Petrographic and geochemical evidence of diagenetic alterations in the Sarvak Formation in an oilfield from the Abadan Plain, west of Iran

The Sarvak Formation, the second important oil reservoir in the Zagros Basin, has a complicated diagenetic history. This study focuses on the diagenetic processes of this formation in the Abadan Plain. To achieve this goal, petrographic investigations of core samples, thin sections, X-ray diffraction, and scanning electron microscopy are integrated with the results of elemental geochemical data. Intensive meteoric dissolution (karstification), paleosol formation, dissolution-collapsed brecciation, meteoric cementation, and silicification are major meteoric diagenetic processes. Such intensive meteoric diagenesis along with the dominance of kaolinite and montmorillonite, as predominant clay types within the paleosol, all indicate a warm and humid paleoclimatic condition at the time of exposure. These diagenetic alterations provided special trends of variations in the elemental contents of altered carbonates. They include a clear increase in Na, Mn, Fe, and Rb along with the decrease in Sr contents recorded below the Cenomanian–Turonian and mid-Turonian disconformities. The variations in Mg contents depend on the original mineralogy of carbonates that can result in variable trends in response to the meteoric diagenesis. Below the mid-Turonian disconformity, the development of mud-dominated facies hampered the free fluid circulation and, consequently, diagenetic alterations and their related geochemical trends are limited within the Turonian sequence.