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SCIENTIFIC RESEARCH - 2026
Criteria and Cutoff Values for Distinguishing Reservoir Rocks in Deeply Buried Zones of the South Caspian Basin
Orkhan Yagubov1 
, Almaz Guliyeva1 , Gunay Vagifgizi2*
Abstract. Reliable discrimination between reservoir and non-reservoir rocks is essential for reserve estimation and field-development planning in the deeply buried productive intervals of the South Caspian Basin. This study evaluates lithological and petrophysical data from the Gunashli, Bahar, and Bulla-Deniz fields to define field-specific cutoff values for porosity, permeability, clay content, and carbonate content. The dataset comprises core descriptions and laboratory measurements from productive-series deposits, including 270 samples and more than 1,200 determinations from Gunashli, horizon-specific core datasets from Bahar, and 47 core samples from Bulla-Deniz. Distribution curves, cross-parameter comparisons, and reservoir/non-reservoir classifications were used to identify threshold values. At Gunashli, the porosity distributions of reservoir and non-reservoir rocks intersect at approximately 10%, while a 9% operational boundary is supported by the broader dataset. For the Bahar productive intervals, porosity cutoffs are generally about 8% and permeability cutoffs about 0.003 μm². For sandy-silty rocks of Bulla-Deniz Horizons V and VII, the adopted criteria are porosity greater than 7.5%, permeability greater than 1 mD, carbonate content below 20%, and clay content below 36%. The results demonstrate that cutoff values are not universal constants; they must be calibrated to lithofacies, burial depth, analytical scale, and production context.
Keywords: South Caspian Basin, reservoir rock, porosity cutoff, permeability cutoff, clay content, carbonate content, core analysis
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