Dolomitization models and related fluid evolution in the carbonate platform of the Asmari Formation

The Oligocene-Miocene dolomitized Asmari Formation has expanded all over the Arabian plate with numerous supergiant and giant hydrocarbons in SW Iran, Iraq, Saudi Arabia, and the United Arab Emirates. Concertedly, the Asmari Formation and age-equivalents in adjacent areas of Middle East include more than 90% of recoverable oil reservoirs (Ghazban, 2007). The best reservoir units of this succession occurs within dolomitic parts exhibiting better reservoir quality than do the intercalated limestones and silisiclastics. Dolomite sequences play an important role in the production of oil and gas in the world's major hydrocarbon basins (Fallah-Bagtash et al, 2020; Noorian et al, 2020; Omidpour et al, 2021; 2022; Fallah-Bagtash et al, 2022). Similarly, in the Asmari reservoir with poor primary reservoir properties, fracturing and dolomitization enhanced porosity and permeability and thus hydrocarbon production (Aqrawi et al, 2006). Shadegan Oil Field is one of the important oilfields of Iran, due to its carbonate- siliciclastic nature, different parts of this formation have been exposed to the process of dolomitization. This has led to the development of porosity and permeability in its different parts. Therefore, in this research, using various data such as detailed petrographic studies along with geochemical studies of dolomites, the dolomites types of the Asmari Formation in the Shadegan Oil Field, dolomitization models, diagenetic history, diagenetic alteration and evolution of dolomitized fluids have been discussed. The results of this study can finally be used to evaluate the effect of dolomitization on the reservoir potential of Asmari Formation in this field.

The present study is based on a petrographic analysis of 1123 thin sections from cores of five wells drilled in the Asmari Formation. All thin sections were stained with potassium ferricyanide and Alizarin Red-S to distinguish carbonate minerals (Dickson, 1965). Dolomites are classified based on dolomite-rock texture classification presented by Sibley and Gregg (1987), Mazzullo (1992) and Chen et al. (2004). Facies analysis and interpretation of the depositional environment was performed using by Burchette and Wright (1992) and Flügel (2010) schemes. Ten uncovered thin sections were also analyzed by cathodoluminescence microscopy. These analyses took place at the Central Laboratories of Ferdowsi University of Mashhad, Iran. Ten gold-coated samples were analyzed with backscattered electron imaging using a Scanning Electron Microscope (SEM) in order to evaluate dolomite types, crystal sizes, micro-textures and pore spaces. Finally, thirty-two dolomitic samples were analyzed for their trace and major element contents using atomic absorption spectrophotometry (AAS) at the Ferdowsi University of Mashhad, Iran. 

The Asmari Formation, in Shadegan Oil Field, with Oligocene-Miocene age, consists of carbonate unit and siliciclastic intervals, which is mainly composed of medium to thick layered limestone and dolomite with interlayers of shale and sandstone. Detailed description of the core samples along with petrographic studies of the Asmari succession led to the identification of 26 carbonate-evaporite microfacies. In general, mineralogical, geochemical, and especially the interaction of facies with the distribution of dolomite indicates dolomitization by five different mechanisms/models in the carbonate platform of the Asmari Formation. These models include: Sabkha model, Seepage-reflux model, Meteoric-mixing zone model, Burial model and Bacterial mediation model. Dolomitization, as the most important diagenetic process in the depositional sequence of the Asmari Formation, has formed in several diagenetic environments, including syndepositional diagenetic realm (near surface), shallow burial, and intermediate to deep burial.

Four texturally and geochemically different types of dolomite include D1 (<10 μm, fabric-retentive), D2 (16-62 μm, fabric-retentive), D3 (62-250 μm, fabric destructive), and D4 (150-250 μm, fabric destructive). The lateral and vertical heterogeneity in dolomite percent indicates that the Asmari reservoir was subject to the multiple dolomitizations that could be categorized by five models in the near-surface to deep burial environments. Thin-layered sabkha dolomites (D1) are formed at or just below the sediment-water interface in mud-supported facies soon after deposition or during shallow burial. The matrix dolomites (D2 and D3) are the most abundant type of dolomites with the most contribution to reservoir porosity. They were formed during intermediate burial stages of the Asmari succession, indicated by their close association with the formation of an early generation of stylolites and fairly high iron concentration. These dolomites formed from warmer and more saline basinal fluids and/or from the dissolution of high-magnesium calcite or earlier dolomites, or recrystallization of D1. The D4 and other dolomites associated with the shaley facies, formed in a deeper burial setting by hydrothermal processes, utilizing hot and slightly-saline fluids that were affected by brine enrichment.