The effects of facies and diagenesis on the pore-throat sizes in Dariyan Formation, Central Persian Gulf

Abstract
This study investigates the effect of facies and diagenesis on the pore-throats in 110 m of carbonate rocks of the Dariyan Formation from one well in the central Persian Gulf. A total of 390 thin sections and 160 core porosity and permeability data were investigated. Petrographic studies led to the identification of nine microfacies in five sub-environments deposited in a carbonate ramp platform. The most important identified diagenetic processes include micritization, cementation, compaction, fracturing and neomorphism that affected the sediments of the Dariyan Formation during different stages of marine, meteoric and burial diagenetic environments. The Winland number has been calculated and compared with measured pore throats. Comparing the Winland rock types with microfacies, it is concluded that two microfacies of laminated mudstone and orbitolina wackestone have the smallest pore-throats (R35< 0.1 µµ). This is due to mud-dominated texture, presence of micrites, presence of clay minerals blocking the pores, and compaction and cementation. In contrast, bioclast peloid packstone to grainstone of shoal environment, due to the grain-dominated texture, low micrite abundance and open fractures, have the highest pore-throats (0.5<R35< 1 µ) among the total microfacies. In some samples, cementation and compaction have reduced the pore-throats and transferred them to units with a value of R35 between 0.1 to 0.5 µ. The upper Dariyan and lower Dariyan contains the microfacies with large (R35> 0.5 µ) and medium (R35> 0.5> 0.1 µ) pore-throats and have the best reservoir quality. In contrast, the Hawar Member corresponds to microfacies with a smaller pore-throats (R35> 0.1 µ) and has almost no reservoir quality.
Keywords: Dariyan Formation, Microfacies, Diagenetic, Pore-throats, Winland diagram
 
 
Porosity and permeability are two important parameters in identifying and understanding the behavior of an oil reservoir. These two parameters show many changes in the shallow carbonate reservoirs (Moore 2001; Lucia 2007). Rock types divide the reservoir rocks into separate units based on similar properties such as permeability and porosity. There are different ways to classify the rock types. The Winland method (Winland 1972) is one of the most widely used approaches for determining these rock types. The method is based on the Winland empirical equation which expresses the relationship between porosity, permeability, and pore-throat sizes at 35% mercury saturation in mercury injection capillary pressure (MICP) test. The diversity of microfacies and the effects of diagenesis, determined the pore-throat sizes and resulted Winland number. Therefore, it is important to study the geology of carbonate reservoirs and the impact of facies and diagenesis on the pore-throat sizes. This study, after examining facies and diagenesis, determines the impact of these two factors on the pore-throat size distribution in the Dariyan Formation. The results could show the most important factors affecting the distribution and changes of the pore-throat sizes in the carbonate rocks of this formation.
 
Materials & In this study, 110 meters of carbonate rocks of the Dariyan Formation in a well from a field in the central part of the Persian Gulf have been studied. At first, petrographic studies were performed using a polarizing microscope (on 390 thin sections) and the facies were named after the Dunham classification method (Dunham 1962). Flügel classification (Flügel 2010) was used to determine the sedimentary environments of the microfacies. Then, the diagenetic processes were investigated. Porosity and permeability of 160 plug samples were measured using Boyle and Darcy methods, respectively. Winland formula has been used to calculate and classify the pore-throat sizes of the samples. At last, the effects of microfacies and diagenetic processes on pore-throat sizes have been investigated through porosity-permeability diagrams and statistical calculations. To check the pore throats using the Winland R35 method, it is necessary to ensure a good correlation between the values of R35 calculated through the experimental formula of Winland using the porosity-permeability values and the actual values of R35 determined by the mercury curves.
 
Discussion of Results & Facies analysis led to the identification of nine microfacies in five sub-environments deposited in a carbonate ramp platform. Micriticization, bioturbation, cementation, especially calcite cement, compaction (physical and chemical), neomorphism and fracturing are observed in the studied formation. These diagenetic processes affected the sediments of the Dariyan Formation during different stages of marine, meteoric and burial diagenesis. Petrographic studies do not show high porosity, but core tests show considerable porosity in these samples. This confirms the presence of microporosity as the main type of porosity in the samples. To investigate the effects of facies and diagenesis on the pore-throat sizes, porosity and permeability data were plotted on the Winland diagram. According to the Winland method, three zones with different pore-throats (R35< 0.1 µ, 0.1 <R35< 0/5 µ, 0.5<R35< 1 µ) were identified. The Winland number has been calculated and compared with measured pore throats. The results show an acceptable correlation between the measured pore throat values and the calculated pore throats. Zone 3 (0.5<R35< 1 µ) shows the best reservoir quality. Comparing the Winland rock types with microfacies, it could be concluded that two microfacies of laminated mudstone (MF1) and orbitolina wackestone (MF3) have the smallest pore-throats (R35< 0.1 µ). This is due to mud-dominated texture, the presence of micrites, the presence of clay minerals that block the pores, compaction and cementation. Bioclast wackestone (MF2) microfacies deposited in lagoon environment have the highest frequency in units with a value of 0.1> R35 µ. Two microfacies of MF5 and MF8 with average pore-throats have the highest frequency in units with a value of 0.5<R35< 1 µ. In contrast, bioclast peloid packstone to grainstone of shoal environment, due to the grain-dominated texture, low micrite abundance and open fractures, have the largest pore-throats (0.5<R35< 1 µ) among the total microfacies. Cementation and compaction have reduced the pore-throats of some samples and moved them to units with a value of R35 between 0.1 to 0.5 µ. The upper Dariyan and lower Dariyan contain the microfacies with large (R35> 0.5 µs) and medium (R35> 0.5> 0.1 µs) pore-throats and have the best reservoir quality. In contrast, the Hawar Member corresponds to microfacies with smaller pore-throats (R35> 0.1 µs) and has almost no reservoir quality. MF6 and MF9 have various pore-throat sizes due to the effect of different diagenetic processes and could be observed in all three zones.