Bottom-current reworked Palaeocene-Eocene deep-water reservoirs of the Campos Basin, Brazil

Abstract

Deep-water reservoirs consisting of turbiditic sandstones moderately to heavily reworked by bottom currents are common in canyon-and trough-filling deep-water (bathyal) Palaeocene-Eocene sequences of the Campos Basin, offshore southeastern Brazil. A number of wells with conventional logs, together with cores, provided the database for the study. Seismic data provide additional support, but low resolution and noise hamper detailed analysis. The sandstones presenting better reservoir quality in these sequences are interpreted as being deposited by turbidity currents, as suggested by the dominance of unstratified normally graded sandstones, with grain sizes ranging from fine to coarse sand, and low clay-matrix content. Sandstones interpreted as bottom-current deposits (mid-water contourites) form poor-quality reservoirs, or baffles and barriers. These rocks are commonly moderately to heavily bioturbated, with variable, frequently high, clay-matrix content. Common trace fossils include Planolites, Palaeophycus and Zoophycos.Locally, these sandstones show faint horizontal stratification and planar cross-stratification. Contourites with thickness ranging from a few decimetres to several metres occur intercalated with turbiditic sandstones. Because they present distinct reservoir qualities, the mapping of the limits between turbidites and contourites is critical for adequate reservoir characterization. Most of this mapping has been performed using well information, constrained by outcrop analogues. The currents responsible for reworking turbiditic sands are interpreted to be deviated geostrophic currents, with velocity enhanced in narrow canyons and troughs.

MTD distribution on a ‘passive’ continental margin: The Espírito Santo Basin (SE Brazil) during the Palaeogene

“…On the well documented Campos basin, Eocene strata similar to those in this study consists of an alternation of turbidite flows, masswasting and bottom-current reworked deposits that led to the development of lensoid sandy reservoirs confined by MTDs (Moraes et al, 2007). Reservoirs in the Campos Basin have very immature lithologies, reflecting a tectonically-active setting from where the sediments are derived (Fetter et al, 2009).…”

“…Resulting strata, commonly termed as Mass Transport Deposits (MTDs), play an important role in shaping the seafloor (e.g. Gee et al, 2006;Moraes et al, 2007;Moscardelli et al, 2006: Moscardelli andWood, 2008). Newly deposited MTDs can infill seafloor depressions or, instead, create new bathymetric relief that will influence later sediment distribution in a basin, controlling the geometry and lateral continuity of reservoir lithologies and stratigraphic traps (Armitage et al, 2009;Moscardelli et al, 2006;Posamentier and Kolla, 2003).…”

Distribution and characterization of failed (mega)blocks along salt ridges, southeast Brazil: Implications for vertical fluid flow on continental margins

“…The Abrolhos Formation comprises vertically stacked MTDs intercalated with siliciclastic and volcaniclastic deposits [Fiduk et al, 2004;Gamboa et al, 2010;Mohriak, 2005] (Figures 1 and 2). Significantly, similar units occur in the Campos basin, where Eocene sand bodies intercalated with MTDs constitute viable reservoirs [Moraes et al, 2007].…”

“…Over growing salt diapirs, overburden strata is commonly either thinned or completely removed by erosional processes, which accumulate eroded strata in peripheral salt withdrawal basins [Giles and Lawton, 2002]. Mass transport deposits (or MTDs) resulting from these erosional processes are usually mud dominated and have low exploration potential [Lee et al, 2004;Moscardelli et al, 2006;Posamentier and Kolla, 2003], except when sand-rich strata is present [Armitage et al, 2009;Beaubouef and Abreu, 2010;Davison et al, 2000b;Dunlap et al, 2010;Moraes et al, 2007;Moscardelli and Wood, 2008;Piper et al, 1997;Shanmugam et al, 1996;Tripsanas et al, 2008]. Important factors controlling the seal competence of MTDs include the presence of a strong structural fabric (including faults) in their interior, and variations in the degree of remobilization experienced by failed strata [Frey-Martínez et al, 2006].…”

The economic importance of contourites

“…Less frequently documented, but also important in appraisal projects, are the internal heterogeneities in oil-bearing reservoirs that can be related to the action of bottom currents. Moraes et al (2007) have presented an example of a giant oilfield in the Campos Basin where thick packages of oil-rich sandstones deposited in structural troughs are separated by an extremely bioturbated anisotropic silt facies, interpreted as being the product of bottom-current reworking along the trough (see Moraes et al 2007, Fig. 5).…”