An analysis of the connection of microfacies, depositional environment, and diagenesis on the reservoir quality of the Mishrif Formation (Early Cenomanian–Turonian) in Esfand Field (Sirri E), northeast of the Persian Gulf

The Mishrif Formation (Cenomanian–Turonian) is one of the most important hydrocarbon reservoirs in the Persian Gulf. In this study, 151 microscopic thin sections and the porosity and permeability data on wells SIE-5 and SIE-6 in Esfand Oilfield were studied to identify the microfacies, depositional environment and the diagenetic processes of the Mishrif reservoir, to evaluate the effect of these processes on the reservoir quality. Lucia's petrophysical classification and the reservoir quality descriptive classification were also used in the optimal analysis of the reservoir characteristics. Petrographic analyses revealed 11 microfacies that belonged to the peritaidal zone, lagoon, shoal, middle and outer ramp sub-environments on a homoclinal carbonate ramp. Most of the microfacies of these two wells are belong to the shoal and lagoon facies belts. The succession in both wells show shallowing upward sequences. The most important diagenetic characteristics of the Mishrif Formation include dissolution, cementation, compaction, dolomitization and stylolitization. Porosity type consists of intercrystalline, vuggy, moldic and fracture porosities. Almost all microfacies in both wells are also in classes one and two of Lucia's petrophysical classification. Furthermore, the facies in the peritidal zone and shoal sub-environments in SIE-5 are in a better reservoir condition than SIE-6. On the other hand, the open marine facies in SIE-6 have a higher quality than the facies in SIE-5. In summary, it could be stated that diagenetic processes have had a larger role in the increase or decrease in the reservoir quality than the depositional environment. Thus, the Mishrif reservoir could be considered a diagenetic reservoir.

The Middle-East has the largest oil and gas reserves in the world, with its major portion being stored in the Arabian Plate (Sharland et al. 2001). The thick Cretaceous-aged sediments accumulated in the Arabian platform and Zagros basin contain enormous and economically valuable hydrocarbon reserves (Setudehnia 1978; Alsharhan and Nairn 1986, 1988; Ghabeishavi et al. 2009, 2010; Hollis 2011; Lapponi et al. 2011). The Cretaceous-aged (Cenomanian–Turonian) Mishrif reservoir is one of the most important hydrocarbon reservoirs in the Persian Gulf. Therefore, a better knowledge of its reservoir properties and depicting the sedimentary environment and diagenetic processes is of high importance.

In this study, 151 thin sections at coring spaces of one m were taken from the Mishrif Formation at SIE-5 and SIE-6 wells respectively, in the Esfand Oilfield. Then, they underwent petrographic examinations with a polarizing microscope to evaluate the microfacies and diagenetic processes. In order to determine the reservoir quality, the porosity-permeability data were plotted in petrophysical classification (Lucia 1983). Subsequently, each microfacies was evaluated separately, with its reservoir status being described based on descriptive classification (Ahr 2008).

Based on the conducted studies on the thin sections prepared from the cores of Mishrif Formation in two wells, 11 microfacies were identified belong to five groups of the intertidal zone, lagoon, shoal, middle ramp, and outer ramp.
The Mishrif Formation (Cenomanian–Turonian) in both wells indicate an evident thinning upward, which is consistent with the trend of sedimentation and its simultaneous depositional sequences in the Persian Gulf and Zagros regions. However, small thinning and thickening cycles showing a local performance in these deposits are also observed in both wells.
The most important diagenetic factors in the Mishrif Formation include dissolution, cementation, dolomitization, stylolitization, bioturbation, pyritization, hematitization, neomorphism, fracturing, and filling. Hematitization, dolomitization, and fracturing are the most important factors, which have increased the total porosity. These processes have created large amounts of intercrystalline, vuggy, and fracturing porosities in the oilfield and have improved the reservoir quality. The significant impact of meteoric diagenetic processes on the Mishrif Formation and upper section of Sarvak Formation has been proved and attributed to the mid-Turonian discontinuity (Hajkazemi et al. 2010; Razin et al. 2010, Taghavi et al. 2006; Rahimpour-Bonab et al. 2012; Hahjkazemi et al. 2017).
By comparing the porosity and permeability data of the microfacies from different facies belts on the petrophysical classification diagram of Lucia, it was found that the tidal zone microfacies of the two wells show a remarkable difference in their reservoir quality. It can be said that although they have deposited in similar sedimentary environments, but the differences in the diagenetic processes have caused the tidal zone microfacies of SIE-5 to have better reservoir quality. The dolomitization process and the presence of fracturing were more and wider in the microfacies of Mishrif Formation at SIE-5 and have been highly effective in the improvement of the reservoir quality. The lagoon microfacies of both wells show a similar trend, indicating the similarity of sedimentary environment and relatively similar effect of diagenetic processes in both wells, although the reservoir status of lagoon microfacies is obviously better at SIE-5. Although the microfacies sub-environment of carbonate shoal in both wells have the same environment, they are affected by diagenetic processes in different ways. This facies belt in SIE-5 is a complete reservoir belt, but in SIE-6 well it is divided into two reservoir and non-reservoir units. The petrographic examinations in the microfacies of this belt show that cementation, as a negative parameter in the reservoir quality, has more intensity in SIE-6, while dissolution and formation of various porosities, as positive reservoir parameters, have happened more at SIE-5. The microfacies of the middle ramp show a similar trend in both wells having a relatively good reservoir quality. The microfacies of the outer ramp, which are only observed in the SIE-5, show poor reservoir quality or have no reservoir status.
The Mishrif Formation with the age of Cenomanian–Turonian is one of the most important reservoirs of the Persian Gulf. The petrographic studies conducted on this formation led to the identification of 11 microfacies. These microfacies have been deposited in five sub-environments of the tidal zone (MF1), lagoon (MF2–MF5), shoal (MF6–MF8), middle ramp (MF9–MF10) and outer ramp (MF11) indicating a homoclinal carbonate ramp. Most of the microfacies of this formation at SIE-5 and SIE-6 wells are deposited in the sub-environments of carbonate shoal and lagoon facies belts, respectively. Dissolution, dolomitization, and fracturing are of the main factors affecting the increase of total porosities. These processes have created large amounts of intercrystalline, vuggy, and fracturing porosities in the studied field and have had a positive effect on increasing the reservoir quality. Evaluation of main diagenetic process has resulted in separation of three marine, meteoric, and burial diagenetic environments. The existing evidence shows that although the diagenetic processes have affected these deposits from the beginning of sedimentation to the deep burial phases, but mostly effected by  burial and meteoric diagenesis environments.
The reservoir evaluation showed that most of the sedimentary microfacies of this formation at both studied wells are plotted in classes one and two of the Lucia, which have a better reservoir status. In addition, the diagenetic processes have highly affected the porosity and permeability of these microfacies. Based on the dispersion of microfacies in different classes of Lucia in both wells, it was found that the sedimentary and diagenetic environments have affected the reservoir quality of sedimentary microfacies, both together and sometimes with different proportions. On the other hand, the results show that the microfacies of the tidal zone, lagoon, and shoal in SIE-5 and those of lagoon and middle ramp in SIE-6 have the best reservoir qualities. This is due to the presence of dissolution and dolomitization processes and formation of interconnected porosities.