Depositional environment, diagenetic evolution, and their impact on the reservoir quality of the carboniferous KT-Ⅱ carbonate in the zhanazhol reservoir, Pre-Caspian Basin, Kazakhstan

Abstract

Abstract The Carboniferous KT-Ⅱ carbonate unit (Late Serpukhovain to Early Moscovian) of the Zhanazhol reservoir in the Pre-Caspian Basin is a giant condensate reservoir with great heterogeneity. Although the depositional characteristics and their impact on reservoir quality have been well documented by previous studies, few studies have focused on diagenesis, the interplay between depositional environment, and their controls on reservoir heterogeneity. In this study, combining sedimentological, petrographic and geochemical analyses, we reconstructed the depositional settings and diagenetic history of the KT-Ⅱ carbonate and assess their impacts on reservoir quality. According to the vertical and lateral distribution of 12 observed microfacies types, we proposed a depositional model of a northward-deepening ramp consisting of tidal flat, inner ramp, and middle ramp, which has played a critical role in controlling the reservoir quality mainly through grain types and matrix content. Fusulinidal grainstones/packstones have the best reservoir quality because of its porous fusulinidal fragments and low micritic matrix content. The major diagenetic events, including cementation (eight cement types), dissolution (fabric and non-fabric-selective dissolution), compaction, pressure dissolution and microfracturing (early and late), were grouped into six diagenetic environments: marine, meteoric, shallow burial, epigenic karstification, intermediate burial, and deep burial. Marine cementation and compaction are the main reason for porosity decreasing in upper inner ramp and middle ramp, respectively. The minor moldic pores created by fabric-selective dissolution during meteoric stage were enlarged by freshwater leaching during epigenic karstification, and its associated cementation occluded pores in the underlying bioclastic grainstones/packstones. Late non-fabric-selective dissolution and cementation likely resulted in local porosity rearrangements without net porosity gain or loss, but certainly reduced reservoir heterogeneity. The interaction between depositional environment and diagenetic evolution played a vital role in the formation of two types of vertical distribution patterns of porosity (types Ⅰ and Ⅱ). Understanding this interaction and its impacts on reservoir quality within a sequence-stratigraphic framework can more reliably predict the distribution of reservoir properties, and provides insights for adjacent fields with similar settings.