Journal of Stratigraphy and Sedimentology Researches (Jun 2023)
The role of relative sea-level fluctuations on dolomitization of carbonate reservoirs Case study: the Oligocene–Miocene Asmari Formation
Abstract
Abstract The Oligocene–Miocene Asmari reservoir is dominated by heterogeneity in various aspects, especially porosity and permeability caused mainly by dolomitization. The Asmari Formation has been deposited along a homoclinal ramp-type platform with a gentle slope divisible into an inner ramp, mid ramp, outer ramp and basinal settings. The distribution of dolomite through the Asmari carbonate platform in the Shadegan Oil Field is not uniform nor is it random. All 12 facies associations are variably affected by dolomitization and have influenced reservoir quality. Inner ramp facies associations are the most dolomitized while, the mid- and outer ramp facies associations are moderate to least dolomitized intervals. Six third-order depositional sequences were recognized within the Oligo–Miocene succession. They are bounded by sequence boundaries (SB) with significant evidence of subaerial exposure, diagenetic alteration, oxidizing conditions, and an abrupt change in facies or facies bathymetry. The stratigraphic distribution of facies associations proves more dolomite percent is formed near the sequence boundary as well as in the high stand system tract (HST) deposits. The main dolomitization model of the Asmari carbonate platform is seepage-reflux. Thus, the highest dolomite percentages occur near the sequence boundary when the sea level was low in the shoreface facies (inner ramp), the lowest percentage and dominant fabric destructive dolomite in the offshore facies, and fabric selective dolomite near the maximum flooding surface (MFS) due to the slow rate of dolomitization and low volume of dolomitizing fluids. Keywords: Dolomitization models, Seepage-reflux, Sequence stratigraphy, Asmari Formation, Oligocene–Miocene. Introduction Dolomitization can affect the reservoir quality of carbonate platforms in both constructive and destructive ways, and according to the model and time of dolomitization, it can lead to an increase or decrease in the porosity and permeability of the reservoir (Rahimpour-Bonab et al. 2010). Dolomitization affected more than 90% of the studied Asmari reservoir sequence. As such, it has played the principal role in shaping the spatial pore space architecture, flow capacity and heterogeneity of the final Asmari reservoir quality (Aqrawi et al. 2006; Fallah-Bagtash et al. 2022; Omidpour et al. 2022). The Oligo–Miocene carbonates of the Asmari Formation form the giant reservoirs in the southwestern fields of Iran, including the Shadegan Oil Field in the Dezful Embayment (Aqrawi et al. 2006). In this formation, the best reservoir units occur within the dolomitized intervals. Therefore, in the Asmari reservoir with poor primary reservoir properties, dolomitization enhanced reservoir quality (Aqrawi et al. 2006; Noorian et al. 2020; 2021; Khazaie et al. 2022; Fallah-Bagtsh et al. 2022). The Asmari Formation has been the subject of many studies, including facies changes and sedimentary environment, paleoenvironmental and sequence stratigraphic reconstruction, investigation of diagenetic processes and reservoir quality in the Zagros Basin (Ehrenberg et al. 2007; Van Buchem et al. 2010; Khodaveisi et al. 2014; Adabi et al. 2016; Noorian et al. 2022; Omidpour and Fallah-Bagtash 2022; Fallah-Bagtash et al. 2021; 2022; Omidpour et al. 2021; 2022; 2023; Ahmadi et al. 2023). These carbonates were deposited in shallow facies belts along a carbonate ramp platform (Fallah-Bagtash et al. 2021; 2022; Omidpour et al. 2021; 2022). Based on previous studies, the age of this formation is Oligocene (Rupelian) to Early Miocene (Burdigalian) in different parts of the Zagros Basin (Ehrenberg et al. 2007; Laursen et al. 2009). According to biostratigraphic studies, the Asmari Formation in the Shadegan Oil Field has been deposited from Oligocene (Chattian) to Lower Miocene (Aquitanian–Burdigalian) (Omidpour et al. 2021). Due to the importance of the Asmari Formation as a most known reservoir in the Zagros region, especially in the Dezful Embayment, and considering the role of dolomitization in the reservoir quality of carbonate reservoirs, a well in the Shadegan Oil Field has been subjected to detailed sedimentology and sequence stratigraphy studies. The present study aims at investigatin the depositional history, the effect of dolomitization on the facies and depositional sequences of the Asmari Formation, and finally the relationship between relative sea level changes and dolomitization in the Asmari carbonate platform. Material & Methods The present study is based on a petrographic analysis of 524 thin sections from core samples of SG-11 well drilled in the Asmari Formation at Shadegan Oil Field. All thin sections were stained with potassium ferricyanide and Alizarin Red-S to distinguish calcite and dolomite minerals (Dickson 1965). Carbonates were classified based on the schemes of Dunham (1962) and Embry and Klovan (1971). Facies analysis and interpretation of the depositional environment were performed using the standard microfacies classification by Wilson (1975), Burchette and Wright (1992), and Flügel (2010). Sequence stratigraphic interpretations of the Asmari Formation were based on Hunt and Tucker (1993) method. Stratal surfaces have been identified according to the changes in lithofacies, fossils and their position relative to each other during the interval, and finally petrophysical logs such as gamma log (SGR and CGR) along these surfaces. Discussion of Results & Conclusion The Asmari reservoir with Oligocene (Chattian) –Miocene (Aquitanian–Burdigalian) age in Shadegan Oil Field with a thickness of 363 meters, includes limestones with interlayers of dolostones, sandstones, shales and evaporites. The detailed thin-section analysis of the carbonate samples resulted in the distinction of 26 carbonate-evaporate microfacies types and 12 facies associations in the studied successions. The Oligocene–Miocene succession was deposited along a homoclinal carbonate ramp setting within inner, mid, and outer ramp, and basin sub-environments. The distribution of dolomite through the facies and depositional sequences of the Oligocene–Miocene succession and the lateral and vertical heterogeneity in the percentage of dolomite indicate that the carbonate platform of the Asmari reservoir has undergone multiple dolomitization, which can be arranged into five dolomitization models from near surface to deep burial settings. The distribution of dolomite through the Oligocene–Miocene succession in the Shadegan Oil Field is not uniform nor is it random. All 12 facies associations are variably affected by dolomitization and have influenced reservoir quality. Inner ramp facies associations are the most dolomitized. Mid- and outer ramp facies associations are moderate to least dolomitized intervals. In turn, the stratigraphic distribution of these facies associations proves more dolomite percent was formed near the sequence boundary as well as in HST deposits. Thin-layered sabkha dolomites are formed at or just below the sediment-water interface in mud-supported facies soon after deposition or during shallow burial. The matrix dolomites (medium to coarse crystalline dolomites) are the most abundant type of dolomites which were formed during the intermediate burial stages of the Asmari succession. 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 fine crystalline dolomites. The very coarse crystalline dolomites 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. Investigation of the relationship between relative sea level changes and dolomitization in carbonate platform shows that towards the sequence boundaries (near shoreface facies), due to the large volume of dolomitizing fluids and high nucleation rate, the dolomitization rate is high, leading to a relatively small dolomite crystals size. On the other hand, toward the MFS (the offshore facies), the dolomitization rate is slowed due to the low volume of dolomitizing fluids and slow nucleation rate, resulting in relatively coarser dolomite crystal sizes.
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