Diverse dolomitization models in the Aptian Dariyan (Shu'aiba) Formation in the Persian Gulf, Iran: Evidence from petrography, geochemistry and impact on reservoir quality

Abstract

This study investigates dolomitization as one of the key diagenetic processes influencing the reservoir quality of the Dariyan Formation (Aptian) in three oil/gas fields in the Persian Gulf. To achieve this goal, petrographic studies of core samples and thin-sections are integrated with cathodoluminescence microscopy, carbon and oxygen isotopic analyses, and fluid inclusion studies. The formation exhibits two main dolomitization models, including hydrothermal-related saddle dolomites and stylolite-associated dolomites, as well as two minor models including bacterial activity-related dolomites and mixed meteoric and marine waters. Hydrothermal dolomitization, associated with deep burial diagenesis, results in saddle dolomites filling fractures and voids, characterized by high-temperature and salinity formation conditions. Stylolite-related dolomitization occurs in mud-dominated facies, as fine to medium crystals with a cloudy center and a clear rim, concentrated along the solution seams and stylolites. Bacterial activity-related dolomitization is distinguished in Lithocodium–algal facies, highlighting the role of microbial mediation in dolomite formation at low temperatures. Mixed-water dolomitization is observed in grain-dominated shoal and lagoonal facies, where the interaction of meteoric and marine waters leads dolomite cementation. Geochemical (δ¹³C and δ¹⁸O) and cathodoluminescence data support the diagenetic model, indicating varying stages and conditions of dolomite formation. Fluid inclusion analyses of saddle and stylolite-related dolomites reveal salinity (9.5 wt% NaCl equiv.) and temperature (105 °C) conditions consistent with burial diagenesis. The paragenetic sequence shows a diagenetic history encompassing marine, meteoric, and burial environments. Marine diagenesis includes early (microbial and mixing type) dolomitization, bioturbation, micritization and isopachous cementation, while meteoric diagenesis comprises dissolution, cementation and recrystallization during prolonged (>1 Myr) exposure at the end of Aptian. Burial diagenesis involves compaction, hydrothermal and stylolite-related dolomitization, cementation, and recrystallization, critically controlling reservoir quality. This study demonstrates that dolomitization significantly enhances permeability, with the highest reservoir quality found in dolomitized facies of the lower carbonate unit of the Dariyan Formation. This study highlights the complexity of diagenetic processes in the Dariyan Formation, providing valuable insights into the factors controlling its reservoir quality. The findings emphasize the importance of integrated geological and geochemical approaches for accurate reservoir evaluation and prediction in dolomitized carbonate formations.