JOSEPH, P., IFP, Rueil Malmaison Cedex, France; O.G. DE SOUZA JR., Petrobras/E&P; R. ESCHARD, D. GRANJEON, O. LERAT, and C. RAVENNE, IFP, Rueil Malmaison Cedex, France

Abstract: Characterization of the 3-D Architecture of Deep-Water Reservoirs from Outcrop Analogues in the French Alps and Application to the Namorado Field (Offshore Brazil)

The Eocene Annot Sandstone in the French Alps corresponds to a turbiditic system up to 1000 m thick, which has filled large morphologic gutters induced by tectonic activity. Detailed field studies have recently been undertaken on well-exposed 3D outcrops at a reservoir scale (kilometric), in order to analyse and reconstruct the architecture of deep-marine gravity deposits. This reconstruction is based on the measurement of vertical sedimentological sections, the mapping of sand units, the calibration and interpretation of continuous photomosaics of the outcrops. Then 3D grid blocks of the geometry and facies distribution of the reservoir units are built using geomodelling and geostatistical tools, and seismic modelling is performed in order to analyze the seismic response.

Some of these sites have been used to provide guidelines for the 3D reservoir modelling of the Namorado field (Upper Albian - Cenomanian), which is one of the most important producing oil fields on the Brazilian shelf (Campos basin). The turbidite reservoirs are trapped in a narrow and sinuous depression (a few kilometers wide), controlled by the movement of the underlying Aptian evaporites.The Namorado reservoir sandstones are organized in three major sequences (U1, U2 and U3), between 20 to I 10 m thick, which are each subdivided in three packages (Fig. 1):

- a sandy unit, with a kilometric extension and up to 30 m thick, beginning frequently by an erosion surface and composed of coarse-grained channelized and amalgamated pebbly sandstones, sometimes graded, resulting from high-density turbiditic flows. Minor amounts of thin Bouma turbidites and hemipelagic mudstones correspond to overbank and inter-channel deposits.

- a mixed unit, composed of alternating shaly and sandy beds, organized in Bouma lowdensity turbidites, associated with carbonate debris-flows and slumps.This unit is interpreted as a channel-levee system, a few meters deep (thinning-upward sequences and lateral evolution to hemipelagic mudstones).

- a shaly unit composed of hemipelagic shales and bioturbated marls, which corresponds to a break in the sand supply and has been interpreted as the equivalent of a maximum flooding surface.

Each sequence shows a characteristic organization: MFS - channel-levees - amalgamated channelized turbidites - channel-levees - MFS, which is interpreted as the result of the shallowing, then deepening of the system.

This organization is very similar to the sequences identified in the Cime Dieu de Delfy site, in the Sanguiniere area (Fig. 2), but in that case the continuous observation of the outcrops enables to better characterize the individual genetic sequences and the lateral evolution of the system, and to quantify useful parameters for the modelling:

- the amalgamated channelized sand unit corresponds to a prograding system during the base level fall. Periodic pauses in the sand supply induced some shale breaks (partly eroded by the subsequent sand deposits), which acts as permeability barriers inside the sand package :.their size and frequency may be quantified using photo-mosaics calibrated with sedimentological sections.

- channel-levee systems are organized in genetic sequences related to the development of overbank deposits during channel activity, followed by the filling and abandonment of the channel. These sequences are organized in compensation cycles in which individual beds pinch out rapidly : their length and thickness may be quantified in order to derive geostatistical parameters for the reservoir modelling (lithofacies proportions, correlation lengths).

- small Bouma turbidites dominated by parallel laminations and shaly interbeds appear as continuous layers at the outcrop scale and confirm the interpretation in terms of major abandonment surfaces (MFS). These heterolithic layers may be distinguished from the overbank deposits of the channel-levee system by their relative abundance in parallel-laminated structures instead of rippled-laminated turbidites well developed in overbank deposits.

A seismic modelling of the outcrop 3D grid block has been performed using the acoustic parameters of the Namorado field and various acquisition frequencies. The result of the analysis enables one to better characterize the seismic signature of the different sand packages (Fig. 4 ) and to provide some guidelines for the seismic interpretation of the field (Fig. 3).
 
 

Fig. 1. Sequence organisation of the Namorado field (Campos basin).

Fig. 2. Architecture of the reservoir sandbodies in Cime Dieu de Delfy (Sanguiniere Massif).

Fig. 3. Seismic section of the Namorado field after inversion (Campos basin).

Fig. 4. Seismic modelling of the 3D facies model of the Cime Dieu de Delfy outcrop.

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