Linking diagenetic history to depositional attributes in a high-frequency sequence stratigraphic framework: A case from upper Jurassic Arab formation in the central Persian Gulf

Abstract

The carbonate Arab Formation (Kimmeridgian-Tithonian) is one of the most prolific hydrocarbon reservoirs in the world. Petrographic studies of this unit in the central Persian Gulf led to identification of eleven sedimentary facies, and facies are grouped into 5 shore-parallel facies belts (supratidal, tidal flat, lagoon, shoal and mid-ramp). Evidences such as the absence of barrier reefs and the low diversity of facies reveals that these facies formed in a ramp-like platform. Postdepositional processes (diagenetic evolution) considerably influenced reservoir properties of sedimentary facies. Depositional and diagenetic features in this formation are strongly controlled by the interplay of glacio-eustasy and climate leading to sea-level fluctuations during the Late Jurassic. Based on the petrographic analyses and previous studies, four third-order sequences are recognized and all sequence boundaries are capped by evaporites. The main diagenetic processes impacting the Arab Formation occurred under marine and hypersaline conditions and burial realms. The hypersaline conditions correspond to sea-level fall and development of brines and are characterized by dolomitization and moldic dissolution in the Upper Arab Formation. Vuggy dissolution is another major reservoir improving factor. Generally, vuggy dissolution along with anhydrite cementation, occurred extensively in the Upper Arab Formation, while in the Lower Arab Formation, these processes are limited to just beneath of the sequence boundary (nodular anhydrite form). Diagenetic features (eogenetic and mesogenetic environments) appear to correlate to depositional environments in the Arab Formation and are controlled by sea-level changes. Consequently, diagenetic processes and reservoir properties can be predicted within a sequence stratigraphic framework.