Substantiation of the petrophysical properties of complex-structured carbonate reservoirs in the Bukhara-Khiva oil and gas region: A case study of the Alan field

Abstract

This study investigates the petrophysical properties of complex carbonate reservoirs in the Alan field, Chorjou step, Bukhara-Khiva oil and gas region, to support spatial distribution and reserve estimation. It aims to identify factors driving the "apparent saturation" effect and low-resistivity pay zones in the Jurassic carbonate sequence (horizons XV-HP, XV-P, XV-ΠΡ, and XV-a), using well logging data (Gamma Ray, Resistivity, Neutron, Sonic) and core analysis linking porosity, permeability, clay content, and fracturing. Results reveal extreme heterogeneity and a bimodal porosity system. Despite relatively uniform total porosity (12–18%) across horizons, permeability and water saturation differ drastically: the upper reef horizons (XV-HP, XV-P) show high permeability (285–400 mD) and low water saturation (11–13%) from vuggy, fractured structures, while the lower sub-reef and basal horizons show reduced permeability (~11.6 mD) and saturation up to 35%. This disparity stems from diagenetic clay minerals (montmorillonite and kaolinite) and isolated micropores, which raise capillary forces and cation exchange capacity, suppressing resistivity and creating an apparent high-saturation effect. Conventional interpretation is insufficient for such systems; integrating Digital Rock Physics and artificial intelligence is recommended to differentiate low-resistivity pay zones, reduce geological uncertainty, and optimize field development, thereby preventing early water breakthrough and maximizing recovery efficiency.