Analysis of Layered Evaporites Within the Santos Basin, Brazil

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Abstract Layered evaporites have been deposited at many times during the earth's past and occur in many places around the world. Quite often the layered evaporites have been deformed by sediment loading, gravitational stresses, or tectonic processes. Deformation of evaporites typically produces a continuum of structures like salt rollers, salt pillows, and salt diapirs which may further evolve into salt tongues, salt sheets, and salt canopies. There may even be multiple generations of this evolution as seen in the Gulf of Mexico. However, in a large part of the deep water Santos Basin, particularly the area of the Sao Paulo Plateau, something fairly unique is seen. In this area the majority of deformation is contained within the layered evaporites. Updip extension, sediment loading, and gravitational stresses have combined to produce a contractional environment on the Sao Paulo Plateau. Contraction may manifest itself in single isolated folds, simple fold trains, isoclinals folds, asymmetric folds, recumbent folds, or thrusted folds. Multiple detachment zones within the layered evaporite partition strain so that more competent intervals deform disharmonicly. This allows the possibility of stacking different strain styles, such as anticlinal structures over synclinal structures. It also allows for polyharmonic folding patterns. Inflation of salt does occur on the Sao Paulo Plateau. However, typical diapiric structures that occur to the northwest and west are uncommon in this area. Inflation more often occurs where fold structures are stacked or thrusted, building into salt walls generally oriented north-south or northeast- southwest. Allochthonous features also occur but are rare in this data set and seen are only in the most eastern part of these data. Introduction The deep water Santos Basin is the site of numerous recently announced petroleum discoveries such as Jupiter, Tupi, Parati, Carioca, Bem-Te-Vi, Caramba, Guara, Iara, and others. All of these discoveries lie within lease blocks BMS-8-9-10-11-21-24 (Figure 1). These lease blocks and intervening areas are covered by several 3D PSTM and PSDM seismic surveys collectively known as the " Cluster??. The Cluster data was collected beginning in 2001 and covers nearly 23,200 km2 in water depths of 1,900 m or greater. The western edge of the Cluster is only about 50 km basinward of the shelf margin but the majority of the data lies 100 to 200 km farther out. Tupi was the first pre-salt discovery announced by Petrobras and BM-S-11 block partners BG and Petrogal in 2007. The petroleum industry gave immediate attention to the deep water Santos Basin when the reserves for Tupi were reported as 5-8 billion BOE (Rigzone, 2007). Tupi was quickly followed by a string of additional discoveries made by Petrobras in partnership with various other companies. Although Petrobras has attempted to be reserved, industry analysts have tossed out potential reserves from 33 billion BOE to 70 billion BOE for the trend (Oil Shalegas.com, 2008; UK.reuters.com, 2008). These are staggering numbers and clearly form the basis of the industry's strong interest. Between the sea floor and the pre-salt reservoir lies an interval of Aptian age layered evaporites and Albian to Late Cretaceous or younger sediment overburden. Over much of the Cluster data the evaporites are 2 km thick, although thickness is variable. Also over much of the Cluster data the evaporites are intensely internally deformed. Companies exploring for petroleum in the Gulf of Mexico know well the problems of imaging sub-salt reservoirs. Imaging the reservoir is an even greater issue for development planning. Although it could be argued that seismic data in the Santos Basin image the section below salt better than in the Gulf of Mexico, anything that can interfere with reservoir imaging can affect development costs and reduce the bottom line. Exploration is inherently risky as attested by three recently announced high profile dry holes, Guarani, Corcovado-2, and a third well in BM-S-17(Rigzone, 2009; Rio Times, 2009; Smith, 2009). Thus, an understanding of cause and effect in deformational processes within layered evaporites should improve risk assessment for exploration prospects and contribute to improved seismic imaging of the pre-salt stratigraphy.