Subsurface geomorphology of wave-dominated nearshore deposits: Contrasting styles of reservoir heterogeneity in response to shoreline trajectory

Abstract

Wave-dominated nearshore deposits are commonly regarded as laterally-continuous, homogeneous, sheet-like reservoirs that can be resolved at the parasequence scale. Many reservoirs, however, have an internal complexity that becomes clearer during production, indicating unexpected internal compartmentalization. Modern wave-dominated nearshore environments are extremely variable due to the effects of allogenic and autogenic processes. Relative sea-level change and shoreline trajectory has been indicated as key controlling factors on the distribution of sand in Quaternary coastal systems worldwide. In this paper we use 3D seismic data to describe the geomorphology of buried, Campanian and Eocene wave-dominated coastal deposits in two stratigraphic intervals within shelf-margin clinoforms in the Santos Basin, SE offshore Brazil. By applying seismic attributes and comparison with modern analogues we interpret the paleoenvironments and correlate them with well-log motifs and styles of shelf-margin trajectories in order to discuss the balance of allogenic and autogenic controls on nearshore reservoir heterogeneity. The sand-rich nearshore deposits appear in the seismic data as distinct topset amplitude anomalies recording shoreline transit across the shelf. The Campanian interval comprises reflectors formed in a predominantly normal-regressive context, while the Eocene interval depicts partially-eroded topset reflectors associated with forced regressions and retrograding reflectors associated with transgressions. Nearshore seismic-geomorphic elements were grouped in associations of strandplains, barrier islands/spits and lagoons, and partially-eroded nearshore. Strandplains are related sub-horizontal to ascending regressive clinoform trajectories where wider strandplains are associated with sub-horizontal trajectories. Nearshore erosion and fluvial incision are associated with descending trajectories within forced regressions, while barrier islands/spits and lagoons are associated with transgressive trends. Transgressive trajectories generated coastal systems with high mud content and discontinuous sand bodies along strike and dip, while forced regressions resulted in sand-rich systems that are complex and highly compartmentalized by subaerial erosion. The most attractive reservoirs are associated with normal-regressive intervals under low rates of relative-sea level rise, forming relatively homogeneous sand-rich strandplains.