Study of Deep-Water Sand-Rich Lobes Variability: Examples from the Annot Sandstones Formation, Eocene-Oligocene, SE France

Etienne, Samuel ¹; Mulder, Thierry ¹; Razin, Philippe ²; Joussiaume, Rémi ²; Tournadour, Elsa ²; Teboulle, Oury ¹; Bez, Martine ³; Desaubliaux, Guy ⁴
(1) UMR CNRS 5805 EPOC, Université de Bordeaux, Talence, France. (2) Institut EGID, Université de Bordeaux, Pessac, France. (3) Total CSTJF, Pau, France. (4) GDF SUEZ, Paris - La Défence, France.

The siliciclastic Annot Sandstones formation (SE France) is composed of a thick series of various types of gravitary deposits and represents the Late Eocene to Early Oligocene northward infill of relatively small foreland basins developed in front of the Alpine orogen. The resulting turbidite system has been considered as a very good analogue for confined sand-rich systems (e. g. Cretaceous reservoirs of Campos Basin, offshore Brazil). It can be summarized as intraslope sub-basins fed by proximal fan deltas leading to transit channels in sub-basins confined zones, distally evolving to depositional lobes and sheets in less confined zones. This study brings new quantitative data on these last architectural elements and focuses on their internal architecture from depositional event scale to elementary object scale. A particular attention has been given to their small scale variability and a significant dataset (made of very high-resolution, multiple scale correlation panels) has been gathered in the Lauzanier, Mont Tournairet and Trois Evêchés outcrops. On the basis of the hierarchical organization of turbidite lobes, results show a high complexity from lobe bed to lobe scale in moderately confined areas and from lobe bed to lobe system in highly confined areas whereas large scale (architectural element/seismic scale), is characterized by apparently homogeneous tabular sandbodies (“sheet-like” sandstones). This complexity implies heterogeneities not only in terms of sediment thickness changes (pinch-outs in onlap zones or compensation features), sedimentary facies and structures distributions but also reveals a large type of individual elementary elements such as internal channels or finger-like lobes. A longitudinal distribution model of elementary objects (from proximal vertically stacked channelized lobes to distal tabular lobes) and associated heterogeneities has been established. Those features have not been accurately described in sand-rich turbidite deposits so far. This high internal variability necessarily implies heterogeneities in terms of petrophysical characteristics (porosity, permeability) and reservoir connection that may have a significant impact on fluid circulation. These results allow reconsidering both sedimentary processes involved in sand-rich lobes and also reservoir models that can be established on field analogues.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.