Provenance, organic geochemistry and sequence stratigraphy of foredeep sediments (Gurpi and Pabdeh formations) in the Zagros pro-foreland sedimentary system, North Dezful embayment, SW Iran

Abstract

The Zagros pro-foreland sedimentary system originated in the Upper Cretaceous and is ongoing. The Pabdeh and Gurpi formations were deposited in the foredeep of the pro-foreland in the northern Dezful embayment. On average, the Gurpi Formation has a thickness of 134 m, whereas the Pabdeh Formation has a thickness of 164 m. The two formations consist mainly of calcareous shales and limestone. Various analytical techniques such as X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscope (SEM), petrophysical logs, Rock-Eval pyrolysis, gas chromatography (GC), and thin sections were used to study the origin of sediments and sedimentary sequences. The predominant minerals found in the shales of these formations include calcite, quartz, dolomite, albite, anorthite, halite, iron minerals, and clay minerals. The highest values of major oxides found in the samples are calcium oxide (CaO), silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), and iron oxide (Fe₂O₃). The source rock of the shales is determined by the quantities of the major oxides present in felsic to intermediate igneous rocks. These sediments are located in a passive continental margin tectonic setting. Clay minerals such as illite, montmorillonite, palygorskite, and sepiolite indicated considerable weathering due to semi-arid to semi-humid conditions, as shown by the CIA index. The average total organic carbon (TOC) content in the samples from the Gurpi Formation is 0.96 wt%, whereas it is 1.27 wt% for the Pabdeh Formation. Both formations include type II kerogen capable of generating oil and gas. The Gurpi and Pabdeh formations have four sedimentary cycles created in a semi-deep to deep environment.