The Pelotas Basin oil province revealed – new interpretation from long offset 2D seismic data

T he Pelotas Basin is a relatively unexplored hydrocarbon province comprising a 500,000 km² passive margin located on the southeast coast of Brazil and north­ern Uruguay (Figure 1). Over much of the area thick Tertiary and Cretaceous clastic deltaic and pro-deltaic sequenc­es are underlain by syn-rift and early-drift seaward dipping reflectors (Abreu, 1998) interpreted as Aptian and pre-Aptian age. The lack of post-Albian volcanics in this basin is markedly different to the more northerly Atlantic marginal basins such as Santos, Campos, Espirito Santo, and Sergipe basins. Sedimentation on the Pelotas Margin Since the inception of rifting between South America and Africa (at 125 Ma), the position of depocenters of paleo trib­utaries of the Rio de la Plata moved along the margin several times. Indeed the Pelotas Basin can broadly be divided into a dominantly aggradational southern delta and an aggradation and progradational northern delta, comprised of numerous discrete deltaic bodies inter-fingering and coalescing through time. The most recent depocenter in the southern delta forms a bathymetric feature named the Rio Grande Cone, which comprises a 4-km thick Tertiary siliciclastic sequence. This is the location of one of the world’s great gas-hydrate accumulations – discussed in the following section. Of the nine wells drilled in the Brazillian Pelotas Basin, five have encountered porous siliciclastic sands. The present-day coastline is lined with abundant quartz-rich sand grains that are derived from the locally eroding crystalline basement.