Sedimentation-controlled double BSRs and implications for paleo hydrate dissociation in the Shenhu area, south China sea

Abstract

Double/multiple bottom-simulating reflectors (BSRs) have been discovered worldwide, but their preservation and formation mechanism remain unclear because most studies are based on seismic data, lacking direct evidence like well logging data. This study introduces a novel well log-based interpretation of double BSR, suggesting that the secondary BSR (BSR2) is the interface between little residual gas and the overlying water-saturated sediment, and the free gas is trapped by capillary seals. The BSR2's amplitude is influenced by the nearby porosity, with lower porosity resulting in stronger reflections. Seismic attributes reveal a dynamic process of hydrate cycle that hydrate dissociation occurs above BSR2, followed by gas migrate to form a new BSR1. The upshift of BSR2 is mainly controlled by sedimentation rate, which varies across different regions, leading to larger upshifts in areas with higher sedimentation rates. Moreover, BSR2's upshift is negatively related to the reflection magnitude of BSR2, because rapidly depositing sediments with higher porosity and lower percolation threshold allow lesser gas sealed beneath the BSR2. Our findings suggest that double BSRs are predominantly formed in muddy sediments under rapid geological events like rapid sedimentation. However, each BSR can be preserved as well as observed to indicate that the BSR has existed for an extended period of time during which the geologic environment has been relatively stable, e.g., low sedimentation rates. This detailed sedimentation-controlled BSR adjustment provides new insights for the interpretation of double BSR and the paleo hydrate dissociation.