Diagenesis, Geochemistry and Reservoir Quality of the Fahliyan Formation in One of the Oil Fields in Southwestern Iran

Document Type : Research Paper

Authors

1 Department of Geology, Faculty of Science, University of Isfahan, Isfahan, Iran

2 School of Geology, College of Science, University of Tehran, Tehran, Iran

3 School of Geology, Upstream Section, Research Institute of Petroleum Industry (RIPI), Tehran, Iran

10.22078/pr.2024.5407.3406

Abstract

The Fahliyan Formation (Early Cretaceous) is considered to be one of the major oil reservoirs in many fields of the Zagros sub-basins such as the Abadan Plain and the Persian Gulf. In order to investigate the diagenesis, geochemistry and reservoir quality of this carbonate succession in one of the fields located in the Abadan Plain, the results of geological-petrophysical studies including petrographic study of microscopic thin sections, conventional core analysis (porosity-permeability), XRD, petrophysical diagrams, carbon-oxygen isotope and elemental geochemical analysis are combined with SEM and CL microscopy. This formation has been affected by various diagenetic processes, including micritization, bioturbation, compaction, cementation, dissolution, dolomitization, and fracturing. Moreover, calcite cementation, compaction and extensive dolomitization are the main factors that have controlled the decrease of porosity and permeability, while dissolution (in the form of voids and molds) and fracturing have increased the reservoir quality of the studied succession. The core porosity and permeability data in the reservoir part of the Fahliyan Formation range from 0.01 to 27.5% and from less than 0.01 to more than 630 mD. These changes in porosity and permeability are strongly dependent on diagenetic processes that follow the pattern of facies changes. In addition, reservoir quality studies also show that diagenesis has had a major positive and negative impact on the Fahliyan Formation and has largely controlled the distribution of the pore system. Using elemental geochemistry and oxygen and carbon isotopic data, a surface corresponding to the maximum relative sea-level fall was identified. The porosity and permeability data associated with the facies of the lower (reservoir) part of the Fahliyan Formation were plotted on the Lucia diagram resulting in the separation of four different rock types, among which the floatstone/bondstone-greenstone has a higher reservoir quality. Moreover, two petrophysical methods were used to determine flow units, including flow zone index and porosity throat radius at 35% mercury saturation. Based on the FZI method, one non-reservoir unit and four reservoir flow units were identified, and based on the porosity throat radius method at 35% mercury saturation, four rock types and one non-reservoir rock type were identified according to the pore size.

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References

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Journal of Petroleum Research
Volume 34, 1403-5 - Serial Number 138
January and February 2025
Pages 175-182

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