Impact of Tar Saturation on Fracture Networks and Reservoir Quality: Integrated Structural and Petrophysical Analysis of the Asmari Formation

Background and Objective: The Asmari fractured reservoir in Iran presents significant challenges for reservoir characterization due to complex structural heterogeneities and the presence of tar. Conventional techniques such as core sampling and standard logging often fall short in predicting permeability and production behaviour within such intricate reservoirs. This study integrates formation micro imager (FMI) data with petrophysical analysis to assess fracture properties, permeability, and evaluate the impact of tar on reservoir performance. Materials and Methods: The MI logs, combined with conventional petrophysical logs, were employed to analyse structural dips, fracture orientations, and reservoir heterogeneity. Bright and conductive features in FMI images were examined using sinusoid fitting techniques, and heterogeneity indices were computed via specialized software. Permeability analyses compared FMI-derived indicators with core measurements, while tar effects were assessed through resistivity contrasts and gamma log responses. Results: Structural analysis identified dips of approximately 46° towards S20W and fracture strikes oriented N83E-S83W. Longitudinal open fractures, predominantly in Zones 6 and 7-1, exhibited apertures up to 1 mm. High FMI resistivity values within porous dolomite layers correlated strongly with the presence of tar and heavy oil, causing permeability overestimation and impaired fluid mobility. Despite elevated porosity in carbonate and dolomite sections, tar occupation of pore spaces significantly reduces effective permeability. Conclusion: The FMI logging offers critical insights into fractures, heterogeneity, and permeability variations influenced by tar saturation. These findings advance the understanding of the Asmari reservoir’s complex carbonate framework, facilitating optimized production strategies for fractured, tar-affected reservoirs.