DEPOSITIONAL ENVIRONMENTS AND DIAGENESIS OF CRETACEOUS (ALBIAN TO MAASTRICHTIAN) STRATA OF THE ABADAN PLAIN OF THE PERSIAN PLATFORM IN SOUTHWESTERN IRAN

Sedimentology, diagenesis, and reservoir quality of Cretaceous (Albian to Maastrichtian) strata were investigated in Azadegan, Kooshk, Khoram Shahr, Darkhoin, Mah Shahr, and Tangoo oil fields of the Abadan Plain province of the Persian Platform in southwestern Iran. Formations studied include (oldest to youngest) Kazhdumi, Sarvak, Ilam, Gurpi, and Tarbur. The overall characteristics of these formations were controlled by the relative sea-level fluctuations, although compressional tectonics associated with the closing of the Neotethys Ocean and basement faulting also played minor roles.

A relative sea-level rise initiated deposition of organic-rich shales of the Kazhdumi Formation (Albian - Cenomanian). The limestones of the overlying Sarvak Formaton (Albian – Turonian) formed during the highstand. A major sea-level fall at the Cenomanian – Turonian boundary exposed carbonates of the Sarvak Formation. However, the uppermost portion of the Sarvak Formation was deposited during an early Turonian sea-level rise, and was subsequently exposed due to a minor sea-level fall. The shales of the Laffan Member (Coniacian) of the Ilam Formation covered the Cenomanian – Turonian unconformity during the early stages of a major sea-level rise. The limestones of the Ilam Formation (Coniacian - Santonian) were deposited during the highstand and possibly early lowestand. A major transgression resulted in deposition of dark gray shales of the Gurpi Formation (Campanian-Maastrichtian). The Tarbur Formation (Maastrichtian) formed patch reefs along the shelf margin. A disconformity separates Maastrichtian and Danian (Early Paleocene) in this area. The study revealed a variety of lithofacies indicative of strand plain, tidal flats, coastal plain, carbonate sand shoals including tidal bar belts, inner shelf, middle shelf, patch reef, barrier complex, outer shelf, mounds, distal deep marine pelagic depositional environments.

Major diagenetic alterations occurred along the Cenomanian - Turonian unconformity. Meteoric processes resulted in karstification, generation of porosity and permeability, and dolomitization, forming good reservoir quality strata in the Sarvak Formation. Burial diagenesis affected all units, in some cases increasing porosity and permeability and in others decreasing them.