چكيده انگليسي :
This study aims to determine the source rock, depositional environment, geological age, and thermal maturity of crude oils from the asmari reservoirs (Dezful Embayment) based on biomarker comparisons. To this end, eleven crude oil samples were collected from Asmari reservoirs.
Using available data from eleven oil fields Pazanan, Rag-e Sefid, Golkhari, Gachsaran, Bibi Hakimeh, Nargesi, Marun, Shadegan, Aghajari, Ahvaz, and Mansouri (Dezful Embayment)—geochemical analyses were conducted through gas chromatography (GC) and gas chromatography–mass spectrometry (GC-MS). Column chromatography revealed that, on average, the saturated and aromatic fractions constitute the highest proportions. Based on the Tissot and Welte diagram, samples from Nargesi, Ahvaz, Marun, and Aghajari fields were classified as paraffinic, while samples from Pazanan, Golkhari, Bibi Hakimeh, Gachsaran, Rag-e Sefid, Shadegan, and Mansouri were identified as paraffinic-naphthenic. The C29 ααα Sterane 20S/(20S+20R) versus C29 Sterane [ββ/(αα+ββ)] ratio, along with the C32 homohopane 22S/(22R+22S) ratio, indicated that the samples are within the oil generation window or at its peak. Additionally, the C29 ααα Sterane 20S/(20S+20R) versus Ts/(Ts+Tm) ratio, CPI index, and methyl dibenzothiophene (MDR) values demonstrated high maturity levels. Elevated Ts/(Ts+Tm), MDR, and TAS values particularly highlighted the higher maturity of the Nargesi oil sample. Using the ternary diagram of regular steranes C27, C28, and C29, the Pr/nC17 versus Ph/nC18 plot, the gammacerane index versus Pr/Ph ratio, TAR index, and oleanane content, the depositional environment of the source rock was determined to be marine, with Type II kerogen as the organic matter across all studied fields. To determine the geological age of the source rock, the C28 sterane abundance versus C28/C29 sterane ratio diagram was employed, revealing that most samples originated from the Lower Cretaceous. The oleanane index also supported this finding, indicating that the source rock predates the Upper Cretaceous. Carbon isotope analysis of asphaltenes further confirmed a Cretaceous age. Cross plots of dibenzothiophene/phenanthrene versus Pr/Ph ratios showed that oils from Nargesi, Pazanan, Gachsaran, Rag-e Sefid, Marun, Golkhari, Shadegan, Aghajari, Mansouri, and Ahvaz fields are derived from carbonate–marly source rocks, while Bibi Hakimeh oil is associated with carbonate–shaly source rocks. Similarly, the C29/C30 hopane versus C35/C34 hopane plot indicated carbonate–marly origins for oils from Nargesi, Shadegan, Marun, Aghajari, Golkhari, Ahvaz, and Mansouri, and carbonate–shaly origins for Bibi Hakimeh, Pazanan, Gachsaran, and Rag-e Sefid. Using the oleanane index and carbon isotope analysis of asphaltenes to determine source rock age, and the dibenzothiophene/phenanthrene versus Pr/Ph ratio to identify lithology, the Kazhdumi Formation was found to be the most likely source rock for the studied oil samples. The dimethylnaphthalene to carbon isotope ratio of asphaltenes further confirmed the Kazhdumi Formation as the source. This diagram also showed that the oil samples belong to a single group, indicating a common origin and classification within the same oil family.
