Detailed Characterisation of Lateral Heterogeneities in Exceptionally Exposed Sand-Rich Turbidite Outcrops from the Grès d'Annot, SE France: Stratal Continuity and Reservoir Simulation

Lomas, S.A., B.T. Cronin, A.J. Hartley, Davide Duranti, Andrew Hurst, Emma Mackay, S.J. Clark, Department of Geology and Petroleum Geology, University of Aberdeen, King's College, Aberdeen AB24 3UE, UK, and Sean Kelly, Shell U K E&P, Altens Farm Road, Aberdeen AB12 3FY, UK

Abstract

The Trois Evêchés outcrop of the Grés d'Annot (Annot Sandstone) represents one the world's best-exposed examples of a confined sandy turbidite system. The Grés d'Annot is a large Eocene-Oligocene sand-prone turbidite basin-fill now exposed as a series of basin remnants fringing the southwest Alps in southern France. The Trois Evêchés remnant has exceptional exposures of sub-seismic to seismic-scale architectural geometries which, despite the scale and quality of the outcrop, have seen little previous work. Our work here shows the Grés d'Annot to be up to 980 m thick with an overall 'net:gross' >75%, a mean sandstone bed thickness of 0.70 m, and a mean sand-body thickness of 26.1 m. 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-bodies for very detailed studies at a scale appropriate to the reservoir modelling scale. A key result is a panel 35 m thick, 1700 m long ('downdip') within which all zones are fully characterised in terms of key properties (grain-size, sorting, cementation, primary and secondary structures) and all key surfaces are absolutely correlated (i.e., directly physically traced). This unique database allows quantificatio n of lateral facies relationships and deterministic definition of both architectural components and stratal hierarchy. Numerical simulations of this outcrop panel are compared with an analogous turbidite field from the central North Sea to evaluate reserv oir modelling constraints and best practice in high-N:G turbidite reservoirs, where the scale and distribution of intra-reservoir heterogeneities are believed to have critical effects on production. Particular emphasis is placed on the modelling and produ ction impact of reservoir heterogeneity on long-reach horizontal wells.