Document Type : Research Paper
Authors
1
Department of Geology, Faculty of Basic Science, Lorestan University, Khorramabad, Iran.
2
- Prof Department of Geology, Faculty of Science, Lorestan University, Khorramabad, Iran
3
Associate Professor, Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, IranSobouti Blvd (Gava Zang St.), Zanjan, Iran
Abstract
. In the Zagros Basin, this climatic shift is recorded as coordinated changes in paleoecological parameters and carbonate associations across the boundary between the Pabdeh Formation (source rock) and the Asmari Formation (the main hydrocarbon reservoir of Iran). Integrated microfacies and foraminiferal analysis of the Tang-e Chug section (Izeh Zone) reveals that during the Rupelian-Chattian interval, normal marine salinity (34–40 Psu) remained stable; however, synchronous changes occurred in depth (decrease from >200 m to 40–80 m), light regime (aphotic to oligophotic), nutrient regime (eutrophic to oligotrophic), and hydrodynamic energy (very low to low-moderate). These environmental shifts drove a fundamental transformation in foraminiferal life strategies, from r-strategy with asexual reproduction in the Pabdeh Formation to K-strategy with sexual reproduction in the Asmari Formation, accompanied by a change in carbonate associations from Nanofer to Foralgal assemblages. This pattern of change is fully consistent with global Eocene-Oligocene cooling, sea-level fall, and ocean circulation reorganization (upwelling intensification), demonstrating that the Zagros Basin, as a fossil analogue of modern upwelling systems, exhibited a sensitive and well-recorded response to global climatic perturbations. The findings have direct applications in hydrocarbon exploration, enabling more accurate prediction of carbonate reservoir quality (Asmari Formation) and source rock potential (Pabdeh Formation), and provide a conceptual framework for basin modeling, organic geochemistry, and stable isotope studies in the Zagros. This study demonstrates that integrated paleoecological and carbonate association analysis is a powerful tool for reconstructing global climatic-oceanographic changes and their relationship to hydrocarbon reservoir formation in tropical carbonate basins.
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