Geochemical Characterization of the Oil Source Rocks in the Ahwaz Oilfield, Southwestern Iran

The Ahwaz oilfield is located in the Dezful embayment of the Zagros fold-thrust belt in SW Iran. The oil source rocks predominantly with the argillaceous lithologies include the Kazhdumi, Gurpi, and Pabdeh formations as well as the basal part of the Asmari Formation. The concentrations of the major, trace, and rare earth elements (REEs) of these lithologies were determined using x-ray fluorescence spectroscopy (XRF) and inductively coupled plasma-emission/mass spectrometer (ICP-ES/MS), in order to infer the geochemical characteristics of the oil source rocks. The North American Shale Composite (NASC)-normalized major oxides displayed a positive anomaly in CaO, negative anomalies in other oxides and no anomaly in P2O5. According to the spider diagram of normalized trace element, there was enrichment in V, Ni, Cu, Cd, Pb, and Th values. In the NASC-normalized REEs pattern, the oil source rocks mostly display a smooth distribution shape, negative cerium anomaly, and slightly positive anomaly in the middle rare earth elements (MREES: Sm-Ho) values. The concentration of V, Ni, Co, and their ratios suggested a mixed marine and terrigenous source input. The ratios of redox-sensitive elements such as Ni/Co, V/Ni, V/V+Ni, Th/U, and Ce/Ce* suggested that the oil source rocks were mainly deposited under anoxic condition.