Timelapse of the geomorphologic and stratigraphic evolution of a Late Cretaceous deep-sea fan, northern Santos basin, Brazil

Abstract

Deep-sea fans are the terminal sinks of clastic sediment, nutrients and pollutants produced in continental landmasses. They are also major targets for oil and gas on continental margins. Several recent studies have applied seismic geomorphology to delineate deep-sea fans, however, most of them describe a relatively short period of the geologic time, disfavoring the observation of long-term geomorphologic changes and the resulting depositional architecture. We analyzed the geomorphologic evolution and architecture of the Maastrichtian, deeply buried, Maricá deep-sea fan in northern Santos Basin, offshore SE Brazil. Using three-dimensional seismic data and logs from three wells, we mapped five horizons (stratal slices), including fan base, fan top and three internal horizons. The examination of lobes and channel networks on each horizon resulted in a timelapse that capture major geomorphologic transformations of the deep-sea fan from inception to abandonment. The Maricá deep-sea fan is a sand-rich system deposited directly onto the irregular topography of the Maricá mass-transport deposit, and its geometry is also influenced by structural highs uplifted by active halokinesis. Six lobes were mapped, which present variable growing patterns and associated stratigraphic signatures indicative of aggradation, progradation, retrogradation, and lateral switching. Four types of channel morphology were identified (straight, sinuous, meandering, and braided-like). Channel length and sinuosity tend to increase with time in the whole fan and within individual lobes. Amplitude maps and well-log motifs suggest that sandy sediments occur mainly as channel-fill deposits and that different channel morphologies produce contrasting stratigraphic architectures and reservoir heterogeneities. Our results show that channel network and sand-body architecture evolve in a predictive fashion mostly controlled by fan aggradation. However, the interaction with the underlying topography causes morphologic changes at the lobe and depositional element scale that often deviate from the overall trend and affect sand distribution across the fan surface.