Multiple scale characterization of sand-rich distal lobe deposit variability: Examples from the Annot Sandstones Formation, Eocene–Oligocene, SE France

Abstract

The present paper investigates the outcrop-scale architecture of submarine gravity flow deposits in both moderately to highly confined settings. A significant dataset was gathered on the sand-rich, confined, siliciclastic turbidite system of the Annot Sandstone Formation (SE France). We focused on thick sedimentary bodies formed by depletive gravity flows, in relatively distal settings (sand-rich lobes). The purpose was to investigate the architectural variability of their small scale components (beds and bedsets) and consequently to evaluate the associated heterogeneities (facies changes, grain size, sedimentary features, sediment thickness, etc.). Therefore, we provide a set of detailed descriptions on well-exposed outcrops, through very high- resolution correlation panels built thanks to multiple vertical logs. These turbidite system end-members have rarely been described in such details and have often been considered as laterally continuous and homogeneous. On the basis of the hierarchical subdivisions of turbidite lobes, results show a high complexity in internal architecture from lobe bed to lobe scale whereas large scale (scenery-seismic scale) is characterized by apparently homogeneous tabular and very extensive sandbodies (thus corresponding to the classical definition of “sheet-sands”). This complexity implies heterogeneities not only in terms of sediment thickness changes (abrupt pinch-outs and variable scale compensation features), but also in terms of facies and associated sedimentary features distributions (tractive structures, internal scours with associated by-pass, freezed mudclast-rich layers, etc.). This facies variability is controlled by the interplay between internal erosive bedforms (i.e. large scours) and constructive bedforms, such as dome-shaped or laterally stacked bedsets that are related to local topographic controls and compensation processes. Variability then appears both laterally and vertically. Besides, results also showed small scale evidences of interactions between turbidite flows and basin floor, only expressed at lobe bed to lobe element scale, especially on the basin sides where paleoslope is subject to erosion and remobilisation and where flows can undergo significant changes. Concerning both seismic and reservoir implications, although those deposits are characterized by a sheet-like geometry, this high internal variability necessarily implies heterogeneities in terms of petrophysical characteristics (porosity, permeability) that may have a significant impact on fluid circulation. These elements lead us to reconsider both sedimentary processes involved in their building and also reservoir models that can be established on field analogues.