Predictive Distributive Deepwater Patterns within a Salt-Influenced Intra Slope Basin — Controls on and Expressions of Evolving Channelized Lobe Complexes and Their Variable Pinch-Out Terminations

Seismic-stratigraphic and -geomorphic analysis providing extraordinary geomorphology detail reveals the sedimentary fill evolution of a salt-influenced intra-slope basin along the South Atlantic Margin. Seismic-stratigraphic extraction and geomorphic visualization captures refined expressions of temporal and spatial organization as well as geomorphic transitions from feeder-channels to distributive channel networks and their genetically linked lobe-elements. Analysis of structural vs. erosional/depositional modification of basin topography suggests that underlying elongate salt ridges control placement of onset channel-branching nodes. Abrupt (narrow) divergent feeder-to-distributive transition zones are observed. This pattern typically correlates to salt-induced topographic contrast from higher to lower local gradient, resulting in depositional conditions for arriving gravity flows. Evolving paleo-topography and external intra-slope basin confinement controls channelized lobe’s placement as well as lateral and frontal termination patterns. In general, seismic termination patterns relative to salt-induced topography can be classified as: i) seismic onlap to static topography (non-kinematic); ii) seismic onlap to dynamic topography (syn-kinematic). In this study, salt driven intra-slope topography generation interacts with topography modification driven by gravity flow deposition. The latter mechanism has a healing effect on any established salt topography. The observed seismic termination patterns indicate that the topography-healing depositional processes operate with significantly higher rate than the salt-mobilization processes generating salt tectonic topography. The dominating termination pattern in the study area is seismic onlap of reservoir units to pre-existing static salt induced topography. Consequently the observed onlap patterns represent a non-kinematic/post-kinematic nature over the narrow time span of episodic sand-prone gravity flow deposition.Observation of tributary channel morphologies “plucking sediment” from syn-kinematic anticline flanks reflects re-sedimentation of unconsolidated sediments brought to an unstable gradient by salt-tectonic deformation. Complete tributrary-straight/bypass-distributary channel patterns are observed along and down-dip of such salt anticline flanks. Related secondary lobes can and should be differentiated from the more significant primary intra-slope basin channelized lobes. The documented temporal and spatial evolution of distributive channelized lobe and lobe complexes has direct exploration implications for intra-slope basin stratigraphic and 3-way trap potential and reservoir risk assessment. Furthermore, the documented generic nature of channel-lobe transition and distal/marginal termination may, with modification, apply in less confined basin settings.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California