Salt Mechanics and its Impact on Pre-Salt Reservoirs in the Santos Basin, Brazil

Abstract

Innovative seismic reprocessing from the Santos basin, Brazil produced enhanced seismic images that provide insights into the flow behavior of the Albian layered evaporite sequence (LES) and impacts the understanding of salt displacement at passive margins, as well as the risk mitigation of underlying pre-salt hydrocarbon reservoirs. The study area covers the San Paulo Plateau, a rift-related basement high which is overlain by three tectono-stratigraphic domains separated by salt architecture: The inboard domain is the Albian Gap, a ~40 km wide zone of roll-over structures that apparently displaced most of the Albian layered evaporite sequence (LES). It 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 and other evaporites of the LES indicate complex internal folding while rather 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 towards the distal section of this domain. The folding intensity 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, the major reservoir rock in the basin, laterally interfering with the LES and/or other pre-salt deposits. These critical observations are based on detailed velocity model building workflows including refraction/ reflection FWI resulting in high resolution 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 #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018