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Palaeotopography Controlled by Salt Tectonics: Initial Methodology and Example for Describing Its Importance to Turbidite Systems Distribution

Albertão, Gilberto A.1; Mulder, Thierry 2; Eschard, Rémi 3
1 Sédimentologie/Stratigraphie, Petrobras/IFP/Bordeaux1, Rueil-Malmaison, France.
2 Géologie et Océanographie, Université de Bordeaux I, Talence, France.
3 Géologie-Géochimie-Géophysique, IFP - Institut Français du Pétrole, Rueil-Malmaison, France.

The dynamic of gravity-driven turbidity currents is strongly influenced by the topography of the seafloor, formed by topographic highs and lows on the margin along the depositional profile. This abstract focuses on the initial methodology and preliminary results of a work on that topic, with an application to an oilfield located at the central part of the Brazilian offshore Campos basin. This basin is located on a typical passive margin and its tectonics is partly controlled by halokinesis; the selected reservoirs are confined Turonian to Campanian siliciclastic turbidite systems.

Available data are a 3D amplitude seismic cube and well information, such as stratigraphic markers, electric logs, biostratigraphy data and core sedimentologic descriptions. The methodology includes, at semi-regional scale, (i)stratigraphic and structural interpretation, (ii)3D modeling and (iii)structural restorations, and, at local scale, (iv)preliminary reservoir interpretation.

Six seismic/lithologic markers were chosen and mapped: Salt Base (Aptian), Salt Top (Aptian), Macaé Formation Top (Albian carbonates), Seismic Cretaceous (Maastrichtian), Blue Marker (Oligocene) and Superior (Miocene). Only major faults were mapped during this work stage and 20 among them were effectively used in the model.

The 3D modeling stage began with the utilization of initial 2D grid files into a step-sequence workflow: fault, horizon and horizon-fault contact modeling. The necessary adjustments on the surface contacts (fault-horizon and fault-fault) as well as on fault throws and gaps are herein performed. A fine edition on the horizons is necessary to prepare them for further steps of the study. Simplifications are inherent to such edition and usually cause the loss of some geological information, such as the perfect adjustment between the interpreted surface and the raw seismic reflector. Anyway, these processes have kept the original seismic-well tying.

Individual surface restorations were performed in all horizons to evaluate the quality of unfolding-unfaulting and the structural coherence. Multi-surface restorations were then carried on in order to determine the related horizon palaeotopographies of each reference depositional time.

First results show that salt tectonics regulates most of the subsequent deposit geometry, and a younger and more direct control on the reservoir is related to the palaeotopography of the Albian carbonates, i.e., its tectonic structures and a detected canyon.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009