Petroleum geochemical properties of the oil shales from the Early Cretaceous Garau Formation, Qalikuh locality, Zagros Mountains, Iran

Abstract

A suite of petroleum geochemical techniques including Rock-Eval VI pyrolysis, carbon analyzer, kerogen petrography and palynofacies, vitrinite reflectance (VRr), elemental analysis of kerogen (CHONSFe), gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), pyrolysis ((PY)-GC) and carbon isotope (δ13C) analyses were performed on 123 outcrop samples from 10 locations to characterize the properties of organic matter and the petroleum potential of the oil shales from the Early Cretaceous Garau Formation at Qalikuh locality in the High Zagros (Iran). To characterize the mineralogy and chemistry of the oil shales X-ray diffraction (XRD) and X-ray fluorescence (XRF) were performed.Together with our previous work (Rasouli et al., 2015), this is the first study in which organic geochemical and petrographical investigations on the Qalikuh oil shales were performed. Total organic carbon (TOC) content of the oil shales varies from 3.83 to 26.4 wt%, with a mean value of 16.5 wt%, suggesting excellent TOC content. The mean value of hydrogen index (HI = 557 mg HC/g TOC) and petroleum potential (S2 = 89 mg HC/g rock) for the oil shales is high, indicating the presence of highly oil-prone organic matter. Elemental analysis of kerogen shows the oil shales are predominantly enriched in Type II-S kerogen. The results of kerogen microscopy observations reveal that homogenous bitumen (as widespread cement by impregnation) and amorphous organic matter are the main organic components. The oil shales of the Garau Formation are mainly consisting of calcite minerals and therefore they are classified as organic-rich/highly bituminous limestone. Additional data from molecular geochemistry and biomarker parameters of GC and GC–MS (e.g. CPI, Pr/Ph, Pr/nC17, Ph/nC18, DBT/Phen and C29/C27 20R sterane) as well as carbon isotope data of extracted bitumen confirm the presence of marine Type II kerogen. These data also show the oil shales were deposited within a marine carbonate environment under anoxic/euxinic conditions.Based on equivalent VRr, elemental analysis, microscopy evidence and biomarker maturity parameters the investigated oil shales from the Garau Formation are early mature and located in the oil generation window. The revised oil-yield calculation from Fischer Assay results confirms the occurrence of moderate to good commercial oil shales with oil yield of 83 l/ton rock on average. Moreover, preliminary analyses of PY-GC data indicate that the Qalikuh oil shales have also very good shale gas potential at over-mature state (i.e. VRr >1.3%).