interbeddings in Faraghun Formation supported the idea of some HC potential for this layer, which requires further research and consideration. As such, in order to investigate the HC generation in the overlying sediments, a total of 266 downhole and surface samples Faraghun and Sarchahan Formations were selected and subjected to pyrolysis analysis(using Rock-Eval 6) and kinetic parameters calculation for chemical characterization. Some previous studies have referred to Sarchahan Formation as the only source rock in the Coastal Far and Persian Gulf Areas. Another method to determine the kerogen characteristics of source rocks and confirm the results obtained from Rock-Eval analysis is combining isotope results (13C and15N) with biomarker analysis results in oil and gas samples, which in this study due to the lack of this type of analysis on the samples cannot be interpreted accordingly (Abiodun B et al., 2020; Koralay D,2021). One of the methods of analyzing the source rocks and determining its spread in a sedimentary basin is the combination of Rock-Eval data with seismic sections. In 2019, Chen JW (2019) analyzed the spread of source and reservoir rocks using seismic sections in the South Yellow Sea.

In their paper, Afshari N et al. (2007) discussed gasgeneration potentials of Faraghun, Sarchahan, and Siahoo Formations. They used Rock-Eval analysis results and suggested that (1) Faraghun Formation at the Kuh-e-Siah 1 Well exhibits HC generation potential, (2) Sarchahan Formation has no HC generation potential, and (3) Siahoo Formation exhibits little HC generation potential. Mirshahani M et al. (2016) undertook a study on the maturity of Sarchahan Formation at Faraghun and Gahkam Mounts. They showed that the samples taken from Sarchahan Formation were in the mid oil window. Vaezian A et al. (2013) published a paper where they investigated geochemical characteristics of source rocks in Salman Field. Using Rock-Eval analysis results, namely TOC and maximum temperature (Tmax), they proved that (1) Kazhdomi, Darian, and Gadvan Formations were of less importance when it came to high-potential source rocks across the field, and (2) TOC, maturity, and HC generation potential of Zakeen Formation highlighted this formation as the most probable source rock across the region.Vaezian A et al. (2013) used Rock-Eval pyrolysis data to study characteristics of potential source rocks across Alborz Region. In this paper, combining kinetic calculation results with the Rock-Eval pyrolysis analysis data, Sarchahan and Faraghun Formations were investigated in further detail.Accordingly, it was demonstrated that, not only Sarchahan Formation, but also Faraghun Formation enjoy very good HC generation potential and thus can serve as source for the regional fields.

2. Geological setting

On the world’s tectonic map, the Iran Plateau represents a small triangular plate delimited by two thrusting faults,namely Alborz to the north and Zagros to the southwest. From the northwest to the south, Zagros Basin has been divided into the following structural divisions: (1) Thrusted or high Zagros, and (2) folded Zagros (Fig. 1). The folded Zagros has been further divided into smaller pieces (Lorestan Area, Izeh Relief, Defuel Embayment, Abadan Plain, Persian Gulf, Fars Relief, and Bandar-Abbas hinterland) (Fadiya S et al., 2020).

Fig. 1. Divisions of Zagros Basin from the northwest to the south.

One of the methods of studying the sedimentary basin and the possibility of hydrocarbon generation by a formation is the use of fossil studies in this formation (MERİÇ E et al., 2020).During the Paleozoic era, studies in the Fars province has shown that the Sarchahan Formation has the most probable conditions for the source rocks. The Fars Relief has been developed between Kazeroon and Nakhiloo Faults, with its northern and southern boundaries being the thrust zone and coastal line of the Persian Gulf, respectively (Fig. 1). In terms of geology, the Fars Relief can be divided into smaller divisions including inner Fars, coastal Fars, and semi-coastal Fars. The Fars Region is well known for its gas reserves hosted by anticlines of various orientations (E-W, NW-SE,and even NE-SW). The variable trends of the HC-bearing structures in the Fars Region has been a result of either fault activity in the bed rock or changes in the direction of motion of the Arabian Plate with respect to the Iranian Plate. The regional folding pattern is mostly concentric with a bottom detachment surface in Hormoz salts and minor intra-formation detachment surfaces in Dashtak evaporates and Pabdeh and Gurpi shales. In total, some 166 TCF of gas-in-place and 3.5×109barrels of oil-in-place have been explored across the region, details of which are presented in Table 2.1.

Table 1. Numbers of the samples used for Rock-Eval pyrolysis analysis.

Table 2. Results of the Rock-Eval pyrolysis analysis on the cuttings taken from Faraghun Formation.

The Persian Gulf Basin is limited by the Abadan Plain and southern part of the Dezful Embayment to the north, the Fars Relief to the northeast, the Arabian Peninsula to the south,and the Oman Highs to the east, extended beneath an area with a width ranging from 75 km (at Strait of Hormoz) to 360 km(Fig. 1). In general, the Persian Gulf structures follow the trend of their Zagros counterparts, namely the NW-SE. From a stratigraphic point of view, the Persian Gulf has many things in common with the Zagros fold and thrust belt (FTB). On this basis, the Persian Gulf can be seen as a part of the FTB.However, their similarities become rare when it comes to structural geology. Based on differences in the stratigraphy and salt dome extension, one can divide the Persian Gulf into two parts, namely northwestern and western Qatar Arches.The sedimentary facies across the Persian Gulf follow either of three fault systems developed along (1) N-S, (2) NW-SE,and (3) NE-SW trends (Chen JQ et al., 2021).

Besides these three fault systems in the Persian Gulf, the genetic tectonics of the Precambrian salts is relatively young and has affected the Qatar and Fars Reliefs on both ends.Known to develop current form of the area in question, the Middle Miocene tectonic movements have further contributed to the formation of salt domes and some distortions in the anticlinal structures within the Persian Gulf Basin.

All across the Persian Gulf, some 95×109barrels of oil,25×109barrels of condensates, and 760 TCF of gas have been already explored (solution gas + gas cap + independent reserves). As of current, some 100 anticlines and pseudoanticlines have been identified across the region, of which 72 anticlines have been drilled. Accordingly, 22, 11, and 39 anticlines have been found to contain oil and gas, gas, and non-economic amounts of HC (i.e. dry), respectively.

3. Stratigraphy of the region

Due to limited and isolated outcrops of the Paleozoic deposits in the region and failure to touch the respective sequences at the studied wells, the Paleozoic petroleum system in Iran is difficult to describe in terms of stratigraphy,making the system a complex one including long oil migrations from the Later Ordovician to the Early Silurian(Mary M and Kuhnel C, 1980). During the Paleozoic, as a stable platform, the land of Iran has been covered by a shallow marine environment, which has been altered to terrestrial area occasionally upon upward movement of the earth crust and regression of the sea. In the meantime, the Caledonian and Hercynian orogeny events have imposed insignificant influences on the Paleozoic platform of Iran(Bordenave M and Hegre J, 2010).

The stratigraphic column in the Zagros Basin is made up of more than 36 formations from the Cambrian to the present.For most part, these are carbonate rocks along with evaporates and clastic rocks (Fig. 2). The studied area has been dominated by lagoon-evaporative deposition environments since the Precambrian to the Early Cambrian. The Cambrian deposits begin with deposit accumulations and continue to shale - red sandstone complexes, indicating a very shallow oxidative environment (Rabbani A, 2007).

Fig. 2. Stratigraphic column of Zagros Basin within the age interval of interest (after Pierre C et al., 2020).

In the Ordovician, the seas have progressed, however, due to the erosional phase of the Caledonian and glacier era of the Hernantian. These rocks exhibit rare outcrops and have developed into four rock units, namely Ilbik, Zard-Kuh,Siahoo, and Dargaz. During the late Ordovician to early Silurian, considering the global extension of glaciers and continuation of the Caledonian event, the land extension increased and the seas were mainly shallow in Iran. Sediments of this age are thus extended over Gahkam and Faraghun Mountains, northern Bandar-Abbas, developing Sarchahan Formation (graptolite-bearing shales of Silurian). Sarchahan Formation (main source rock of Paleozoic era) is 102 m thick at its type section located 120 km to the north of Bandar-Abbas, with its sediments beginning with conglomeratic sandstone, sandy lime, and limestone and further formed dark grey and green, laminated shales containing sandstone alterations with rare amounts of brachiopods-bearing sandy limestone.

During the Late Ordovician to Early Silurian, a strong eustatic sea level rise occurred that flooded wide areas,triggering the deposition of black, organic-rich shales in many places in Iran, North Africa and the Arabian Peninsula(Lewan M and Ruble T, 2002). The Sarchahan Formation represents a good hydrocarbon source rock and has generated large amounts of oil and gas for various Paleozoic and Triassic units. the Sarchahan Formation can be divided into two main zones, a poor organic matter zone with low gammaray response and a rich organic matter zone with high gammaray response, each with their own organic geochemical signatures. In the Arabian Peninsula, the poor organic matter and low gamma-ray zone has almost no source potential and is called the Sharawra Member of the Qalibah Formation(Mangotra S et al., 1995). The rich organic matter and high gamma-ray zone consists of dark gray to black, oil-prone,marine shales and is called the Qusaiba Member of the Qalibah Formation. This member contains abundant amorphous organic matter, marine algae (Saberi M et al., 2016).

During the Devonian, as a result of the Caledonian drought over a major portion of Iran, clastic rocks and carbonate rocks were developed in the Early and Middle-Late Devonian, respectively, together constituting the Zakeen Formation.

In the Early-Late Carboniferous, upon the start of the tectonic motions (equivalent to the Hercynian), the extended areas of Iran were exposed and erosional phase dominated across Zagros Basin for millions of years (Karimi A et al., 2016).

In Iran, the Permian deposits are well extended, indicating significant progression of the sea in this era and the several million-year sedimentary hiatus separating the Upper Devonian clastic sequences (Zakeen Formation) from the Early Permian horizons (Faraghun Formation). Type section of the Faraghun Formation has been defined at Faraghun Mount, 80 km to the north of Bandar-Abbas. The formation has a thickness ranging from 53 m at Faraghun Mount to 500 m at Chalisheh. In terms of lithology, it is composed of clastic sediments including conglomerate, sandstone, siltstone, and coal-bearing shales along with small layers of lime (in the upper part). This formation overlays white sandstones of Zakeen Formation along an erosional unconformity and rather underlies the Dalan limestones homoclinically (Moattari M et al., 2012).

Dalan Formation exhibits the carbonate facies of the Permian sequences found in the upper Zagros. With a thickness of 748 m, type section of the formation has been defined at well Kuh-e-Sefid 1. At its base, the Dalan Formation alters, gradually yet continuously, to the clastic Faraghun Formation. Faraghun Formation is made up of three parts: lower carbonates, Nar evaporates, and upper carbonates(Mary M and Kuhnel C, 1980).

In the Triassic, depths of the sea were not identical across Zagros Basin. Accordingly, high Zagros has experienced the deepest marine environment while folded Zagros and the Persian Gulf have witnessed much shallower depths(evaporative conditions). The decreased sea depth during the Middle Triassic along with the accumulation of gypsum and lack of Upper Triassic rocks could indicate the action of Early Cimmerian events and detachment of Zagros from the Central Iran. Sediments of this stage include Kangan Formation at the base and Dashtak Formation on the top, which exposures at folded Zagros and the Persian Gulf, respectively, with Khanekat Formation outcropped at high Zagros.

The Kangan Formation is 178 m in thickness with its type section defined at Well Kuh-e-Siah 1. This formation can be characterized by three distinctive facies: (1) Clean carbonates,(2) clayey and shaly layers at the base, and (3) evaporative carbonates.

Being 814 m thick, the Dashtak Formation has been type sectioned at Well Kuh-e-Siah 1 and is known to be composed of two parts: (1) Aghar shales, and (2) Sefidar dolomites.

4. Materials and method

Numerous methods have been proposed for HC source rock characterization by OM maturity assessment, HC generation timing analysis, and extension studies. Among the conventional methods for this purpose, one may refer to“Rock-Eval pyrolysis”, which was introduced by Espitalie in 1977, where a rock sample is analyzed by heating in absence of oxygen (Chen JQ et al., 2021).

In the present work, two approaches were followed for determining the geochemical characteristics of samples from Faraghun and Sarchahan Formations, namely Rock-Eval pyrolysis and kinetic analysis. The Rock-Eval pyrolysis was performed on 100 mg powdered samples for determining their HC generation potential, OM maturity, and type of the OM by having them heated at 25 bonateusing a standard Rock-Eval 6 utilizing the OPTKIN Software. The Rock-Eval instrument was calibrated using 50 mg of the sample prepared according to IFP 160000. Once finished with calibrating the instrument,the sample was placed in the pyrolysis oven where it was heated in absence of oxygen under an atmosphere of nitrogen,with the emitted gas recorded at a flame ionized detector(FID). Next, the sample was moved to the oxidant oven where it was burnt in presence of oxygen and the emitted gas was recorded by an infrared cell (IR Cell) detector (Behar F et al.,2001). The values recorded at the detectors were presented in terms of S3peaks:

S1: About 300℃, representing the existing OM content of the rock sample;

S2: 300℃-650℃, representing the residual potential of the rock sample;

S3: 300℃-390℃, representing the CO2(mineral carbon)content of the rock sample;

S4: 300℃-600℃, representing organic CO2content of the rock sample;

S5: 600℃-850℃, representing mineral CO2content of the rock sample (Chen JQ et al., 2021).

4.1. Rock-Eval pyrolysis results

Drill cuttings of Sarchahan (Ordovician, Silurian) and Faraghun Formations (Permian) in the Salman Oilfield and Zireh, Assaluyeh, Golshan, Nar, and Homa Gasfields as well as the Darang and Kuh-e-Siah Fields and also the surfacetaken samples of these formations at the Faraghun Mount were analyzed in this study. The location and details of the samples are presented in Table 1, with a summary of results detailed in Tables 2-4.

Table 3. Results of the Rock-Eval pyrolysis analysis on the cuttings taken from Sarchahan Formation.

Table 4. Results of the Rock-Eval pyrolysis analysis on the surface samples of Faraghun and Sarchahan Formations taken at Faraghun Mount.

4.2. Kinetic theory

Optkin is based on the kinetic model of Tissot B and Espitalié I (1975) which predicts the amount of hydrocarbon(oil and gas) generated by primary cracking of a kerogen when temperature increases through time. The degradation of kerogen into hydrocarbons is described by a series of n parallel chemical reacts.

Each chemical reaction i obeys a first-order kinetics which is characterized by the Arrhenius Law.

where:

dXi/dt= hydrocarbon generation rate (S2 signal);

t= time (sec);

Xi= residual petroleum potential of the organic matter involved in reaction i (mg/g Total Organic Carbon);

ki= reaction rate parameter depending on absolute temperature T by the law (ki=A(-Ei/RT));

A= Arrhenius constant (or pre-exponential factor)depending on the type of organic matter (sec-1);

Ei= Activation energy related to reaction i (kcal/mole);

R= molar gas constant (R= 0.00199 kcal/mole);

T= absolute temperature (degrees K);

So the complete Arrhenius Law for each chemical reaction i may be written:

The total initial petroleum potential of the kerogen[Hydrogen Index (HI) in mg HC/g TOC] is expressed by:

whereXio= initial petroleum potential of the organic matter involved in reaction i (mg/g TOC)

= value ofXiat t = 0

The amount Q of generated hydrocarbons (in mg HC/g TOC) is expressed by:

So each chemical reaction may be characterized by:

the Arrhenius constantA;

the activation energyEi;

the initial petroleum potentialXio;

Therefore, a given kerogen is characterized by the Arrhenius constantA, the number n of chemical reactions and the distribution of the activation energies (i.e. the relative abundance of each petroleumXio, corresponding to each activation energyEi). This distribution is directly related to the chemical composition of the kerogen (Fig. 3).

Fig. 3. The distribution of activation energy of different type of kerogen.

The distribution is extremely narrow in type I Kerogen(Green River Shales, Utah) made of cross-linked aliphatic chains where only C-C bonds are broken. It is wider in the type II Kerogen (Paris Basin) due to a greater variety of chemical bonds, and the distribution of type III (coals,Mahakam delta) is much smaller and dissymetric (Sweeney JJ et al., 1987).

The aim of optimization is to determine the best fit for the activation energy distribution (A, Ei, Xio).

So the activation energies Ei are regularly spaced with an interval of 1 kcal/mole or 2 kcal/mole in a given range(40 kcal/mole to 80 kcal/mole, for example). The optimization program uses various random values (A,Xio) in order to minimize an error function: this error function is obtained by summation of the quadratic differences between the computed and observed data. The optimization, which is nonlinear, is carried out by a gradient method with variable steps and allows to obtain the best fit for (A,Ei,Xio).

First case of optimization:

Only pyrolysis curves of immature samples

The error function is obtained by summation of the quadratic differences between computed and observed rates of release of hydrocarbons during pyrolysis (S2 peak).

Second case of optimization:

Pyrolysis curves of immature samples residual petroleum potential of mature samples (sedimentary natural series or artificially matured samples in autoclave).

The error function is obtained by summation of the quadratic difference between:

-computed and observed rates of release of hydrocarbons during pyrolysis (S2 peak);

-computed and observed residual petroleum potentials of the mature samples;

The kinetics refers to the study of the rate of chemical processes and the factors affecting the rate. In the field of petroleum engineering, the HC generation can be estimated by evaluating kinetic parameters of the source rock (Ghazban F,2007). These parameters can then be used in a basin modeling software to predict the produced amount of HC as a function of time and temperature. The rate of HC production from kerogen is determined by the kerogen structure and affects the gross kinetic parameters (the activation energy and the frequency coefficient) (Konert G et al., 2001).

Given the various sources of input, sedimentary environment and geological evolution, it is important to estimate the history of HC production from the kerogen before the kinetic parameters can be evaluated (Hunt J, 1996).The kinetic parameters of oil production are usually determined experimentally and incorporated, using Arrhenius equation, into the thermal-burial oil production history to specify the timing and extension of oil production from a particular source rock within a petroleum system (Espitalié J et al., 1985).where the time and temperature values are determined according to the thermal-burial history and theEa(activation energy) andA0(frequency coefficient) are determined by experimentation on the OM content of the source rock.Depending on the bonding energy, the activation energy is the minimum energy required to turn a composite into the product. Frequency coefficient is a physical quantity corresponding to collision frequency. According to these definitions, the higher the activation energy, the more energy is required for obtaining the final product, rising the process temperature to higher levels, which, in turn, accelerates the particle motion and hence the frequency coefficient (Ghazban F, 2007).

The Rockint software was used to interpret the raw data and prepare the input for OPTKIN Software - an optimization program based on kinetic model - for estimating the produced amount of HC upon initial cracking of the kerogen with increasing the temperature over time. The aim of optimization was to determine the optimum activation energy distribution based on the results of several rock-Eval pyrolysis on the samples of mature or immature source rocks. Determined by the Arrhenius equation (frequency coefficient) and the activation energy distribution, the kerogen distribution was found to be directly related to the chemical composition of the kerogen (Rabbani A, 2008).

According to kinetic model proposed by Spitaley and Tissot:

(i). Extremely narrow distribution of kinetic parameters characterizes the type-I kerogen;

(ii). Broad distribution of kinetic parameters refers to type-II kerogen;

(iii). Limited and asymmetric distribution of kinetic parameters is related to type-III kerogen (Espitalié J et al.,1985).

Herein, the optimization is a nonlinear function that gives a good, weak, or unideal solution if the value of the error function falls below 1, ranges between 1 and 10, or exceeds 10, respectively (Espitalié J et al., 1985).

The OPTKIN software uses hydrocarbon production data at different temperature (at least three temperatures) using the non-linear optimization method to solve the Arinus equation based on two sample temperatures and the results are evaluated with third point information and determine the amount of solution error (Pierre C et al., 2020).

In this study, using the Rock-Eval system, 100 mg samples of six source rocks from Faraghun and Sarchahan Formations were investigated at three temperature rates: 5℃/min, 15℃/min, 25℃/min. The optimization was then performed to obtain kinetic parameters (activation energy and frequency coefficient). Similar studies were performed on two Ordovician and one Silurian surface samples taken from Sarchahan Formation at Faraghun Mount, and also on four samples of cuttings from Faraghun Formation at the Salman Well (Fig. 4).

Fig. 4. Location map the studied fields and the sampling points at Faraghun Mount.

Based on the results of the Rock-Eval pyrolysis, the following criteria were applied for choosing the eight samples:

(i). High TOC, indicating the HC generation potential of the sample;

(ii). Low maximum temperature at which the sample remained immature and started getting matured artificially by the system;

(iii). High S2peak, indicating the HC generation capability of the sample; and

(iv). Low oxygen index (OI) to opt for non-oxidized samples.

Table 5 presents details of the kinetic analysis on these samples.

Table 5. The related analysis data to the selected samples for Optkin method.

5. Discussion

5.1. The Rock-Eval pyrolysis analysis

The Van Krevelen diagram was used to obtain the type of OM. The samples were firstly investigated in terms of residual HCs and OM richness followed by removing the nonresidual HC and omitting the OM-poor samples to evaluate the HC generation potential of the Sarchahan and Faraghun Formations across the study area.

The plot of S1versus TOC was used to evaluate the nonresidual (migrated) HCs from residual (non-migrated) HCs(Fig. 5). Based on the results, the samples taken from Sarchahan Formation at Kuh-e-Siah Well and some of those from Zireh and Golshan Wells exhibited non-residual(migrated) HCs, but those from other wells and surface samples contained residual HCs. The samples from Faraghun Formation at West Asalouye, Kuh-e-Siah and Homa Wells had non-residual (migrated) HCs, while the other samples from the Faraghun Formation contained residual HCs.

Fig. 5. Plot of S1 vs. TOC for samples from Sarchahan and Faraghun Formations.

The plot of S1+S2vs.TOC was drawn to determine the HC generation potential of the studied source rocks and thus rank them (Fig. 6). The samples from Sarchahan Formation at Kuh-e-Siah Well and surface samples of upper Sarchahan and some of those from lower Sarchahan exhibited very good HC generation potentials, but the samples at Golshan and Zireh Wells showed only poor HC generation potential. The samples from Faraghun Formation at the Salman and Kuh-e-Siah Wells had good potentials for HC generation while the samples at other wells and surface samples exhibited merely weak to very weak HC generation potentials.

Fig. 6. S1+S2 vs. TOC of samples taken from Sarchahan and Faraghun Formations.

Chen JQ et al. suggested that the plot of hydrogen index(HI) versus maximum temperature can be used to identify the rate of maturity and the OM type (Chen JQ et al., 2021),though the kerogen type determination was challenging using this figure. Drawing the plot for the cuttings and surface samples of Sarchahan and Faraghun Formations (Fig. 7), the surface samples from the Sarchahan Formation exhibited type-II kerogen at the beginning of the gas generation window, where any increase in the maximum temperature added to the rate of maturity while the produced amount of hydrogen decreased. When it came to Faraghun Formation,the samples at Salman Well showed type-III kerogen at the beginning of the oil generation window.

Fig. 7. HI vs. Tmax of samples taken from Sarchahan and Faraghun Formations.

The plot of S2vs.TOC was herein utilized to identify the OM type (Fig. 8). According to this plot, the kerogen contents of the upper and lower portions of Sarchahan Formation were found to be of type II, with some shale in the surface samples interpreted mistakenly as type-III kerogen (i.e. gas producer).Indeed, the fact that the rate of maturity increased and HI decreased with increasing the maximum temperature made it difficult to properly identify the kerogen type. The kerogen content of the samples from Faraghun Formation at the Salman Well was of type III, i.e. gas producer.

Fig. 8. Plot of S2 vs. TOC for samples taken from Sarchahan and Faraghun Formations.

Further in this study, the plot of productivity index (PI)vs.Tmaxwas used to assess the OM maturity and probable source rock (Fig. 9). The samples taken from the upper and lower portions of the Sarchahan Formation (surface samples) were in the region corresponding to the condensate and wet gas generation. The samples taken from Faraghun Formation at the Salman Well were identified as being at the beginning of the oil production window.

Fig. 9. Plot of PI vs. Tmax for samples taken from Sarchahan and Faraghun Formations.

5.2. The results of kinetic analysis

Four samples from Faraghun Formation were taken from Salman Well. Fig. 10 shows the distribution of the kinetic parameters resulted from the optimization process. Since the error value remained below 1 for all samples, the optimization performance was evaluated as good (Table 6). In general, the activation energy distributions were limited and asymmetric for all of the four samples, with mean values in the range of 40-60 kcal/mol, indicating type-III kerogen. Fig. 11 presents the rate of HC generation at the three heating rates, comparing the results of Rock-Eval pyrolysis and OPTKIN Software.Herein the dashed line denotes the measured curve while the solid line introduces the calculated curve.

Fig. 10. Distribution of kinetic parameters for the samples taken from Faraghun Formation: a-Sample A, b-Sample B, c-Sample C, and d-Sample D.

Table 6. Kinetic results of the samples taken from Faraghun Formation at Salman Well.

Fig. 11. The rate of HC generation for the samples taken from Faraghun Formation: a-sample A, b-sample B, c-sample C, and d-sample D.

The four samples taken from Faraghun Formation generated HCs in an approximate temperature range of 120-150℃ temperature range broad bell curve that characterizes type-III kerogen. Table 7 shows total produced amount of HC for each sample; the higher the value corresponding to a sample, the lower the maturity of the sample. Fig. 12 is related to the rate of converting kerogen in three temperature rates which is a comparison between measured pyrolysis curve by rock-Eval system 6 and pyrolysis curve calculated by OPTKIN software. Measured curve has been shown with dashed line and calculated curve with continues line in figure.

Table 7. The amount of hydrocarbons produced per gram of organic matter.

Fig. 12. The rate of kerogen to HC conversion for the samples taken from Faraghun Formation: a-Sample A, b-Sample B, c-Sample C, and d-Sample D.

In this figure, the rate of kerogen to HC conversion is stated in percentages as a function of temperature. Since all samples had their kerogen contents converted to HC within a temperature range of 370℃-520℃, kerogen content of Faraghun Formation was found to be of type III.

The results of kinetic analysis on the three samples taken from Sarchahan Formation (surface samples collected at Faraghun Mount) are shown in Fig. 13, where distribution of different kinetic parameters upon optimization is shown. With the obtained values of error falling below 1, the optimization performance was evaluated as good for all samples. Table 8 reports a summary of the results of kinetic parameters for the surface samples taken from Sarchahan Formation. The asymmetric nature of the first figure (upper Sarchahan) and the obtained values of activation energy (48-52 kcal/mol)indicated type-II kerogen.

Fig. 13. Distributions of kinetic parameters for the samples taken from Sarchahan Formation: a-sample A, b-sample B, and c-sample C.

Table 8. Results of kinetic study on the samples taken from Sarchahan Formation.

Fig. 14 demonstrates the HC generation rate at the three heating rates for the samples taken from Sarchahan formation.Considering the temperature range within which the kerogento-HC conversion occurred, Samples 2 and 3 from Sarchahan Formation (Silurian) exhibited a conversion temperature ranging within 110℃-120℃ a conversion temperatureand hence indicating type-II kerogen; given the relatively high HC generation rates exhibited by these samples, they could be concluded as being relatively mature. For Sample 1(Ordovician), the high conversion temperature and low amount of produced HC (24 mg/g of OM) showed the excellent maturity of the sample (Table 9).

Fig. 14. The kerogen-to-HC conversion temperature range and produced amount of HC for the samples taken from Sarchahan Formation:a-Sample A, b-Sample B, and c-Sample C.

Table 9. The amount of hydrocarbons produced per gram of organic matter for the samples taken from Sarchahan Formation.

Fig. 15 shows the kerogen-to-HC conversion temperature ranges at the three heating rates. On this figure, the conversion temperature range of the Sample 1 is shown to be 360℃-700℃. This shows that the upper Sarchahan was dominated by type-III kerogen. The Samples 2 and 3 had their kerogen contents converted to HC within a temperature range of 420℃-540℃, indicating type-II kerogen.

Fig. 15. The kerogen-to-HC conversion temperature range for the samples taken from Sarchahan Formation. a-Sample A, b-Sample B, and c-Sample C.

6. Conclusions

Two geochemical analysis methods were applied on a number of samples taken from Sarchahan and Faraghun Formations in southern Iran to study their hydrocarbon (HC)generation potential and the type and maturity of their organic matter (OM). For this purpose, Rock-Eval 6 pyrolysis was conducted on 187 and 72 samples from Sarchahan and Faraghun Formations, respectively. Moreover, OM type and kinetic parameters were more accurately evaluated using kinetic analyses on 3 and 4 samples from Sarchahan and Faraghun Formations, respectively. Accordingly, the following conclusions were drawn:

(i) Based on the results of Rock-Eval analyses, Sarchahan Formation was divided into two parts, namely Upper Sarchahan (Silurian, poor in OM) and Lower Sarchahan(Ordovician, rich in OM). The lower Sarchahan exhibited a hydrogen index (HI) of about 73 mg of HC/g of rock, a total organic carbon (TOC) of about 2.5%, and a maximum temperature of 459℃ on kerogen types II and II/III.Altogether, these findings characterized good HC generation potential parts of Coastal Fars where the samples were found to be in the late oil window.

(ii) According to the obtained results from the Rock-Eval analysis, Faraghun Formation exhibited HC generation potential in some of its coal interbeddings, which were assessed to be in early oil window and exhibit kerogen type III with a HI of about 120 mg of HC/g of rock, a TOC of about 3.9%, and a maximum temperature of 427℃ and the Salman Oilfield, the HC content of this formation was authigenous and exhibited a high value of TOC, indicating that the interbeddings of this formation could have served as a good source rock for the Salman Oilfield.

(iii) According to the results of the kinetic analysis on Sarchahan Formation, upper part of the formation exhibited an activation energy of about 55-80 kcal/mol and type-III kerogen. This was while, the lower Sarchahan had an activation energy of about 48-52 kcal/mol and type-II kerogen, indicating relatively mature samples.

(iv) According to the results of the kinetic analysis,Faraghun Formation contained type-III kerogen with an activation energy of about 40-60 kcal/mol, indicating that, in terms of maturity, the samples were in early oil window.

(v) Results of the analyses at different wells showed that the Faraghun Formation is richer in OM than the Sarchahan Formation, in south of Iran, and that the OM content of Faraghun Formation becomes further mature as one moves toward the middle of Persian Gulf. Maturity of the Sarchahan Formation was also found to increase from southern Iran toward the Coastal Fars. As such, it can be expected that,these formations serve as a good source rock within the mentioned areas.

CRediT authorship contribution statement

Younes Jalilian carried out the experiment and wrote the manuscript with support from Mohammad Hossein Saberi.Mohammad Hossein Saberi supervised the project.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

The authors gratefully acknowledge the Editor-in-Chief of the China Geology and reviewers for their very constructive comments on this article.