Challenging the Horizontal Assumption: Tilted Fluid Contacts in a Complex Carbonate Field

Tilted fluid contacts refer to oil-water or gas-water interfaces that are not level or horizontal across the reservoir. These deviations can result from structural gradients, capillary pressure variation, or hydrodynamic flow and introduce substantial complexity into reservoir characterization. In carbonate settings, this complexity is further amplified due to inherent heterogeneities in rock properties and subtle structural features. Unlike conventional models that assume fluid level equilibrium, tilted contacts represent spatial variations that must be interpreted with a geologically consistent framework. Identifying and understanding such contacts is critical for accurate reserves estimation, optimal well placement, and robust static and dynamic modeling (Kirkham et al., 2010). This case study focuses on Field G, a mature carbonate reservoir in the Middle East, where spatial variation in oil-water contact (OWC) depths across the Central and Eastern areas triggered an investigation into the potential presence of a tilted contact. The study utilizes a comprehensive dataset, integrating petrophysical analysis, core-based capillary pressure measurements, and 3D seismic interpretation. Although pressure data were reviewed, they did not provide conclusive evidence of a consistent Free Water Level (FWL), reinforcing reliance on log-based interpretations. The study evaluates the interplay of facies, permeability, and structural features and their collective role in governing contact geometry. The resulting observations suggest that a facies-controlled tilted contact, possibly influenced by long-term structural tilting, is a plausible explanation for the observed OWC variation. This integrated analysis enhances understanding of the reservoir's architecture and highlights the need to revisit assumptions regarding fluid level equilibrium in similar carbonate systems. The implications are significant for reserves estimation, well planning, and full- field development strategies in mature, heterogeneous carbonate settings.