Reservoir quality of the Jahrum carbonate succession; a case study from the Fars region of Zagros Basin, SW Iran

Abstract

Abstract Eocene to Oligocene–Miocene carbonates of Jahrum and Asmari formations form the reservoirs of the Khesht Field in the Fars Province, the Zagros Foreland basin. In this study, the reservoir characteristics of the upper parts Jahrum Formation in the Khesht Oil Field have been investigated based on the combination of facies analysis and diagenetic features of samples in the porosity-permeability framework. The Jahrum Formation in the Fars Province is mainly composed of limestones that formed on a laterally continuous carbonate ramp with significant variations in reservoir heterogeneity and quality. Petrographic studies led to the identification of five carbonate microfacies. Five petrophysical rock types (RT) in KH-02 well are recognized by considering primary and secondary controls on pore type and size distribution. From RT1 with no reservoir property toward RT5 reservoir quality is enhanced. Diagenetic features have significantly affected reservoir properties by both enhancing and destroying porosity and permeability. Bivariate plots of porosity and permeability, combined with thin-section petrography indicate that pervasive anhydrite cement and compaction had the greatest negative impact on reservoir quality, whereas, dolomitization, fracturing, and dissolution of Nummulites played the most positive role. Therefore, the general characteristics of the Jahrum reservoir in the Khesht Field are mainly shaped by diagenetic features. Using CycloLog software in the two studied wells led to the identification of two sedimentary cycles within the Jahrum Formation. The negative trend of the INPEFA log (sea-level fall) in the second sedimentary cycle includes the upper part of the Jahrum Formation which has medium to high reservoir quality. Keywords: Reservoir zonation, Rock types, Diagenesis, Cyclolog Software, Jahrum Formation. Introduction Carbonate reservoirs are difficult to describe and produce a realistic picture of reservoir properties due to the vertical and lateral heterogeneity in various categories including lateral distribution of facies (Alsharhan 2006; Dou et al. 2011) and complex diagenetic evolution of carbonates (Lucia 2007; Ahr 2008; Rong et al. 2012; Moore 2013). Carbonates of the Jahrum Formation in Fars province together with the Asmari Formation form the main part of reservoirs in SW Iran. So far, no study has been conducted to investigate the reservoir characteristics of the Jahrum Formation in the Khesht oil field. These carbonates are generally deposited in a shallow marine environment, and the estimated porosity from thin-section petrographic studies show that due to the abundance of lime mud and low textural maturity, their reservoir capacity was low in the time of deposition (Hassanvand 2016). Therefore, due to the low primary porosity in most parts of the Jahrum Formation, it seems that the reservoir quality of this formation largely depends on the development of secondary porosity as a result of the performance of various diagenetic processes such as dolomitization (Azomani et al. 2013), fracturing and dissolution. Due to the importance of reservoir studies and the investigation of the distribution of reservoir properties, the combination of facies analysis and diagenetic characteristics of the Jahrum Formation samples in the framework of porosity and permeability were used for reservoir zonation in Khesht-2 well and the studied rocks were divided into different rock types. Also, the identifying sedimentary cycles of the Jahrum carbonate succession and reservoir correlation in KH-02 and KH-03 wells were performed using Cyclolog software. Material and Methods Petrographic analysis of thin sections together with petrophysical wire logs, porperm data, and core descriptions were used for Jahrum reservoir zonation in the Khesht oil field. In this way, 40 m of cores and 80 thin sections from KH-02 well were used to investigate facies distribution and diagenetic processes in the studied interval. All thin sections stained for dolomite identification using Dickson (1965) method and limestone classification has been done according to Dunham's (1962) and Embry and Klovan (1971) schemes. Facies analysis and interpretation of the depositional environment was performed using the standard microfacies classification by Wilson (1975) and Flügel (2010). We followed Lucia (1995, 2007) for reservoir quality characterization and identification of rock types. Determination of sedimentary cycles and reservoir correlation between the studied wells has been done by using CycloLog software and gamma-ray log. Discussion of Results & Conclusion The Late Eocene Jahrum Formation in the Khesht oil field, SW Iran has mainly composed of limestones lithology. Petrographical studies led to identifying five carbonate microfacies in the Jahrum succession which have been deposited on a homoclinal carbonate ramp. These microfacies are consists of F1) Echinoid Orbitolites Nummulites wacke/pack/rudstone - inner ramp (restricted lagoon); F2) Nummulites Echinoid packstone to grainstone - inner ramp (shoal); F3) coral boundstone - inner to mid ramp; F4) Echinoid Nummulites wackestone - mid ramp (open marine); F5) Bioclast dolostone (shallow burial environment). The Jahrum carbonates have been subjected to a variety of diagenetic processes including micritization, dolomitization, cementation, dissolution, fracturing, and styloliteration in the three diagenetic realms including marine, meteoric, and shallow to deep burial. The wide range of porosity and permeability values in most facies of the Jahrum Formation shows that the sedimentary texture of the studied samples alone did not control the reservoir quality. The reservoir quality of the studied succession is mainly controlled by different diagenetic processes. Petrographic studies along with porosity and permeability cross plots for the studied samples show that the combination of the dolomitization, dissolution, and fracture processes played the greatest role in improving the reservoir quality of the studied formation, but calcite and anhydrite cementation; especially pervasive dolomitization type and compaction, have reduced the reservoir quality of the studied succession. The INPEFA stratigraphy approach has identified two sedimentary cycles within the Jahrum succession with good correlation in both KH-02 and KH-03 wells. Reservoir quality is significantly better during sea-level fall than when sea level was high in both sedimentary cycles. Overall, the Jahrum Formation is deposited in a regression cycle that begins with the deposition of open marine microfacies on the Pabdeh Formation and ends with the deposition of echinoid Orbitolites Nummulites wackestone/packstone (F1) at the end of the sequence in a lagoonal environment at the boundary of the Asmari Formation. Using CycloLog software and correlation between the reservoir zones of the Jahrum Formation in Khesht Oil Field and the identified timelines reveals that some reservoir zones show good compatibility. However, some of these zones do not correspond to the timelines. By considering other cyclo-stratigraphic factors along with lithology, porosity, and permeability data, revision in Jahrum reservoir zonation in the Khesht Oil Field is needed.

Keywords

• reservoir zonation
• rock types
• diagenesis
• cyclolog software
• jahrum formation