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

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