Organic geochemistry and petrography of the early Cretaceous Garau Formation in the Kabir-Kuh Anticline, Ilam

Abstract

AbstractIn this research, the Neocomian–Cenomanian Garau Formation, in the surface outcrop of Kabir-Kuh Anticline (Ilam), has been subjected to geochemical evaluation using the Rock-Eval pyrolysis and organic petrographic techniques. For this purpose, 19 surface samples were taken at regular intervals of 30 meters. Results from this study indicated that there is a highly organic-rich zone at the base of the Garau Formation. A relatively organic-rich zone exists in the middle parts of the Garau Formation, which progressively changes into organic-poor succession upward in the studied section. Based on the Rock-Eval pyrolysis results, the studied samples from the Garau Formation have little hydrocarbon generation potential and are highly mature in the studied section. Also, according to organic petrographic observations, the organic-rich succession of the Garau Formation contains abundant solid bitumen with framboidal pyrite. Accordingly, it can be concluded that anoxic conditions existed in the paleo-depositional environment of the Garau Formation. At present-day conditions, the studied formation can be important in terms of unconventional hydrocarbon resources (gaseous hydrocarbons trapped inside organic porosity).Keywords: Garau Formation, Organic geochemical evaluation, Organic petrography, Kabir-Kuh Anticline, Outcrop IntroductionIn the present study, for the first time, the organic geochemistry of the Garau Formation (Neocomian–Albian) has been evaluated using the outcrop samples collected from the Kabir-Kuh Anticline in Ilam Province. These samples were analyzed using Rock-Eval pyrolysis and organic petrographic methods.Previous studies on the Garau Formation have examined this formation in terms of stratigraphy (Barmaki Zadeh 2017), palaeodepositional environments (Sharafi, 2021), and microfacies (Hosseinpour 2007; Mahanipour 2018). Also, organic geochemical characteristics of the Garau Formation have been investigated in several of wells drilled in the Lurestan Area (Ala et al. 1980; Tavangar Ranjbar 2014; Mahbobipour 2016; Lotfiyar 2017; Hosseinpour 2021). It is worth mentioning that the Garau Formation has been studied in the outcrops in the Aligudarz Area (Shabrang 2021). However, this formation has not been geochemically evaluated in the outcrops of the Ilam Province. In this study, for the first time, 19 samples from the outcrop of the Garau Formation in the Kabir-Kuh Anticline of Ilam have been analyzed by organic petrography and Rock-Eval pyrolysis techniques. This information can be very helpful in better understanding the organic facies, the source rock characteristics, and palaeodepositional environments of the Garau Formation. Material & MethodsRock-Eval pyrolysis is a relatively easy and cost-effective method for rapid source rock evaluation in the oil industry (Behar et al. 2001). Among the most important geochemical parameters obtained from Rock-Eval pyrolysis are TOC, hydrogen index, oxygen index, S1, S2, and Tmax. These parameters can be used for evaluating the amount, type and maturity of the organic matter contained in sedimentary rocks (Behar et al. 2001). On the other hand, organic petrographic methods provide a direct means of observing organic constituents under the microscope. These techniques can provide valuable information about the type and thermal maturity of the organic matter contained in rock samples (Taylor et al. 1998). In the present study, results from both techniques are combined for better geochemical evaluation of samples collected from the Garau Formation. Discussion of Results and & ConclusionsResults from this study suggest that the basal parts of the Garau Formation are highly organic-rich. The TOC and S2 parameters for the studied samples vary between 0.5-40 (wt%) and 0.1-14 (mg HC/g rock) respectively. In addition, the high Tmax values (460-520 ֯C) are consistent with the very high thermal maturity of the studied samples. This highly organic-rich facies at the base of the Garau Formation gradually changes into moderately-rich facies in the middle parts and eventually into organic-poor facies at the top of the studied section. The organic-rich succession is characterized by abundant quantities of solid bitumen and framboidal pyrite under the microscope. The latter provides robust evidence that the palaeodepositional conditions were anoxic during the deposition of organic-rich facies. This is further supported by field evidence (including the presence of dark lamination and the lack of bioturbation) indicating the prevalence of anoxic conditions during the deposition of the organic-rich facies of the Garau Formation. In addition, results from this study indicate that the studied samples are highly mature (the late gas window). This suggests that the remaining organic matter in the Garau Formation consists of inert carbon, which has a limited hydrocarbon generation potential. Therefore, the remaining organic matter in the Garau Formation is the residue from primary organic matter that was preserved during the sedimentation of the Garau Formation. This implies that large volumes of hydrocarbons were generated by the Garau Formation in the earlier geological time. However, a detailed geohistory analysis of petroleum system elements and processes is required to better understand the fate of generated hydrocarbons.

Keywords

• garau formation
• organic geochemical evaluation
• organic petrography
• kabir-kuh anticline
• outcrop