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Abstract

The Trois Evêchés outcrop of the Grès d’Annot (Annot Sandstones: Eocene-Oligocene, SE France) represents one of the world’s best exposed examples of a confined sandy turbidite system and has exceptional exposures of ‘channelised lobe’ or ‘amalgamated sheet’ sand-bodies. The sand bodies have simple tabular external geometries (lobe/ sheet-like) but show complex internal organisation characterised by a combination of scouring, bypass and aggradational features (channel-like). We have targeted the best exposed, most laterally continuous sand body (termed the FB unit) for very detailed studies at a scale appropriate to reservoir modeling. The result is a west northwest-east southeast panel 35 m thick, 1700 m long parallel to palaeocurrent direction (‘downdip’), within which all zones are characterised in terms of key properties (grain-size, sorting, cementation, and primary and secondary structures) and all key surfaces are absolutely correlated (i.e., directly physically traced). This unique database allows quantification of lateral facies relationships and deterministic definition of both architectural components and stratal hierarchy. The FB body is a thick, tabular, high-net:gross unit having a simple tabular external geometry but a complex internal structure. Key observations are that: (1) the system shows considerable lateral variability; (2) the well-defined base of the FB sand body is a different genetic surface in different places; (3) a few beds and surfaces are laterally persistent but, because of erosion, many are not traceable for more than 100-200 m; (4) packages within the sand body are erosive based and vary laterally in thickness, having an element of compensation between successive packages; (5) the erosive-based packages show a back-stepping arrangement of successive points of deepest erosion (i.e., migrating towards the east-southeast, uppalaeocurrent). The FB sand body appears relatively uniform at a distance but in detail contains numerous heterogeneities at variable scales which would have a considerable impact on fluid flow through an apparently homogeneous sandstone.

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