Recent Development and Understanding of Evaporites on Regional 3D Seismic Data Regarding Sealing and Trapping Potential, Santos Basin, Brazil

Pyett, Matthew¹; Berryman, Jake G.¹; Borsato, Ron¹; Hawkins, Lisa¹; Martin, Mark²; Zhu, Hongwei¹
¹Reservoir, Petroleum Geo-Services, Weybridge, United Kingdom.
²Multi-Client, Petroleum Geo-Services, Weybridge, United Kingdom.

Introduction

Petroleum Geo-Services has merged 2D/3D seismic data in the Campos/Santos Basins of Brazil to review the offshore regional potential. The 3D alone encompasses 85,000 sq km made up of the Campos Basin (37,000 sq km) and the Santos Basin (48,000 sq km).

Geology

The Southern Brazilian Salt Basin comprises three sub-basins: Santos, Campos and Espirito Santo, all of which developed during the opening of the South Atlantic margin during the Lower Cretaceous (Barremian- c.130-125 Ma). Syn-rift clastic fill post-dates the flood basalts and; good source rocks are present within the Lagoa Feia/Picarras and Itapema Formations.

Source rocks are overlain by an early sag phase basin containing coarse clastic alluvial fans which grade laterally into shallow water carbonates, which formed the main pre-salt reservoirs, Lula is an example of these shallow water carbonates on a structural high, contained in a stratigraphic pinch out sealed by the overlying salts of the Ariri Formation.

Development in understanding the Ariri formation

The Ariri Formation has been dated to the Late-Early Aptian (c. 116-111 Ma). Deposition has been calculated to a period of 400-600 ka for the whole evaporite sequence. Deposition rates appear rapid but similar rates are known in Lake Assal (Imbert & Yann, 2005), Ethiopia & the Messinian evaporites of the Mediterranean (Clauzon et al. 1996). The absence of any carbonates or clastics also suggests rapid deposition. Basin margin topography was present but sediments did not deposit between salt cycles proven from recent wells. A pre existing depression was present aiding this rapid salt deposition (Davison, 2007). Early movement of the salt would have created differential loading over the basin allowing significant fault reactivation causing the final salt thickness to reach 4.5 km within local half grabens where only 1-2 km of salt was deposited.

Conclusions

This thick evaporite sequence has had a profound effect over the basin; each kilometre of salt reduces the geothermal gradient across the basin by 20°c allowing source rocks and reservoirs to still be in the oil window at greater depths. Deeper regions of the basin are blanketed with up to 2 km of salt. The salt acts as a great seal across the basin, also creating traps along faults and along stratigraphic pinchouts. Adding to the exploration and production of pre-salt plays, adding an estimated 8-20 bboe to Brazilian offshore reserves (Chakhmakchev & Rushworth, 2010).

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012