Investigation of factors influencing geological heterogeneity in tight gas carbonates, Permian reservoir of the Persian Gulf

Understanding the spatial distribution of petrophysical properties in carbonate reservoirs is complicated due to their heterogeneity and sensitivity to diagenesis along with important facies variations. The reservoir evaluation in tight carbonates is even more complicated. The Permian Dalan Formation in the central part of Persian Gulf is generally considered as conventional carbonate reservoir. However, in this study, a part of K3 unit of this formation is introduced as a tight carbonate reservoir. To identify the factors influencing the quality of this unit, the sedimentary facies, depositional environments, diagenetic processes, and pore-type distribution were studied in details on 402 thin sections. Porosity and permeability of K3 are mainly controlled by dolomitization, dissolution, compaction, and cementation. MICP-based (mercury injection capillary pressure) methods, FZI*(FZI-star), and rock fabric numbers were considered to recognize the factors controlling heterogeneity of this unit. The MICP-based methods of Winland, Pittman, and Rezaee were combined with 3 porosity cutoffs to detect the heterogeneities of tight carbonates. None of these methods provided a high degree of correlation between porosity and permeability of the samples. The results of rock fabric number showed that samples with similar reservoir quality are the product of different diagenetic processes. Results obtained in this study demonstrate that the FZI*(FZI-star) and rock fabric number classifications are the most suitable methods for differentiating the tight zone and non-reservoir/reservoir parts in carbonates of K3 unit.