Oil–oil and oil–source correlations in the Abadan Plain, SW Iran: Updated insights into middle Jurassic – Lower Cretaceous petroleum systems through biomarker analysis and chemometrics

Abstract

This study investigates the organic characterization of three source rocks in the Abadan Plain (namely, Lower Cretaceous–Albian Kazhdumi, Lower Cretaceous–Neocomian Garau, and Middle Jurassic Sargelu formations), located in the western Zagros fold–and–thrust belt, southwest of Iran. A total of 82 source rock samples (33 from the Kazhdumi, 38 from the Garau, and 11 from the Sargelu) were analyzed using Rock–Eval pyrolysis and organic petrography to evaluate organic matter quantity, quality, and thermal maturity. The Kazhdumi source rock predominantly contains immature type II/III kerogens with macerals such as vitrinite, amorphinite, inertinite, alginite, cutinite, and solid bitumen. The Garau source rock primarily consists of moderately mature marine type II kerogen, with macerals including alginite, cutinite, and solid bitumen. The Sargelu source rock exhibits highly mature type II–S kerogen, dominated by late-oil solid bitumen and pyrobitumen macerals. Molecular analysis was performed on 26 source rock extracts and 58 crude oils from various reservoirs across the Abadan Plain. Using isoprenoids, triterpanes, steranes, and aromatic biomarkers, a thermal sequence of Kazhdumi < Garau < Sargelu or Kazhdumi < Sargelu < Garau was identified in source rock extracts, while crude oils followed Sarvak < Azadegan Sandstone < Gadvan < Fahliyan. Principal component and hierarchical cluster analyses revealed that crude oils align with the thermal maturity of their source rocks. Strong correlations were observed between the crude oils and Garau and Sargelu source rocks, distinctly separating from the Kazhdumi source rock, emphasizing their dominant role in the Abadan Plain's petroleum systems.