Abstract:Organic matter is abundant in the shales and marls of the Pabdeh Formation in the southwestern Iran . They can be considered as a conventional or unconventional petroleum system in the region. This study evaluates the hydrocarbon generating potential and the feasibility of unconventional hydrocarbon resources of this formation from the viewpoint of petroleum geochemistry in the vast areas of southwest Iran . For this purpose, the results of Rock-Eval and vitrinite reflectance of the Pabdeh Formation from 43 oil fields in North Dezful Embayment, South Dezful Embayment, Abadan Plain, and northwestern Persian Gulf have been used. Geochemical parameters indicate that Pabdeh has good quantity and quality of organic matter in the region. However, it has not reached the maturity required for hydrocarbon generation in most areas. From the perspective of unconventional hydrocarbon resources, some layers of the Pabdeh Formation in the Dezful Embayment, especially in central areas, have the potential for oil shales. The shale oil potential is not suitable due to the low maturity and hydrocarbon saturation index of the Pabdeh Formation in the study area.Keywords: Pabdeh Formation, hydrocarbon generating potential, oil shale, shale oil IntroductionThe Zagros sedimentary basin hosts a significant amount of the world’s oil and gas reserves. This high amount of hydrocarbon results from several active petroleum systems in this basin over geological time. The Pabdeh Formation in some regions of southwest Iran contains a good quantity and quality of organic matters that, can act as either a conventional or unconventional petroleum system under suitable thermal conditions. This study aims to assess the hydrocarbon generation potential and the feasibility of unconventional hydrocarbon resources of this formation across the vast areas of southwest Iran from the viewpoint of petroleum geochemistry. For this purpose, Rock-Eval and vitrinite reflectance results of the Pabdeh Formation in southwest Iran from 43 fields, covering 353 kilometers in width and 596 kilometers in length, have been used.Materials & MethodsIn order to evaluate the hydrocarbon generation potential and the feasibility of the associated conventional and unconventional resources of the Pabdeh Formation on a regional scale, the Rock-Eval pyrolysis and vitrinite reflectance results of 641 samples within 43 fields across the Dezful Embayment, Abadan Plain, and northwest of the Persian Gulf were applied. Contamination can significantly impact the Rock-Eval pyrolysis results. In this research, the reliability and absence of contamination were assured before using the Rock-Eval data. For this purpose, samples with S1/TOC ratios greater than 1.5 and production indexes higher than 0.5 were excluded from the study. All Rock-Eval pyrolysis and vitrinite reflectance information were uploaded, analyzed, and interpreted using the geochemical software p: IGI 3.5.1.Discussion of Results & ConclusionThe Total Organic Carbon (TOC) levels in the studied samples vary from 0.09 to 1.6 wt%. Approximately 70% of the samples have a TOC greater than one, indicating that the Pabdeh Formation can quantitatively be considered a good source rock in the southwest of Iran. The highest organic matter content is found in the center of the Dezful Embayment. The Petroleum Potential Index is the sum of free hydrocarbons and those generated during the pyrolysis process. Samples from the Dezful Embayment exhibit the highest potential, while those from the northwest of the Persian Gulf show the lowest. Samples with good and very good potential contain Type II kerogen. The highest quality of organic matter in the Pabdeh Formation pertains to the central part of the Dezful Embayment.Vitrinite reflectance is one of the standard and efficient methods for evaluating organic matter maturity. The vitrinite reflectance values of the studied samples vary between 0.2 and 1. Most samples have values less than 0.6 or close to it, indicating immature to the early stages of the oil window. Moreover, other maturity parameters such as Tmax and Production Index confirm this result. Therefore, although the quantity and quality of organic matter in the Pabdeh Formation in southwest Iran are very good, the low maturity limits its contribution to hydrocarbon generation in the region. It should be noted that the samples were taken from the anticlines. It is therefore likely that this source rock could have matured in synclines and depressions and produced some hydrocarbons.Growing demand for energy, coupled with depleting conventional hydrocarbon reserves, has focused attention on unconventional hydrocarbon resources.. Oil shales are sedimentary rocks containing significant amounts of organic matter that have yet to reach the maturity stage necessary for hydrocarbon production. The TOC in oil shales ranges between 3 and 30 wt%. (Hosseiny & Mohseni 2023). It is evident that the greater the quantity and quality of organic matter in the oil shale, the greater and more economically viable the recoverable oil will be. Oil shales are rocks rich in organic substances with vitrinite reflectance below 0.6% and Tmax lower than 435 C° (Hinrichs et al. 2010). Accordingly, the strata of the Pabdeh Formation in the Dezful Embayment have the potential to be an oil shale resource especially in the central parts where the Aghajari, Rameshir, Gachsaran, and RageSefid fields are located. It should be noted that, , in addition to geochemical characteristics, the depth and thickness of the shale layer, mineralogical and petrological characteristics, geology, energy requirements and costs as well as current technology, are incredibly important in assessing the feasibility and capability of oil shales.Shale oils are liquid hydrocarbons in a free, soluble, or adsorbed state in marls or shales within the oil window. In other words, shale oils are petroleum resources found in source rocks that have not migrated or have migrated just a short distance from the source rock (Jiang et al. 2016). The quantity of organic matter plays a fundamental role in shale production potential and storage capacity. The minimum TOC to form a shale oil is two wt%. Since Type III kerogen is mainly associated with gas generation, source rocks containing this kerogen type have a lower potential for shale oil resources. In shale oils, the source rock must be situated within the oil window (Ro = 0.6%-1.2%) (Jiang et al. 2016). The first step in determining the sweet spots for shale oils is identifying the source rocks within the oil-generating window (Jarvie 2012). Determining the source rock's oil saturation is also highly important alongside maturity assessment. The amount of free hydrocarbons in the “S1” peak of Rock-Eval directly reflects the hydrocarbon saturation in the source rock (Jarvie 2012). The source rock's Oil Saturation Index (OSI) is obtained through the formula OSI=S1*100/TOC. Layers with an OSI greater than 100 mg HC/g TOC are considered to have productive potential (Jarvie 2012). The Pabdeh Formation has appropriate conditions for shale oil resources regarding the quantity and quality of organic materials in the Dezful embayment, especially in central areas. However, the maturity level in these areas is not sufficient. In the northern parts of the Dezful embayment, the Pabdeh could be within the oil window. However, samples from these areas show TOC and OSI less than two percent and 100 mg HC/g TOC, respectively. more...