The paper investigates the geological structure of the Middle–Upper Jurassic carbonate formation in the northwestern part of the Chardzhou Step, with the Birgutlin and Karakul troughs and the Kulbeshkak–Pitnyak swell taken as key examples. The study presents comprehensive results on the tectonic setting, lithological–stratigraphic characteristics, and facies composition of the carbonate sequence, as well as the petrophysical properties of reservoir rocks derived from deep-well core analyses. Based on these data, the main reservoir parameters and threshold values of productive intervals were determined. The findings indicate that the lagoonal zone developed in the study area is composed mainly of carbonate and carbonate–sulfate chemogenic deposits, rich in evaporites, dolomites, and terrigenous materials of open shallow-water origin. It was revealed that a reliable understanding of porous and permeable rock distribution within the carbonate formation requires identifying the relationships between the genetic types of carbonate rocks and their reservoir properties, which depend on structural and textural features. Petrophysical analysis and the determination of threshold values for open porosity and permeability were performed using graphical–analytical methods, producing cumulative reservoir–non-reservoir curves. The reservoir rocks of lagoonal deposits were found to be porous–fractured and highly heterogeneous, as reflected by a wide range of porosity and permeability variations. A schematic map showing the predicted spatial trends of reservoir properties within the Jurassic carbonate formation was developed. The results demonstrate that the distribution of reservoir properties is consistent and reveal zones with favorable parameters for further oil and gas exploration.

Keywords: Jurassic, carbonate formation, lithology, tectonics, reservoir, stratigraphy, porosity, permeability, boundary values

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