JOSEPH, P., R. ESCHARD, D. GRANJEON, 0. LERAT, C. RAVENNE - IFP

Abstract: The Annot Sandstone Outcrops (French Alps): Quantification of Sedimentological Organization and 3D Reservoir Modelling

Detailed field studies at a reservoir scale have recently been undertaken on well-exposed 3D outcrops of the Eocene Annot Sandstone in the French Alps, in order to analyse and reconstruct the architecture of deep-marine gravity deposits (slumps, turbidites and debris-flows). This series corresponds to a sand-rich turbiditic system up to 1000 m thick, which has filled large morphologic furrows induced by alpine tectonic activity.

The studied sites have been chosen in order to cover a large range of sedimentary settings (proximal, mid and distal fans), morphological configurations (narrow furrow controlled by faulting, unconfined basin), and sources of sediment (Southern Corso-Sardinian massif, Eastern Alps).

The reservoir architecture has been reconstructed from the measurement of vertical sedimentological sections, the mapping of sand units, the calibration and interpretation of continuous photo-mosaics of the outcrops. In a sequence stratigraphy framework, a quantification of the size and vertical / horizontal distribution of facies and architectural elements has been performed, then 3D grid blocks of the geometry and facies distribution of the reservoir units have been reconstructed, using geomodelling and geostatistical tools.

These 3D models may be used to optimize the development of specific deep-water reservoirs, by providing guidelines for correlation, and giving access to quantitative parameters used for the stochastic reservoir modelling: geometrical characteristics of reservoir bodies (channel, lobes) or permeability barriers (shale breaks, calcite concretions); geostatistical parameters of the lithofacies distribution (proportion, correlation length ... ).

The detailed studies of various sites evidence the distinct features of the different parts of the Annot Sandstone system.

The westward system (corresponding to the Annot and Trois Eveches sub-basins) is mainly fed by Corso-Sardinian material coming from South. The proximal area (Annot, Fig. 1) is very sand-rich and related to the infill of a confined fan valley, trapped in a narrow depression due to the folding of the older << Nummulitic Limestone >> and << Blue Marls >> deposits (Fig. 2). This valley is characterized by deeply incised channels, filled by coarse-grained to pebbly high-density turbidites, which laterally develop terrace deposits (thick sand beds alternating with heterolithic fine-grained low-density turbidites). The heterogeneity inside the sand package mostly corresponds to shale breaks within thin-bedded turbidites and mud-clast breccias at the bottom of amalgamated channels.

The distal part (Chalufy, Fig. 3) is mud-rich and characterized by an alternation of tabular bodies (interpreted as channelized or depositional lobes) and thick heterolithic or shaly layers. The lobes are made of coarse-grained high-density turbidites, which may be confined in deep (50 m), but very narrow depressions probably related to slump scars on the slope. The whole system is onlapping the << Blue Marls >> paleoslope on the southern border of the basin (Fig. 4). The main permeability barriers between the sand units correspond to the thick and extended heterolithic or shaly layers made of thin-bedded low-density turbidites.

The eastward system (Allos-Sanguiniere sub-basin) is characterized by a more complex organization, with a larger development of tractive processes. Genetic units may be evidenced with a lower part corresponding to slope channels fed by hyperpycnal flows, and building.prograding tongue-shaped sand lobes, and an upper part corresponding to a lobate system built by turbidity currents.This organization seems to be controlled by accommodation variations, and the depositional system is probably directly connected to shallow environments (braided deltas at the border of the Alpine continent).

The difference between the two systems is related to the nature and origin of the feeding material (Corso-Sardinian massif at South or Alpine massif at East), and to the basin configuration, which is controlled by the deformation regime, probably more active in the Eastward part of the basin.

Detailed 3D reconstruction of kilometric-scale outcrops shows that the << classic >> Annot Sandstone does not present an homogeneous organization, but displays very specific geometrical and heterogeneity patterns, which may be related to distinct depositional environments.
 
 

Fig. 1. Outcrop view of the Annot Sandstone in Annot.

Fig. 2. Schematic architecture of the low sinuosity fan channel in Annot.

Fig. 3. Outcrop view of the Chalufy onlaps (Trois Eveches Massif).

Fig. 4. Architecture of the onlapping sandbodies in Chalufy (Trois Eveches Massif).

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England