Research paperGeological reservoir characterization of the Lower Cretaceous Dariyan Formation (Shu'aiba equivalent) in the Persian Gulf, southern Iran

- The Dariyan Formation is among the most important reservoirs in the Persian Gulf.
- Depositional and diagenetic histories of this formation are investigated.
- 2nd and 3rd order depositional sequences of this reservoir are differentiated.
- Reservoir units are positioned at two stratigraphic intervals in this formation.

The Aptian carbonate reservoirs of Persian Gulf, known as the Dariyan Formation (Shu'aiba equivalent), are among the most important oil reservoirs of Iran. Despite its significance, a little is known about the facies characteristics, diagenetic history, sequence stratigraphy, and reservoir quality of this formation. Using the new core data, this study presents an integrated geological reservoir characterization of this formation in four giant fields in the Persian Gulf. Using the results of the facies analysis it could be suggested that this unit formed in a shallow carbonate platform surrounding intra-shelf basins of Kazhdumi and Bab in the western and eastern Persian Gulf, respectively. A major relative sea-level fall in the Late Aptian resulted in exposure of this unit and subsequent clastic influx as well as meteoric diagenesis. This phase of meteoric diagenesis has resulted in some important diagenetic features, special geochemical trends, and considerable variations in reservoir quality of the Dariyan Formation. Sequence stratigraphic interpretation of this formation has resulted in the recognition of 2nd and 3rd order depositional sequences in the studied wells. They are closely correlated across the studied wells in the Persian Gulf, SW Iran, and other places in the Arabian Plate. Reservoir characterization has revealed that the high quality units are positioned at two stratigraphic intervals in the concerned reservoir. The first reservoir zone is located beneath the unconformity surfaces, where meteoric dissolution of grain-to mud-dominated facies has resulted in considerable amounts (10-40%) of vuggy, moldic, and micro-porosity in the highstand systems tracts (HSTs). The second reservoir zone of this formation has formed within the Lithocodium-algal facies, floatstone and boundstone, of inner ramp setting. The latter reservoir facies are mainly concentrated within the fields of the eastern Persian Gulf.