A New Practical Anhydrite Classification Scheme: Implications for Depositional Environments and Seal Assessment

Abstract The absence of a comprehensive classification system for anhydrites has led to oversimplified assumptions of homogeneity, which undermines accurate seal evaluations. This study presents a new classification scheme that captures the heterogeneity of anhydrite deposits and enhances interpretations of depositional environments and seal integrity. A 70-meter Late Jurassic core (Arab A, B, and C members) near Riyadh, Saudi Arabia, was analyzed using sedimentological and mineralogical techniques, including over 120 thin sections, high-resolution gamma-ray readings, and porosity measurements. Four structural anhydrite groups with distinct gamma-ray signatures were identified and categorized by their anhydrite-to-carbonate ratios into eleven classes. These classes reflect varying depositional and diagenetic settings with distinct petrophysical properties. Additionally, a novel anhydrite logging sheet is introduced to aid in core and outcrop analysis, improve facies recognition, and predict gamma-ray responses. The study emphasizes the importance of detailed anhydrite characterization for reliable subsurface seal assessments.