Influences of Geological and Petrophysical Attributes on Electrical Resistivity–Based Reserve Evaluation: Enhancing Carbonate Reservoir Classification, Permian-Triassic Reservoirs of Southern Iran
Authors: Omrani Sajjad , Tavakoli Vahid 
Published: July 2024

Summary
The strong heterogeneity of carbonate reservoirs makes it challenging to assess the spatial distribution of fluid behavior, geological attributes, petrophysical properties, and estimate oil reserves. The diversity in facies and diagenetic processes contributes significantly to the heterogeneity in these reservoirs. The ability to accurately characterize and manage hydrocarbon reservoirs heterogeneity hinges on a comprehensive understanding of geological and petrophysical attributes such as water saturation, porosity, permeability, and electrical conductivity.
In the quest for optimizing hydrocarbon reservoir identification and management, understanding the intricate relationship between the geological, petrophysical characteristics, and electrical conductivity of reservoir rocks is paramount. Electrical conductivity serves as an indicator of the structural attributes of pore networks, reflecting sedimentary and diagenetic influences on reservoir quality. Despite the extensive use of electrical resistivity in hydrocarbon reservoir assessments, accurate interpretation of its variations remains a significant challenge, which is due to complex geological factors. Thus, acquiring a thorough insight into the impact of geological and petrophysical attributes on electrical resistivity is crucial for a dependable assessment of hydrocarbon reservoirs. In this article, we explore the complex interplay between various geological and petrophysical factors and their impact on the electrical conductivity of rocks, which serves as a vital parameter in the assessment of hydrocarbon reservoirs.
The focus is on how sediment texture, pore types, depositional environments, diagenetic processes, and characteristics of pore throat radius influence the electrical properties of rocks. The objective of this comprehensive approach is to decipher the geological and petrophysical modifications using electrical data, thereby improving the analysis of electrical discrepancies. Moreover, the accuracy of categorizing rocks based on their electrical characteristics is evaluated to effectively manage reservoir heterogeneity. Different data were gathered from an exploratory well situated in the western Persian Gulf.
This data set comprised evaluations of 1,370 thin sections, 1,110 porosity and permeability data, 32 scanning electron microscopy (SEM) analyses, wireline logs, 29 mercury injection capillary pressure (MICP) data, 58 formation resistivity factors (FRFs), 20 formation resistivity indexes (FRIs), and 157 Dean-Stark measurements. Rocks were classified into different groups with similar electrical behavior utilizing the electrical quality index (EQI) approach. The efficiency of this method in managing reservoir heterogeneity, as one of the techniques for determining electrical rock type, was assessed, particularly by comparing the precision of predicted water saturation with Dean-Stark saturation data.
The findings of this study demonstrated that diagenetic processes, especially dolomitization and dissolution, have the most significant impact on variations in the electrical conductivity of rocks. These processes govern pore size, distribution, pore types, and the radii of pore throats. According to the findings of this research, categorizing rocks based on their electrical characteristics improves the precision of water saturation predictions. In additionally, the use of constant Archie coefficients in water saturation calculations results in an overestimation of water saturation, leading to an underestimation of hydrocarbon reserves in the reservoir. The results of this research allow for more knowledgeable decision-making concerning reservoir efficiency, hydrocarbon reserves, production techniques, and increased oil recovery.