Geometric Factor Optimization for Compaction State Analysis of Clay in Rapid Sedimentation Zones: A Case Study from the Yinggehai Basin

The lower-velocity clay in the Yinggehai Basin (northwestern South China Sea) forms under rapid depositional conditions. These clays are typically buried at depths of 1.3–4.0 km, with P-wave velocities ranging from 2.5 to 3.0 km/s. They produce pseudo-bright spots on seismic images, often mistaken for gas sand reservoirs, thus complicating reservoir identification. When quantifying the geometric factor that characterizes the compaction state of clay using log porosity data, we found that the geometric factor calculated from the critical porosity could not effectively describe the elastic characteristics of clay. As a result, the bulk modulus was used to optimize the calculation method for the geometric factor and to improve its accuracy. A rock physics model incorporating the optimized geometric factor successfully synthesized sonic velocity curves showing higher consistency with measured acoustic logs. The refined model further elucidates the elastic and anisotropic characteristics of lower-velocity clays. Core-scale inversion of geometric factors demonstrated remarkable correlation with mineralogical composition, specifically illite and smectite content, revealing systematic alignment between geometric flattening patterns and mineralogical diagenesis. This integrated approach provides a tool for distinguishing clays from gas sand reservoirs, significantly enhancing the reliability of seismic interpretation in similar depositional environments. The findings offer critical insights for improving reservoir identification accuracy and reducing exploration risks in rapidly deposited sedimentary basins.