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Styles of Pre-Salt Carbonate Platforms, Brazil Santos Basin


Pre-salt carbonates in Brazil have become a commercial success story with proven in place volumes reaching 130 GBO and production rates often exceeding 40,000 boe per day. The carbonate reservoirs are mostly preserved on depositional highs within an extensive lake system developed during the Barremian-Aptian. The lake system was the result of a complex extensional tectonics within a volcanic province; normal faults controlled the position and size of the highs and the position and depth of the lake centers. Normal faulting was intense during early stages of lake evolution and diminished during main phases of carbonate deposition, and with local post-salt depositional movement. Field-closures, as proxy for depositional highs, ranged from 50 to 1,000 Km2; 50% of are between 120 to 200 Km2 and the remainder are equally divided between <100 Km2 and >300 Km2. The size of the depositional high and the duration of faulting has major impact on carbonate deposition and results in different styles of carbonate platforms. A review of many of the Santos fields using 3D seismic data supports the grouping of carbonate platforms into several styles of deposition. Although the styles of deposition may characterize a field, most fields are characterized as having several depositional styles. Within a field, the style of carbonate platform may change temporally and/or geographically. One style of platform is dominated by seismically defined mounding. Mounds can be isolated, or organized forming ridges or clusters. Mound thickness varies 200 m to 500 m, with synoptical relief up to 700 m and slope angles of 10-30°. Often, the ridges are associated with faults. Mound clusters are common within platform highs and represent 40-50% of the area. Mounds are common in the Alagoas and generally appears to be larger in the Lower Alagoas (Aptian). A second style of platform is flat-topped with parallel reflectors grading to a shelf edge and slope/basin transition. The transition from flat-top to inclined reflectors can be very abrupt with mounding and slope angles of 15-200. Other flat-topped platforms have a gradual transition with slope angles of 5-80. The flat-top platforms show both progradational and retrogradational geometries. Collapse margins near faults are also common. Although reservoir quality is highly variable, preliminary observations suggest that the platform styles have similar facies associations and diagenetic trends that may allow for predictions of different reservoir quality scenarios.