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Introduction

The Fahliyan Formation, a thick carbonate sequence of the Lower Cretaceous, is mainly a shallow marine carbonate succession deposited on the Arabian plate (Adabi et al., 2010). The studied area is located in the central part of the Persian Gulf (Figure 1).

The goals of this study are to determine the main facies, diagenetic process, and porosity types, to reconstruct depositional environment, and to delineate facies with good reservoir quality in the Fahliyan Formation.

Figure 1: Location map of the studied area

Summary

The Lower Cretaceous Fahliyan Formation, part of the Khami Group, overlies the Hith Formation and is overlain by the Gadvan Formation (Figure 2). 16 microfacies types in the Fahliyan Formation of the studied core data were defined (Figure 2). The studied facies are described briefly below:

Tidal flat facies association: Only one microfacies (Fh-MF1) was recognized in this sub-environment (Figure 2; Table 1). Dolomitization, micritization, physical compaction, and cementation are diagenetic processes of this facies belt. Fenestral porosity is the main pore type of this facies belt; moreover, interparticle is observable in some intervals. This facies belt has good reservoir quality (Table 1).

Lagoon facies association: Five microfacies are associated with lagoon sub-environment (Figure 2; Table 1). Cementation, chemical compaction, dolomitization, and dissolution are the main diagenetic processes of this facies belt. Microporosity is the main pore types of this facies belt in shallower facies (Fh-MF2, Fh-MF3, and Fh-MF4); while interparticle is predominant porosity in the facies near shoal and reef sub-environments (Fh-MF5 and Fh-MF6; Table 1). As a result, Fh-MF2, Fh-MF3 and Fh-MF4, microfacies mainly have poor to moderate reservoir quality; whereas, Fh-MF5 and Fh-MF6 are mostly of good reservoir characteristics (Table 1).

Reef facies association: One microfacies is associated with this sub-environment (Figure 2; Table 1). Lithocodium aggregatum has built framework in this facies.

Figure 2: Sedimentological log of the Fahliyan Formation.

Cementation, micritization, dissolution, and physical compaction are diagenetic processes in this facies. While interparticle is the main porosity of this facies, vuggy, channel, and microporosity are also observable. This facies has poor to moderate reservoir quality (Table 1).

Shoal facies association: This facies belt includes three facies (Figure 2; Table 1). Dissolution, micritization, cementation, and physical compaction are major diagenetic fabrics. The main porosity of this facies belts is interparticle and hence this facies belt has good to very good reservoir quality (Table 1).

Channel facies association: One microfacies (Fh-MF11) is recognized in this sub-environment (Figure 2; Table 1). Peloids with sparry cement forms a peloid grainstone facies characteristic of channels which cut shoals. Cementation, dissolution, physical compaction, and micritization are diagenetic features of this facies. Fenestral and interparticle are the main pore type of this facies. This facies has poor to moderate reservoir quality (Table 1).

Shallow open marine facies association: This facies belt includes five facies (Figure 2; Table 1). The dominance of sponge spicules and the great amount of micrite accompanied by pyritization suggest a shallow open-marine and low-energy depositional environment. Cementation, chemical compaction, dissolution, micritization, and physical compaction are diagenetic processes of this facies. Microporosity is the main porosity of this facies belt. Due to lack of effective porosity, this facies belt mainly has poor reservoir quality (Table 1).

The Fahliyan Formation in the studied well was probably deposited on a rim-like carbonate platform in which more or less discontinuous marginal algal build-ups separate shallow water platform from deep marine settings (Figure 3).

Figure 3: Schematic diagram of the Fahliyan carbonate platform.

Platform margin originates from Lithocodium aggregatum. Because these algae are not frame-builder, this margin would not be able to build a high rim or strict margin. As a result, one can presume a carbonate platform with a smooth margin along which shallow water could be related deep marine (Figure 3). Accordingly, the carbonate platform should be considered as a transitional type between a rimmed shelf and a ramp.

Absence of major break of slope from shoreline into deeper water, no evidence of resedimentation (e.g., calciturbidites) and the presence of high-energy grainstone facies are consistent with the Fahliyan Formation having been deposited on a carbonate ramp. This is a possible explanation for the existence of some minor shallow marine biota within open marine facies and vice versa.

Assessment of porosity and permeability according to microfacies, SEM, and convential core analysis indicates that microfacies of shoal and channel facies belts (Fh-MF8, Fh-MF9, Fh-MF10, and Fh-MF11) and one microfacies of lagoon with grain-supported texture (Fh-MF6) have good reservoir quality due to abundance of interparticle porosity. Microfacies of tidal flat facies belt has also good reservoir quality due to fanestral porosity.

Table 1: The results of thin section study and CCAL analysis

CCAL analysis

Diagenesis has mainly had a destructive role in the Fahliayn Formation in the studied well because physical and chemical compaction, cementation, and micritization are the predominant diagenetic processes that affected the original texture of facies in the studied well. In spite of this, dissolution has increased porosity and permeability and improved reservoir quality in some intervals.

In the studied area, the Fahliyan Formation can be subdivided into upper and lower parts. Generally, the lower part of the Fahliyan Formation in the studied well has better reservoir quality in comparison with the upper part. It should be noted that interparticle and fenestral are the main pore types of the lower Fahliyan; while microporosity is mainly observed in upper part (Figure 2). According to CCAL data, porosity in microfacies bearing microporosity is relatively high; while permeability is low due to poor connection between empty spaces. As a result, lagoon and shallow open marine facies belts have poor reservoir quality because microporosity is the main pore type of these facies belts (Goodarzi et al., 2016; Table 1). Interparticle is the main porosity of shoal facies belt. Such type of porosity creates effective connection between pores hence high permeability in the sample.

Conclusions

Core and petrographic analyses of the Fahliyan Formation led to the recognition of tidal flat, lagoon, reef, shoal, channel, and shallow open-marine facies belts in the studied well.

There are microfacies types in the Fahliyan Formation based on lithology, sedimentary characteristic, texture, and fossil content of the studied samples.

The Fahliyan Formation in the studied well was probably deposited on a rim-like carbonate platform where more or less discontinuous marginal algal build-ups separate shallow water platform from deep marine settings. The most prevalent diagenesis processes in the studied well are physical and chemical compaction, micritization, cementation, neomorphism, dissolution, dolomitization, and pyritization.

The lower part of the Fahliyan Formation in the studied well has better reservoir quality in comparison with the upper part.

Petrographic evidences show that reservoir quality of the Fahliyan Formation in the studied well depends on preservation of primary sedimentary porosity (interparticle porosity) and diagenesis impact. It should be noted that diagenetic processes (except dissolution and dolomitization) are mainly responsible for reservoir quality decrement.

Shallow open-marine and lagoon facies belts have poor to moderate reservoir quality because microporosity is the main pore type in them. It is also noteworthy that interparticle porosity, mainly observed in shoal facies belt, is the main reason for creating the best reservoir quality.

Acknowledgements

• PEDEC geology department is thanked for the help in providing data and technical advice.

References

• Adabi, M.H., Salehi, M.A., Ghabeishavi, A., 2010. Depositional environment, sequence stratigraphy and geochemistry of Lower Cretaceous carbonates (Fahliyan Formation), south-west Iran. Journal of Asian Earth Sciences, 39: 148-160.
• Flügel, E., 2004. Microfacies of carbonate rocks: analysis, interpretation and application. Springer-Verlag, Berlin, Heidelberg, 976p.
• Goorarzi, M, Jamalian, M., Amirsardari, D., Jamalian, M., Shoghi, J., 2016. Relationship between Rock Typing and Petrography: A Case Study on the Fahliyan Formation in the Persian Gulf. 78th EAGE Conference & Exhibition 2016, Vienna, Austria.
• Jamalian M., Adabi M.H., Moussavi M.R., Sadeghi A., Baghbani D., Ariyafar B., 2011. Facies characteristic and paleoenvironmental reconstruction of the Fahliyan Formation, Lower Cretaceous, in the Kuh-e Siah area, Zagros Basin, southern Iran. Facies, 57: 101-122.