Microfacies, Sedimentary Environment, Diagenesis and Petrophysical Evaluation of the Sarvak Formation in the framework of Sequence Stratigraphy in North-Dezful Embayment, southwestern Iran

Abstract

Abstract The subsurface section of the Sarvak Formation with a thickness of 455 m is located in the North-Dezful Embayment. The Sarvak Formation is underlain by the Ilam Formation with erosional discontinuity and is overlain probably by the Shale of the Kazhdumi Formation (due to the lack of drilling to the underlying boundary). The probable age of this formation is Albian–Turonian. Based on petrographic studies of 2111 thin sections from cutting, the dominant lithologies are limestones and dolomitic limestones. Facies analysis has led to the identification of nine microfacies in three facies belts of the lagoon, bioclastic shoal and shallow open marine. The gradual changes in microfacies, lack of calciturbidite, slump structure and absence of large reefs structure, confirm the ramp type-carbonate platform. Diagenetic processes consist of micritization, neomorphism, physical and chemical compactions and dissolution. These processes have changed the reservoir quality of the Sarvak Formation. The quality of the reservoir is carried out in the framework of sequence stratigraphy and the Sarvak Formation is divided into third depositional sequences and six system tracts, and the reservoir potential of each system tracts was calculated and the result have shown 6 reservoir zones, which transgressive systems tract (TST)from the second depositional sequence has the highest reservoir potential in this well. Keywords: Sedimentary environment, Petrophysical evaluation, Sequence stratigraphy, Sarvak Formation Introduction The Albian–Turonian Sarvak Formation is the reservoir rock of many reservoirs in Zagros Basin and the Persian Gulf that holds significant parts of hydrocarbon reservoirs. Carbonate rocks are one of the most common reservoirs in the world, often in the form of limestones and dolostones (Motiei 1993). Iran's oil and gas reserves, such as the Asmari, Ilam and Sarvak formations located in the south and southwest fields of Iran, are accumulated in carbonate reservoirs. Dissolution of limestone and also brittleness have caused joints and fractures in them, resulting in increased porosity and permeability, which has provided suitable conditions for the formation of reservoir rock (Motiei 1993; Ghabeishavi et al. 2010; Asadzade et al. 2017). Studies conducted in central and eastern Zagros show that with the relative transgressive of sea level in the Middle Cretaceous, the sediments of the Sarvak Formation have been deposited on a homoclinal carbonate ramp with rudist patch reefs (Mehrabi et al. 2022). The position of studied oil field is located in the southwest of Iran, north of the Dezful Embayment. The structure has a northwest-southeast trend. The thickness of the drilling part of this formation in this well is about 455 meters, and its upper boundary with the Ilam Formation is erosional and the lower boundary is probably the Kazhdomi Formation. The study aims to understand the sedimentary environment, microfacies, diagenetic processes and reservoir quality of the Sarvak Formation. Material & Methods In order to determine sedimentary microfacies and diagenetic processes affecting carbonates of the Sarvak Formation, petrographic studies have been carried out on 2111 thin sections prepared from cuttings in the studied field. Classification for carbonate rocks used in this study is the terminology introduced by Dunham (1962) and its modified classification by Embry and Klovan (1971). The facies analysis and interpretation of the depositional environments were conducted using the standard microfacies of Flügel (2016). The quality of the reservoir has been evaluated by Geolog software and the multimin method. The sedimentary sequences are based on changes in microfacies, gamma ray logs peaks and biozones. The data in the palaeologs were used to identify the age of the Sarvak Formation. Discussion of Results & Conclusions The drilled thickness of the Sarvak Formation in this well is 455 meters. It consists of limestones, cherty limestones, dolomitic limestones and some shales. Generally, in order to determine the depositional environment of carbonates, it is necessary to study the constituent components and sedimentary facies. Therefore, petrographic analysis of the constituent components and identification of the facies have been conducted to determine the depositional environment of the Sarvak Formation in the studied field. The probable age of this formation is Albin–Turonian and it is unconformably overlain on the Ilam Formation and possibly resets on the Kazhdumi Formation. The petrographic studies led to the determination of nine facies associations deposited on three facies belts: the lagoon, shoal and the shallow open marine. The lagoonal microfacies are more abundant than the other facies belts. Considering the gradual changes of facies, the absence of large reefal structures, slump structures, and calciturbidite, the sedimentary environment of the Sarvak Formation in this study is a carbonate ramp setting. One of the most important diagenetic processes that led to a reduction in porosity in this formation is cementation. Processes such as stylolitization, dissolution, dolomitization and fracturing have resulted in increased porosity. The expansion of diagenetic processes including fracture porosity and vuggy porosity has improved the reservoir quality of the fine-grained sedimentary microfacies. Intergranular and fracture porosity predominantly occur in the grainstone facies, while vuggy porosity is mainly associated with the wackestone facies. Sequence stratigraphic studies have led to the identification of three 3rd-order depositional sequences and based on net-to-gross ratio, shale volume, porosity and water saturation, the TST of the second depositional sequence shows the best reservoir potential. The presence of detailed facies in conjunction with the depositional sequences and zones with reservoir potential indicates a direct relationship between the fine-grained grainstone microfacies associated with bioclastic units and vuggy microfacies associated with the lagoon environment with the zones exhibiting high reservoir potential.

Keywords

• sedimentary environment
• petrophysical evaluation
• sequence stratigraphy
• sarvak formation