f Rift-Associated Magmatism in the Sergipe-Alagoas Basin and its Influence on Local Petroleum System

Sergipe-Alagoas Basin is located in Brazil’s northeast region and is one of many basins formed as a result of West Gondwana break-up. It has a well-known onshore portion that have being drilled since 1939 and contains an exposed and complete geological record since Jurassic’s pre-rift sequences (Souza-Lima et al. 2002 and Campos Neto et al. 2007 ). The shallow water region is also well described, being a pioneer in hydrocarbons discoveries at Brazil’s offshore in 1968. The constitution of ultra-deep-water sequences that occurs below the drift sequences, however, is extensively debated since 90’s works and will be the focus of this work. It was already interpreted as containing (i) a continuation of the pre-rift + berriasian-aptian rift sequences ( Pontes et al. 1991 ) and as (ii) thinning of the rift sequences with seaward dipping reflectors (SDR) marking the emplacement of oceanic crust ( Mohriak et al. 1997 , 1998 , 2000 ).

During the latest exploratory phase taking place in the basin, the wildcat well 1-SES-158 (1-BRSA-851-SES, as named by regulatory agency - ANP) brought more clarity to this matter, when a sequence of eoalbian pillow basalts (104Ma) was drilled and indicated alkaline composition ( Caixeta et al. 2015 ). Recent works incorporating this data proposed that Sergipe-Alagoas basin has been affected by multi-phased rifting processes at least until Albian ( Caixeta et al. 2015 ). Whereas in onshore and shallow water portion no evidence of magmatism was identified and salt deposition occurred, in ultra-deep water there are evidence of vulcanism covering a hyperextended continental crust ( Caixeta et al. 2015 ). This model proposes that the region where magmatism occurred at first corresponded to a basement high, caused by the rise of mantle material that limited the transitional sequences deposition in the proximal portion. This region was submitted then to hyperextension and volcanic and volcano-sedimentary sequences filled generated depocenters. Syn-depositional tectonism during transitional sequences deposition (aptian-albian sequences) was also described in both onshore and offshore portions, especially close to the basin faulted border and along aptian hinge line ( Cruz, 2008 ).

The crustal hyperextension resulted in a variety of volcanic geometries whose development culminated on the oceanic crust formation. These geometries include the earlier identified SDR ( Mohriak et al. 1997 , 1998 , 2000 ), lava-deltas, submarine mounts and volcanoes that provide interesting information about the final rifting dynamic as well as the environment conditions they were formed. The resulting relief has an important role in recently exploration of ultradeep portion of this basin since it has a strong influence in its petroleum system: either by forming depocentres for marine source rocks deposition or traps development for hydrocarbon in turbidites through differential compaction process. This subtle resulting structuration might have played an important role in oil and gas discoveries in offshore of Sergipe-Alagoas basin and is an key analogue for post-salt opportunities in basins without any type of strong tectonism forming traps.