Revisiting Salt Mechanics in the Light of Enhanced Seismic Imaging in the Santos Basin, Brazil

Abstract

Enhanced seismic imaging based on innovative reprocessing of seismic data from the Santos basin, Brazil, indicates flow behavior in the Albian layered evaporite sequence (LES) that impacts the understanding of salt displacement, and hence the risk mitigation of underlying hydrocarbon reservoirs associated with the pre-salt section.

The study area has been subdivided into three tectono-stratigraphic domains, with the inboard extensional domain characterized by the Albian Gap, an approximately 40 km wide zone of roll-over structures that apparently displaced most of the LES. The Albian Gap terminates along predominately counter regional faults against a thickened section of Albian LES. Frequent down building mini basins dominate this central domain and are occasionally welded against pre-salt sequences. Highly reflective layers of Anhydrite or other evaporites of the LES indicate complex internal folding while transparent sections are interpreted as accumulations of intra-formational mobilized halite. The occurrence of mini basins diminishes in the adjacent, outboard extensional domain that reveals increasingly well imaged folds within the LES which are gradually transient into asymmetric and then upright folds in the distal section of this domain.

In general the intensity of folding increases towards the top of the LES whereas the base appears unaffected, as evident by flat reflectors associated with a prevalent basal anhydrite. In addition, intensive late-stage faulting at the base of the Albian Gap and LES contribute to a rugose morphology, which is further enhanced by carbonate build ups such as Microbiolites, Coquinas or Travertine, the major reservoir rock in the basin, laterally interfering with the LES and/or pre-salt deposits.

These critical observations are based on detailed velocity model building workflows inclusive of refraction and reflection FWI and well tie analysis resulting in high resolution enhanced seismic imaging of the pre-salt section. It indicates little to no lateral movement along the base of LES and provides no evidence for a distinguished detachment fault as suggested by common models accounting for the genesis of the Albian Gap. The lateral flow component in the LES appears to be rather related to a continuous flow profile with particle movement increasing toward the top of the LES. The suggested process for the lateral flow in the LES is interpreted as displacement loading caused by the progressive formation of the Albian Gap.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018