An Integrated Method for Quantifying Karst Volume in Carbonate Reservoirs

The accurate quantification of karst volume is essential for evaluating storage capacity and fluid flow behavior in carbonate reservoirs, which are often highly heterogeneous due to complex diagenetic and karstification processes. However, conventional approaches to karst characterization frequently rely on qualitative descriptions or isolated datasets, lacking an integrated framework that effectively combines petrophysical properties with lithological controls. To address this gap, this study proposes a novel and practical workflow that systematically integrates routine well logs and core data to quantitatively estimate karst-dominated porosity in carbonate sequences. The methodology is designed to be both log-based and scalable, making it suitable for field-wide application even in data-constrained environments.

The proposed method is structured into four sequential and interpretative steps: (1) Data Integration and Quality Control: Multi-source data—including conventional well logs, core measurements, geological models, and lithology logs—are carefully depth-matched and subjected to rigorous quality checks to ensure consistency and reliability. Special attention is paid to correcting for borehole environmental effects and log normalization across multiple wells. (2) Definition of Karst Porosity Threshold: Based on systematic analysis of core-derived porosity-permeability cross-plots, a porosity value greater than 15% is established as a robust indicator for significant karst development. This threshold effectively distinguishes karst-related pore space from matrix porosity and is validated through thin-section and CT-scan observations. (3) Identification of Host Lithology: Lithofacies modeling and petrographic analysis are employed to confirm that karst features are predominantly hosted within dolomite intervals, highlighting the lithological control on karst distribution. Log-based facies classification is calibrated with core data to ensure accurate lithology discrimination in non-cored sections. (4) Calculation of Karst Volume Percentage: The proportion of karst volume is quantitatively computed as the ratio of high-porosity (>15%) dolomite volume to the total pore volume, which includes matrix pores, vugs, and fractures. This volumetric approach enables a more realistic representation of karst contribution to total porosity and reservoir performance.

This workflow was applied to a carbonate reservoir in the Pre-Caspian Basin, where it yielded a karst volume proportion of 11.1%. This result aligns closely with independent core statistics and regional geological understanding, validating the method’s accuracy and applicability. Sensitivity analyses were conducted on the porosity threshold, confirming that the selected 15% cutoff optimally balances discrimination capability and geological plausibility. The integrated approach not only enhances the reliability of karst assessment but also offers a scalable and reproducible tool for reservoir characterization. It supports improved geological modeling, reservoir performance prediction, and development planning in complex carbonate settings, ultimately contributing to more efficient hydrocarbon recovery. Future work will focus on extending the workflow to incorporate seismic attributes and dynamic production data for enhanced 3D karst modeling.

Keywords: Karst Volume; Carbonate; Well Logging; Porosity; Integrated Workflow