Reservoir characterization is of key importance in upstream industry as it ensures maximum productivity of crude along with the selection of an appropriate method for economic recovery from the sub-surface reservoir. Proper identification of hydraulic flow units and a detailed understanding of pore characteristics are essential in reservoir characterization studies. Here, hydraulic flow unit concept is applied for in-depth characterization of a complex petroleum reservoir from Indian sub-continent having fine-grained sandstone, siltstone together with shale. A semi-logarithmic porosity – permeability plot exhibited large heterogeneity of the reservoir under study. An improved relationship between porosity – permeability was developed through classification of the reservoir formation under study into three hydraulic flow units based on unique flow zone indicator value. Pore size classification of studied core samples were investigated using mercury intrusion capillary pressure curves and later, validated using fractal dimensions calculated using capillary pressure fractal model. Finally, factors affecting the heterogeneity in permeability of subjected reservoir were analyzed. The study indicates that quantitative evaluation of pore system by mercury intrusion porosimetry along with the fractal dimensional analysis can efficiently be applied for verification of hydraulic flow unit classification and hence, for the detailed characterization of heterogeneous petroleum reservoir. © 2021 Elsevier B.V.