Introduction

The Aptian–Albian Kazhdumi Formation of southwestern Iran is a significant hydrocarbon source rock that contributes substantially to the discovered oil and gas reserves in the Dezful Embayment (Van Buchem et al. 2010; Bordenave 2014; Alipour 2022, 2024). Previous studies have extensively explored its organic geochemistry (Bordenave and Burwood 1995; Bordenave and Huc 1995; Bordenave and Hegre 2010; Alizadeh et al. 2012) and sequence stratigraphy (Van Buchem et al. 2010; Vincent et al. 2010; Shabani et al. 2022). Recently, Alipour (2022) conducted a detailed study of the palaeo-depositional environments of this critical source rock in the Zagros Basin of Iran.

An overwhelming body of research exists on the organic geochemistry of the Kazhdumi Formation in the Zagros Basin (Table 1). These studies provide an extensive Rock-Eval pyrolysis data set that can be applied to the regional characterization of the Kazhdumi source rock (Table 2). However, caution is necessary when interpreting these analytical data, as many measurements are influenced by notably high concentrations of free hydrocarbons from oil-based mud (i.e., S₁ values exceeding S₂ parameters in pyrolysis) (Hunt 1996).

In particular, most previously published studies have overlooked that proper screening of the Rock-Eval pyrolysis datasets lies at the core of reliable geochemical interpretations. This is evidenced by the reports of the Rock-Eval pyrolysis S₁ being greater than the S₂ parameter in most of the previously reported datasets (Tables 1 and 2). Therefore, our main concern in the present study was to apply strict database screening methods to filter out unreliable values from geochemical interpretations. With this approach, it was possible to identify significant trends of variation in the geochemical data of the Kazhdumi source rock. The new findings about organofacies heterogeneity of the Kazhdumi source rock can have practical implications for basin and petroleum system modeling studies in the Zagros Basin. In addition, they may be useful in fine-tuning the mass balance estimates in the Kazhdumi-Ilam/Sarvak petroleum system, which in turn may help to identify low-risk zones of interest for future exploration/production activities.

Table 1- A summary of previously published studies reporting geochemical information of the Kazhdumi Formation in the Zagros Basin.

Geological background

The Cretaceous period was a time of worldwide sea-level high associated with a warm and humid climate. The sedimentary record of the Zagros Basin during the Cretaceous period is composed predominantly of carbonates due to its positioning near the palaeo-equator (Sharland et al. 2001). However, strong siliciclastic influxes entered into the carbonate depositional environments due to warm and humid palaeo-climatic conditions. This resulted in the development of mixed clastic-carbonate successions in which the lithofacies distribution was controlled by sea-level fluctuations (Davies et al. 2019). Periods of relative sea-level fall corresponded to “strong clastic influxes” in the proximal areas and “exposure/karstification of platform tops” in the distal settings (Davies et al. 2002). On the other hand, the clastic wedge was pushed back landward during times of relative sea-level rise, which corresponded to the deposition of highly organic-rich sediments in the basinal areas (Alipour 2022). The Kazhdumi organic-rich marls were deposited under a period of sea-level high at the center of an intrashelf basin which is known as the Kazhdumi Intrashelf Basin (KISB) (Fig 1).

Fig 1- Generalized location of oilfields from which geochemical information on the Kazhdumi Formation is available.

The KISB was separated from the open Neo-Tethyan oceanic waters by shallow-water carbonates of the Dariyan Formation (Fig 2). This rimmed intra-shelf setting also received considerable clastic influxes from the southwest through the Burgan deltaic system (Van Buchem et al. 2010; Davies et al. 2019; Shabani et al. 2022) (Fig 2). Anoxic bottom conditions favoring organic matter preservation prevailed at the center of the KISB and led to the deposition of highly organic-rich marls of the Kazhdumi Formation (Davies et al. 2002; Van Buchem et al. 2010; Vincent et al. 2010; Alipour 2022). Oxygenation of these settings during times of relative sea-level fall resulted in the periodic development of suboxic conditions and deposition of organic-poor carbonate-rich horizons within the Kazhdumi Formation.

Fig 2- Generalized chrono-stratigraphic section through the Kazhdumi intra-shelf basin (see Figure 1 for section location) based on previously published information (Davies et al. 2002; Van Buchem et al. 2010; Davies et al. 2019). Maximum flooding surfaces and the relative sea-level curve are adapted from Sharland et al. (2001).

 Material and methods

Rock-Eval pyrolysis

Our database of Rock-Eval pyrolysis parameters included analytical data from 382 samples, including those reported in previous publications and those from our proprietary database (Table 2). Rock-Eval pyrolysis is a cost-effective analytical technique for acquiring information on the bulk geochemical characteristics of source rock samples (Lafargue et al. 1998). This instrument uses ~70 mg of powdered rock samples for programmed heating in a pyrolysis oven. The first 3 minutes include iso-thermal heating at ~300 ֯C, which releases the free hydrocarbons in the sample (Fig 3). These vaporized hydrocarbons are recorded as peak S₁ (mg HC/g rock). In the next step, the temperature of the pyrolysis oven is increased at a rate of 25 ֯C/min, and this causes the generation of hydrocarbons from thermal cracking of the associated kerogen (Fig 3). The released hydrocarbons are recorded as the S₂ peak (mg HC/g rock), which can generally be considered as an indicator of source rock generative potential (Hunt 1996). The temperature corresponding to the peak hydrocarbon generation at the pyrolysis step is called the Tmax which is used toevaluate the thermal maturity of the source rock samples (Behar et al. 2001). Other useful parameters calculated from the Rock-Eval pyrolysis data, which include the Hydrogen Index (HI), Oxygen Index (OI), and Production Index (PI) (Peters 1986) (Fig 3).

Table 2- A list of Rock-Eval pyrolysis data (from previously published papers and our proprietary data) are used in this study highlighting the average values for several important parameters and indicating the number of samples before and after the screening procedure.

 Fig 3- Schematic representation of the Rock-Eval pyrolysis analytical procedure (Hunt 1996).

 Database screening

In this study, preliminary steps were taken to eliminate anomalous samples from the database, following the standard guidelines established in earlier studies (e.g., Katz 1983; Peters 1986; Peters and Cassa 1994; Dembicki 2009). Chemometric methods can be used to enhance the screening of bulk geochemical information, especially for datasets containing large numbers of values (Alipour et al. 2019). However, in the present study, a simpler screening procedure was applied for the preparatory screening of the Kazhdumi database by using plots of the PI versus Tmax (Fig 4a and b). It is generally known that PI values increase with increasing thermal maturity (Peters 1986; Peters and Cassa 1994). According to these authors, immature samples (i.e., Tmax < 435˚C) exhibit very low PI values (i.e., PI ≤ 0.1), while PI values as high as 0.4 are expected at peak maturity (Fig 4a). Therefore, low-maturity samples exhibiting anomalously high PI values should be excluded from further consideration (Fig 4b).

Fig 4- Data set structure captured by the Production Index (PI) versus Tmax (˚C) diagrams and bar diagrams of S₁ and S₂ parameters showing the raw database of 382 samples before screening (a) and removal of anomalous values after the screening which leaves a total of 135 samples for further processing (b).

 A significant number of samples were excluded due to anomalously high concentrations of free hydrocarbons in their matrices. Nevertheless, the resulting dataset provided adequate sample coverage from 22 different oilfields (Figure 1 and Table 2) and was suitable for understanding the regional geochemical characteristics of the Kazhdumi source rock in the study area.

The screening step applied in this study ensured the omission of samples contaminated with non-indigenous hydrocarbons (Figure 5). Following these preparatory procedures, 135 samples remained out of 382 samples in our database, which were subsequently used for the geochemical evaluation of the Kazhdumi source rock (Table 2).

It is worth mentioning that samples with S₂ values less than 1 mg HC/g rock should also be excluded from the database because these samples represent unreliable Tmax values (Peters 1986; Peters and Cassa 1994; Lafargue et al. 1998; Dembicki 2009). However, this step was not necessary in the present study because the main aim was to highlight the organic geochemical heterogeneity within the Kazhdumi Formation.

Fig 5- Diagrams of S₁ versus TOC for studied Kazhdumi samples before (a) and after (b) screening steps.

 Organic petrographic examinations

Organic petrography is a powerful technique for the visual examination of organic matter contained in source rock samples (Gonçalves et al. 2024). This approach provides useful insights into the type and maturity of the organic matter contained in the source rock samples, especially when combined with bulk rock pyrolysis data (Amiri and Alipour 2023, Ammari and Alipour 2024). In this study, we used a Zeiss Axioplan-II microscope to study the organic matter contained in the Kazhdumi samples under oil immersion and reflected white light with 100x objective magnification.

Results and discussion

Source rock evaluation by Rock-Eval pyrolysis data

The geochemical evaluation of petroleum source rocks includes three main steps that are necessary to define the quantity, quality, and thermal maturity of the contained organic matter (Hunt 1996; Dembicki 2017). The amount of organic matter in the source rock is a critical factor, typically reported as Total Organic Carbon (TOC) in weight percent. Nevertheless, it must be emphasized that TOC alone cannot be a reliable indicator of source rock potential (Hunt 1996). Therefore, other parameters such as HI and S₂ must be considered in combination with TOC to better understand the petroleum generative potential of source rock samples. A comparison of TOC histograms of the Kazhdumi samples indicates that the screening procedure has satisfactorily removed anomalous values (Fig 6a). Most of the remaining samples are characterized by TOC values in the range of 1–1.5 (wt%).

Additionally, plots of HI versus TOC indicate that a more linear correlation can be obtained after screening the anomalous pyrolysis values (Figure 6b). This indicates that the higher TOC samples in the Kazhdumi samples are also characterized with greater generative potential, which in turn suggests that the S₁ cross-over effect is mostly cured by the screening procedure (Figure 6b).

Fig 6- TOC histograms of the Kazhdumi samples before and after screening (a) and a comparison of the HI versus TOC plots for Kazhdumi samples before and after screening (b).

The type of organic matter in source rock samples can be assessed using HI versus OI diagrams (Hunt 1996). A 3D plot of HI, OI, and S₁+S₂ parameters for the Kazhdumi samples after screening reveals that the organofacies vary within each of the studied oilfields (Fig 7). A trend of variation from hydrogen-rich (i.e., highly oil-prone) to hydrogen-poor (i.e., less oil-prone) organic matter is observed for most of the studied oilfields (Fig 7). This conclusion agrees well with previously published results on the geochemical characteristics of the Kazhdumi source rock in the Zagros Basin (Alipour 2022).

Fig 7- 3D plot of HI, OI, and S₁+S₂ parameters for Kazhdumi samples after preparatory screening steps.

Organic petrographic observations

Additional support for the organofacies heterogeneity of the Kazhdumi source rock is provided by independent lines of evidence coming from the organic petrographic observations (Fig 8). The hydrogen-rich organofacies are characterized by the widespread occurrence of amorphous organic matter and framboidal pyrite (Fig 8a and b). These petrographic signs are consistent with favorable conditions for organic matter preservation in the palaeo-depositional environments of the Kazhdumi Formation (Alipour 2022). On the other hand, the hydrogen-poor organofacies are mostly characterized by the lack of amorphous organic matter, minor occurrences of inertinites, and abundant fossil remains (Fig 8c,d). These petrographic signs suggest that periods of dysoxic conditions prevailed in the palaeo-depositional environments of the Kazhdumi Formation.

The level of thermal maturity can be assessed using HI versus Tmax diagrams (Hunt 1996). This is based on the assumption that due to the generation of hydrocarbons from the sedimentary organic matter, HI values progressively decrease with increasing Tmax (Tissot and Welte 1984). However, under a certain geothermal regime, different types of organic matter follow specific pathways on these plots due to their differing reactivity (Behar et al. 1997). In the present study, the hydrogen-rich oil-prone organofacies of the Kazhdumi Formation exhibit a thermal evolutionary pathway distinct from the hydrogen-poor organofacies (Fig 9). This supports the conclusion that the Kazhdumi Formation is composed of different organo-facies with different reactivity (i.e., kinetic properties) in the study area.

Fig 8- Photomicrographs of the Kazhdumi hydrogen-rich organofacies (a,b) and hydrogen-poor organofacies (c,d) in the Rag-e-Sefid Oilfield under oil immersion and reflected white light with x100 magnification.

Fig 9- 3D plot of the HI, Tmax, and S₁+S₂ parameters for the Kazhdumi samples after preparatory screening steps.

Based on the above discussion, it appears that the Kazhdumi Formation has greater source potential in certain intervals than in others, reflecting the inherent organic-facies heterogeneity of this important source rock. These conclusions have practical implications for future basin and petroleum system modeling studies in the Zagros Basin. For example, the whole thickness of the Kazhdumi Formation cannot be considered a source horizon in these models because this will exaggerate the volume of in-place oil in identified prospects. In addition, the source rock heterogeneity reported in this study can potentially affect petroleum expulsion/migration processes from the Kazhdumi source rock. Therefore, additional research is required to carefully constrain the spatial distribution (i.e., geographic and stratigraphic) of the Kazhdumi oil-prone organofacies in the Zagros Basin.

Conclusions

A synthesis of the available geochemical information for the Kazhdumi Formation of SW Iran was performed with careful preliminaryscreening steps. The results indicate that the organofacies properties of the Kazhdumi Formation vary throughout the whole thickness of this formation. This organofacies heterogeneity is evidenced by the occurrenceof hydrogen-rich highly oil-prone organofacies and hydrogen-poor less oil-prone organofacies in different stratigraphic horizons within the Kazhdumi Formation. Different organofaceis of the Kazhdumi Formation also exhibit different thermal evolutionpaths on HI versus Tmax diagrams, reflecting different reactivity of the associated organic matter. The results of this study suggest that additional research is required to map the hydrogen-rich organofacies and determine the effective thickness of the Kazhdumi source rock in the Zagros Basin.

Acknowledgments The authors are grateful for the valuable and constructive comments from anonymous reviewers which greatly enhanced the quality of this manuscript.