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Diapiric structures in the Peniche Basin (Portugal) - location, deformation timings and importance to petroleum exploration


The present paper addresses the issue of salt tectonics in the Peniche Basin, which is located in the deep offshore of the West Iberian Margin. The main purpose of the current work was not only to map diapiric structures in the Peniche Basin, but also to date the time of the salt deformation. Twenty-six 2D seismic reflection profiles of the Peniche Basin (courtesy from PETROBRAS) and two of the Lusitanian Basin, were interpreted in order to characterize the existing evaporite bodies. In this study we addressed: i) the diapirs location; ii) the sedimentary infill deformation related with the diapiric growth; iii) the seismic units involved. Due to the fact that there isn't any borehole to validate the stratigraphy of the sedimentary infill, it was assumed that seismic units could be correlated with those identified in the Lusitanian Basin. Seven seismic units were interpreted in the Peniche Basin, spaning from the Triassic to the present. The main seismic unit considered was unit 1, Late Triassic to Hettangian, which includes the Silves Sandstones (lower part) and the evaporitic Dagorda Formation (upper part). We identified nine diapiric structures originated in the Dagorda Formation, located north of the Nazaré canyon. The interpretation of the structures was based on the internal configuration of the seismic reflectors of this formation, often chaotic. To date the salt movement, one of the main purposes of this work, the age of the younger layer affected by the deformation has been considered. Based on the interpretation of salt movement ages, we identified what appears to be two different halokinesis phases: i) Mesozoic, mainly related with extension, subsidence and overburden; ii) Cenozoic, mainly related with compression and uplift. This study of salt tectonics proves to be increasingly important in the petroleum industry, given its key role in the success of a oil system exploration. The salt withdrawal movement allowed the creation of accommodation space to keep infilling the basin and promoted overburden and maturation of the source rock, as well as the formation of several trapping mechanisms, both structural and stratigraphic. On the other hand, the salt motion led to the fracturing of the overlying units creating migration paths.