Identification of Porosity Types and Geological Features of the Ilam Formation Based on FMI Image Logs, velocity deviation log and Petrographic Thin Section Data and in a Southwestern Iranian Field
Subject Areas : Petroleum Geology
1 - MSc in Petroleum Engineering, Sahand University of Technology
Keywords: Velocity Deviation Log, Thin Section Petrography, FMI Log, Structural Analysis of Reservoir,
Abstract :
The identification of porosity types and structural geological studies of hydrocarbon reservoirs is one of the key petrophysical characteristics. To this end, in the first phase of the present study, the porosity types in the Ilam Formation, an oil well located in southwestern Iran, were determined from the constructed velocity deviation log using well logging data as a cost-effective alternative to identify porosity types; subsequently, thin sections were examined through petrographic analysis for verification. Petrographic examination of five available thin section samples from the studied well revealed that the microfacies of this formation contain intercrystalline, intergranular, and moldic porosities, with no evidence of fracturing in the reservoir. According to the velocity deviation log, the main reservoir zones of the Ilam Formation exhibit velocities close to zero, indicating the presence of intercrystalline and intergranular porosities with good permeability. Negative velocity deviations observed in this well indicate borehole breakouts, while zones with positive velocity deviations are associated with intragranular and moldic porosities, mostly present in non-reservoir intervals. Subsequently, in the second phase, structural geological interpretation of the reservoir was performed using the FMI imaging log. Accordingly, 69 bedding planes were interpreted in the studied reservoir, 9 of which with reliable dips with confident bedding contacts and 60 having dips with unreliable bedding boundaries. The average dip of the layers was determined approximately 4 degrees toward S30W, with a strike of N30W. Of the identified stylolites, 50 were of the bedding-parallel type, with an average dip of 4.5 degrees and a strike of N30W–S30W. Borehole breakouts occurred in the direction of the minimum horizontal stress acting on the borehole, with a strike of N20W–S20E, approximately corresponding to the general trend of the Zagros fold belt. Based on the analysis of conventional logs, velocity deviation, and FMI data, no natural or induced fractures were observed in the Ilam reservoir of the studied well. This study demonstrated that intercrystalline and intergranular porosities had the greatest influence on the reservoir quality of the Ilam Formation, whereas moldic porosity had a relatively minor effect.
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