Gas Migration and Slope Instability in the Danube Fan: Insights from integrated OBS-MCS Seismic Analysis

Abstract. Gas hydrates and deltaic deep-sea fans are main features in continental margin systems, influencing slope stability, fluid migration, and carbon cycling. In the northwestern Black Sea, the Danube Fan remains poorly constrained with respect to subsurface structure, sediment strength, and hydrate dynamics. Here, we present high-resolution multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data to characterise sedimentary structure and fluid-related features. Two integrated OBS–MCS profiles reveal underconsolidated, clay-rich levee deposits interspersed with mass-transport units, chaotic facies, and gas-related anomalies. Derived P- and S-wave velocity models indicate low shear strength and high Vp/Vs-ratios in shallow units, consistent with soft, water-saturated sediments. Deeper layers display compaction-driven velocity increases but remain mechanically weak, rendering the slope prone to failure. Our findings suggest that vertical gas migration is widespread, expressed by seismic chimneys, polarity reversals, and velocity pull-downs, with free gas confined below bottom simulating reflectors and in stratigraphic traps. Hydrates likely occur as sparse, patchy pore-filling accumulations, and the lack of S-wave velocity anomalies suggests they do not act as cementing phases, implying little direct influence on sediment strength or slope stability. The hydrate system appears hydrate-poor, possibly reflecting post-glacial re-equilibration. Overall, lithology, gas migration pathways, and high sedimentation rates emerge as primary controls on hydrate formation and slope instability in the Danube Fan.