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Lithology, Stratigraphic Architecture and Salt-related Structural Styles within the Enigmatic Evaporites of the Aptian Ariri Formation, Santos Basin, Offshore Brazil

Rodriguez, Clara; Jackson, Christopher A.; Bell, Rebecca; Rotevatn, Atle; Francis, Malcolm

The Santos Basin holds some of the largest hydrocarbon discoveries made in the last decade. The reservoirs are located beneath an up to 2 km thick evaporitic sequence, the Aptian-age Ariri Formation, which acts as the main seal in the basin. This thick salt layer is characterized by a variety of structural styles attributed to gravity driven deformation during passive margin development. Even though several wells have been drilled through the salt, the stratigraphic heterogeneity and its impact on salt-related deformation are poorly understood.

We have integrated and analysed 16000 km2 of time-migrated, 3-D seismic reflection data and well logs to understand the lithology, stratigraphic architecture and structural styles of the enigmatic Ariri Formation. Seismic facies are used to identify and map key stratigraphic intervals and wireline log data allow us to probe the sub-seismic character, indicating that it is composed of interbedded halite, anhydrite and K-Mg salts. Seismic attributes analysis and seismic inversion were also utilized to identify lithology variations and salt-related structural styles away from areas of well control. Individual layers of evaporites at the wells were also correlated with a relative acoustic impedance volume.

Overall, four key stratigraphic intervals can be recognised based on seismic facies, thickness variations and well log character within the Ariri Fm. Our observations suggest syndepositional deformation and a multi-stage tectono-stratigraphic history for this sequence characterised by: (i) movement of a basal, variably thick, chaotic halite-rich unit and the formation of a series of minibasins filled and capped by (ii) a thick strongly-reflective evaporites interbedded unit (iii) minor movement of an upper halite-rich unit; and (iii) filling of remnant relief by strongly-reflective stratified unit. The latter thins and it is truncated near the larger salt ridges, possibly due to post-depositional, thin-skinned extension in the Albian.

We suggest a close relationship between the depositional stratigraphy, initial salt basin configuration and the initiation and further development of salt-related structural styles in the Santos Basin.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013