Integrated geological study for reservoir modeling and characterization of the Shurijeh gas bearing sandstones in one of the Northeast Iran gas fields

The sandstones deposited in the sequence of Shurijeh Formation comprise a giant gas reservoir in the Northeast of Iran. Five primary facies (petrofacies) were identified in the deposits of Shurijeh Formation such that the entire sedimentary sequence was divided into eight zones. Diagenetic processes have affected the pore properties of sandstone (zone D2). In this research, the petrophysical parameters (porosity and water saturation) and lithology were gamma-ray in the correlation with the petrofacies and core data. The areas with good reservoir properties for each zone and the total reservoir volume were simulated by exerting an appropriate threshold cutoff limit for the reservoir parameter (porosity and water saturation) and shale volume. For determine the economic hydrocarbon volume of the field, the uncertainty analysis in volumetric calculations was carried out utilizing the Monte Carlo method, with the production of Case50, gas-in-place was calculated as 28 × 10⁶ cubic meters. The sensitivity analysis was conducted to determine the impact of each parameter on gas-in-place. Based on the results of this study, the sedimentary facies and diagenesis processes directly affect the in gas-in-place the reservoir. According to the sensitivity analysis, the map of underground lines (UGC) and fluid contact were the most sensitive compared to porosity and water saturation. That the fluid contact is related to the sedimentary facies and diagenetic processes. In fine-grained sedimentary facies, due to the transition zone increase, it is difficult to accurately determine the fluid boundary of the reservoir, while in coarse-grained sediments, which have high permeability, due to the transition zone reduction, the fluid boundary is more accurately determined. Also, diagenetic processes such as cementation, dissolution, and dolomitic processes cause great complexity in determining fluid boundaries. Therefore, comprehensive geological and reservoir studies must be done before any field development work. In this study, the spread of the reservoir zones B and D1, which have the highest volume of in-situ hydrocarbons, with lithofacies that are associated with high-energy environments (mostly sandstone) and have excellent petrophysical parameters in terms of reservoir quality was demonstrated. In accordance with these results, initial depositional processes control the distribution of porosity, water saturation, thickness and extension of prolific areas at the field scale.