Diagenetic History and Isotope Geochemistry of Pabdeh Formation in Dezful Embayment (Zagros Basin) SW Iran

Abstract

Pabdeh Formation (upper Eocene-Oligocene) is a carbonate dominated sedimentary package with shale -marl intervals. This Formation was studied the type section (Kuh-E-Gurpi) and four boreholes located in Dezful Embayment (Zagros Basin). The Pabdeh Formation comprises three depositional sequences bounded by Type I sequence boundary in lower part and both Type I and type II sequence boundaries in upper part. Uppermost sequence encompassed a subsea marine phreatic diagenetic environment, whereas sequence one and two evidently experienced burial diagenesis with moderately reducing conditions in a relatively enclosed system. Sr87/ Sr86 ratios represents a sharp separations between sequence two and three, whereas low Rb content of these samples suggesting these sediments are not affected by meteoric fluids in an open system. A double behaviour is expected from the Pabdeh Formation as the lithology are combination of carbonates and shale alternations, as shales could be considered as potential source rocks, whereas grainstones of tempestite facies have reservoir characteristics. Hence change of stratigraphic trap exploration is a scenario for these facies changes within the Pabdeh Formation. Furthermore, extensive fracturing in upper parts of second sequence implies reservoir porosity development in these parts. Evidences of meteoric water flushing implies in third (last) sequence, leads porosity development in this sequence.