Submarine fan morphology and lithology distribution: a predictable function of sediment delivery, gross shelf-to-basin relief, slope gradient and basin topography

Abstract

Over the last decade comprehensive exploration for turbidite plays has been rewarded by a significant number of hydrocarbon discoveries. However, the industry is still critically challenged with respect to appropriate and refined prediction of sand/shale distributions within these depositional systems. Along with a strong and sustained commercial interest in these plays, extensive subsurface datasets, including a new generation of core- and well/log-calibrated seismic facies suites, have become available. Based on this valuable information, systematic 2D and 3D seismic sequence stratigraphical analysis with particular focus on seismic facies and seismic geomorphology was performed in five different basins along North and South Atlantic margins. Each subsurface example was characterized with reference to its relative sequence stratigraphic order within a hierarchy. Evolving geological parameters like shelf accommodation, shelf-break position, slope gradient, slope and basin bathymetry/topography and third-, fourth- and occasionally fifth-order stacking patterns were assessed within second-order cycles in each case. Based on these observations, recognition criteria for subsurface lithology prediction were derived. Conceptual knowledge from outcrop analysis was an essential and integral part of the interpretation process throughout the study. The subsurface recognition criteria, highlighted in this study, emphasize similarity and variability between two end-member basin styles: high shelf-to-basin relief/sediment under-filled basins where sand-prone nearshore systems do not prograde over deep-water fan systems in a second-order cycle context; low shelf-to-basin relief/sediment over-filled basins where sand-prone nearshore systems do prograde over deep-water fan systems in a second-order cycle context. The most striking contrast with respect to deep-water reservoir sandstone distribution in the two basin categories relates to the fourth- and fifth-order stacking patterns of fan building-block cycles within second- and third-order cycles, which tend to show low progradation/aggradation ratio in the high relief basins and high progradation/aggradation ratio in the low relief basins. Given an understanding of geological context, these seismically identifiable patterns provide more accurate prediction of sand and shale distribution within deepwater clastic systems.