"Deep-Water Sedimentation in Salt-Influenced Basins: Insights Beyond the ‘Fill and Spill’ Model"
Dr. Clara Rodriguez
Exploration Geoscientist, Schlumberger WesternGeco
Understanding the spatial distribution and temporal evolution of deep-water depositional systems relative to salt-related deformation is key to improving the ability to predict reservoir presence, architecture and potential trapping styles. Previous studies suggest that deep-water sedimentation progressively fill or ‘heal’ highly-mobile salt-influenced slopes; i.e.: classical ‘fill and spill’ model. The stratigraphic development of salt-influenced sedimentary basins offshore Angola, Brazil and Mexico, displays many of the characteristics captured in the ‘fill and spill’ model. Our analysis of 3D seismic stratigraphy and geomorphology offshore Angola and Brazil indicate that spilling between minibasins tends to occur in locations where salt-cored structures trend perpendicular to sediment transport, whereas long-distance, axial confinement characterizes locations where salt structures are oriented parallel or at a low angle to sediment transport.
Furthermore, our seismic-based analysis of long-term (> 60 Myr) stratigraphic development in the Santos Basin indicates that the pattern of minibasin filling can be influenced by minibasin welding and growing intra-minibasin structures leading to deep-water systems being diverted away from updip minibasins, and resulting in downdip minibasins filling first, contrary to the classical ‘fill and spill’ model. Furthermore, a key result of our work is that post-depositional growth of salt diapirs can cause dissection of previously through-going depositional systems, resulting in ‘pseudo-onlap’ onto the diapir flanks. This process has been overlooked in previous studies focusing on the short-term development (< 1 Myr) of relatively young (i.e. Neogene-Quaternary) systems presently still overlying or occurring on the flanks of salt structures.
Tectonically active salt-influenced basins offshore Mexico are characterized by a complex and counter-intuitive depositional pattern, which is not captured by the ‘fill and spill’ model. The structural and stratigraphic complexity in Salina del Istmo Basin is illustrated by the folding, thrusting and dissection of deep-water depositional systems; thus, it is unclear how salt tectonics, alone, and in combination with orogenic-driven shortening controlled sedimentation. Nevertheless, detailed 3D seismic analysis suggests primary salt movement locally (10s km) controlled sedimentation similarly to what has been observed offshore Angola and Brazil.
Our results highlight key aspects of the filling and depositional patterns in highly mobile substrate where structures continue to grow and deform after the deposition of any specific stratigraphic interval. These observations have direct applied implications and should be considered when understanding reservoir distribution in salt-influenced basins.