Title : Controls of facies on reservoir quality of the asmari formation in a supergiant gas field, Zagros foreland basin
Abstract:
The Oligocene-Miocene aged Asmari formation, holding approximately half of Iran's oil in place and accounting for over 80% of the country's oil production for more than a century, stands as the most significant reservoir rock in Iran. In the Asmari reservoirs, the high oil production rate from fractures has led to less attention being paid to facies and diagenetic controls on reservoir quality.In order to investigate the facies controls on reservoir quality in the Asmari formation within one of the highly fractured carbonates reservoirs, facies, diagenesis, and stratigraphic sequence were examined using core data, petrophysical logs, and geochemical data.Petrographic studies indicate that this formation is dominated by foraminiferal and coralgal facies which have been significantly influenced by diagenetic processes such as dolomitization, compaction, cementation, and lesser dissolution. Eight sedimentary facies have been identified in this formation, which are interpreted to have been deposited in a carbonate ramp with coral patch reefs. Based on strontium isotope dating (87Sr/86Sr ratios) and index fossils, these formations were deposited during the Chattian to Burdigalian (26.1 to 18.3 Ma).
According to facies stacking pattern, fossil assemblages, diagenetic modifications in the cored wells, five depositional sequences have been identified in the Asmari Formation. These sequences include two Oligocene sequences (lower and upper Chattian in age) and three Miocene sequences are lower Aquitanian, upper Aquitanian and lower Burdigalian in age.Poroperm data from core samples show that the matrix porosity of the Asmari Formation ranges from 0 to approximately 30 percent with an average of 10 percent, while permeability varies from very low to about 100 millidarcies with an average of about 20 millidarcies. The better reservoir properties occur in red-algae dominates, mid ramp facies with progradational geometry (F-1: large hyaline foram, echinoid, red algae wacke/packstone; F-2: coralgal boundstone/floatstone/rudstone). The facies with average porosity exceeding 10% and average permeability exceeding 20 millidarcies, have relatively better reservoir quality compared to other facies. These facies were developed in early Chattian and occur at the basal units of the formation in the studied field.Moreover, dolowackestone/mudstone with anhydrite nodule (F-8) with an average porosity of 14% and an average permeability of 40 millidarcies constitute the most significant reservoir facies, highlighting the dolomitization effects on reservoir quality. Findings indicate that facies variations exert significant control over the reservoir quality of the Asmari Formation and the better-quality reservoir facies primarily extend within the Oligocene depositional sequences where sedimentation rates are relatively high.
Audience Takeaway Notes:
- Through this study, audience become more acquainted with the vast reservoirs of the Asmari Formation. The dominant sedimentary facies and the diagenetic processes affecting these facies are elucidated within the framework of sedimentary sequences.
- The findings of this study can be extrapolated to other carbonate reservoirs worldwide, and the lessons learned from it can be examined in similar reservoirs.
- This study highlights the importance of facies variations on reservoir quality in fractured reservoir rocks.
