Print this pageUpper Cretaceous Clastic Reservoirs of the Santos Basin: Depositional Style and Sequence Stratigraphy
Upper Cretaceous sandstones hold significant gas reserves in the Santos Basin of offshore Brazil, including fields such as Merluza, Lagosta and Mexilhão. The most prolific reservoirs are deeply buried, tight sandstones of turbiditic origin. Stratigraphic analysis based on seismic, well logs and cores has been carried out in order to determine and predict the spatial and temporal distribution of these reservoirs in regional scale. The lower section of the Santonian/Early Campanian depositional sequence is composed of a thick lowstand deltaic complex generated by supply dominated alluvial systems that formed at the basin margin in response of a Late Cretaceous to Tertiary uplift phase. The high amount of sand delivered to the basin by rivers in flood formed thick and extensive turbidite units. Enhanced accommodation in the shelf was mainly controlled by mini-basins created due to the displacement of Aptian salt. Sand-rich fans and, in less instance, isolated channels, take place sandwiched within muddy, highly bioturbated, frequently slumped prodelta strata. Such sand-rich fans can be considered depositional elements of the deltaic systems since they lie on the bottomsets of delta clinoforms and, if seen in well logs, stack vertically as progradational successions. The sandstones show plant fragments concentrated near the top of event beds, testifying the fluvial heritage. Muddy intraclasts and shell debris are also present, being evidence of submarine erosion by the passage of turbulent hyperpycnal flows. Loading promoted by strong sediment supply was, at least partially, responsible for salt domes at the basinward edges of the deltaic systems. Most of the prodelta sand-rich fans were kept behind these positive structures but some fans, probably those deposited during the final stages of delta progradation, passed the salt domes reaching more external depocenters. A thick transgressive to highstand mud-rich package is a regional seal for the turbidite reservoirs. Highstand siltstones and shales may form a more than one thousand thick progradational wedge within some mini-basins, providing excellent sealing conditions.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil