Analysis of Fahliyan Reservoir Fractures by combining conventional petrophysical logs and FMI image log in an oil field in Southwest Iran

In this study, the fractures of the Fahliyan limestone formation, the third hydrocarbon-bearing reservoir in southwestern Iran, were investigated. Despite previous studies on the Fahliyan Formation, a comprehensive workflow integrating conventional petrophysical logs, FMI image analysis, and velocity deviation logs for systematic fracture characterization has not yet been fully developed. In the first step, petrophysical parameters, including water and hydrocarbon saturation and the overall lithology of the reservoir (limestone with minor shale content), were evaluated using conventional petrophysical logs, such as gamma ray (GR), density (RHOB), neutron (NPHI), photoelectric factor (PEF), and resistivity logs (MSFL, LLS, LLD). In the second step, processing of the Formation Micro Scanner (FMI) image logs led to the identification of bedding with dips less than 30° toward the northeast and fractures, which mainly extended with dips of 75° in the directions S65E/N65W and S25W. A more detailed analysis of the fracture patterns and their relationship with bedding was made possible by combining manual and automated methods. Analysis of the in-situ stresses indicated that borehole breakouts had mirror-dip orientations of N40W–S40E, and induced fractures had mirror-dip orientations of N70E–S70W, which is consistent with the Zagros trend. In the third step, the velocity deviation log (VDL) results showed negative values in the fractured upper part and zero to positive values in the lower part of the Fahliyan formation. This study, through integration of conventional petrophysical logs and FMI images, enabled the quantification and classification of fracture systems in the Fahliyan formation and demonstrated their direct relationship with the velocity deviation logs and in-situ stress orientations. This methodology facilitates the prediction of high-permeability fractured zones and optimization of reservoir development.