Abstract: Physical Modeling of-Salt-Related-Structures-of the Brazilian Continental Margin

Guerra, Marta; Mônica Pequeno; Peter Szatmari; Ernani Porsche - Petrobras/ Cenpes/E&P

In order to improve the study of the origin of salt-related structures a new method of physical modeling was developed at the Geotectonics Laboratory of the Petrobras Research Centre, Brazil.

The analogue material used was silicone, to represent salt, and sand to represent the sedimentary cover. The fundamental difference from the traditional subaerial modeling is that, following an early suggestion by Peter Cobbold, in this new method the experiments are run in a subaqueous environment. Replacing the air by a water column, the sand layer becomes saturated and its shear strength is reduced due to pore pressure. Also, the bulk of the overburden weight increases. This facilitates the rise of silicone through the overlying sand layer, generating diapirs.

This technique has also been suitable to study the influence of salt tectonics on turbidite distribution, since subaqueous modeling permits the simulation of turbidity currents. Experimental results showed that most of the sediments eroded from the shelf were carried down the slope by turbulent flow and deposited as sand bodies in deep water. At the beginning, these sand bodies led to the formation of silicone highs by the escape of the underlying silicone towards regions of less overburden. The distribution of subsequent sediments entering the basin was then controlled by the topography resultant from silicone movement.

Vertical sections of the analogue models were compared to seismic lines from the SE-Brazilian continental margin and showed to be very close to reality, reproducing a large number of salt-related structures observed in the evaporite basins. Diapirs with variable sizes and shapes (Fig. 1) formed in compressional and extensional settings. Some of these diapirs were mushroom-shaped, with overhangs intercalating with adjacent sediments. Silicone lenses were observed in some vertical sections of the models. They turned out to be connected to the mother layer by out-of-section stems. Such lense-shaped structures were interpreted also in seismic profiles and, as in the experiments, their stems may be found in a parallel section. Frequently bulbs of diapirs coalesced to form canopies, giving rise to columnar structures completely surrounding sand bodies (Fig. 2). The silicone extrusion over a large area in some places created a secondary salt layer at a higher stratigraphic level. Sometimes new diapirs were observed growing from this secondary layer. In the sedimentary cover, "turtle-back" structures, folds, normal faults and growth of the sedimentary sequence towards the diapir flanks were observed. The sand eroded from the source area and transported by turbidity currents was deposited adjacent to silicone domes formed during the experiment and observed both in strike and dip sections.

Subaqueous physical modeling is thus a pioneering technique that has proved to be very useful in the study of salt tectonics. This technique provides a variety of structural patterns that can be successfully applied to the interpretation of seismic sections in evaporite basins. Structures observed in the experiments, such as sand bodies surrounded by salt, if verified in nature, may add to the petroleum potential of some basins.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil