Fault sealing capacity of mudstone caprocks under contrasting brittle–ductile regimes

Evaluating the vertical sealing capacity of faulted mudstone caprocks is critical for assessing fluid preservation and trap integrity. However, common quantitative indices, such as hydrocarbon fault-connection thickness (Hfc) and Shale Smear Factor (SSF), often fail to provide reliable sealing thresholds within the brittle–ductile transition zone. To address this, we propose an integrated quantitative framework combining rock mechanical behavior with fault-zone compositional variations. We take the Central Structural Belt of the Xihu Sag (East China Sea Basin) as a case study. Using an integrated stress characterization based on core and logging data, calibrated with core measurements, we optimize log-derived predictions of rock brittle–ductile behavior and quantitatively define critical brittle–ductile transition depths of 3150 m and 8250 m. Subsequently, we incorporate the Shale Gouge Ratio (SGR) into an enhanced Fault-Throw-Knipe-Diagram (FKTD) to continuously quantify vertical shale content along fault planes. FKTD analysis effectively eliminates the threshold ambiguity inherent in isolated Hfc and SSF evaluations. Furthermore, it reveals a depth-controlled fluid flow system: open faults in structural lows serve as vertical fluid conduits, while sealed caprocks in shallower structural highs prevent upward migration. Furthermore, our analysis indicates that the clay content threshold required for effective fault sealing increases systematically with regional tectonic intensity. These findings establish a robust, geomechanically constrained predictive model for fault seal evaluation, providing new insights for deep petroleum exploration and secure CO2 geological storage in tectonically active basins worldwide.