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New Approaches for Diagenetic Characterization of Carbonate Reservoirs: Examples From Tengiz Field, Republic of Kazakhstan


Characterizing diagenesis, in addition to facies arrangements and stratigraphic architecture, is critical for predicting the distribution of reservoir quality in many carbonate reservoirs. Carbonate diagenetic histories are often extremely complex, reflecting syndepositional to deep burial processes with multiple superimposed cementation and dissolution events. Traditionally, these records are captured in paragenetic charts that focus only on the presence of the various phases in a rock but lack linkage to reservoir quality. We here propose an approach for carbonate diagenetic characterization that, in addition to traditional documentation of the paragenetic sequence of cements and pore types, also 1) distills the multifaceted paragenesis into the event(s) that most impacted present-day reservoir quality (Key Paragenetic Step, KPS); 2) graphically depicts the starting point, diagenetic path, and end-product for a particular rock (Paragenogram); and 3) links to the distribution of reservoir quality (flow units) through integration of diagenetic profiles, cumulative-thickness plots, depositional facies, and sequence stratigraphy. To demonstrate this approach, we present examples from carbonate margin and slope facies of the Paleozoic Tengiz field in the Republic of Kazakhstan, where a complex diagenetic history heavily impacted matrix reservoir quality and non-matrix flow characteristics. The dataset consists of 16 cores with thin sections that sample margin grainstone/boundstone-, upper slope boundstone-, middle slope breccia/grainstone-, and lower slope grainstone/mudstone facies assemblages. Using our new approach, we developed a framework that ties diagenetic styles to resulting rock types and improves the linkages between depositional facies, diagenetic overprints, volumetrically significant pore families, and the origins and distribution of enhanced matrix reservoir quality. For example, this work underscored the importance of burial dissolution and its link to improved matrix porosity, permeability, and flow behavior. These findings provide geological context to better understand MICP-derived pore throat data, porosity-permeability distributions, and vertical flow behavior along the borehole, as well as valuable input for log-based Petrophysical Rock Typing.