Study of an anomalous Evaporitic Unit in the Eastern Algarve Basin (Portugal), and implications of this unit for hydrocarbon exploration.

Abstract

The geological history of the Algarve basin includes extension episodes through Triassic, Jurassic and Early Cretaceous, followed by tectonic inversion since Late Cretaceous due to the Europe-Africa compression. The recent studies of the basin (Matias et al., 2011) identify the presence of two separate evaporitic units. The deepest one is an autochthonous unit deposited during the first stages of extension, Late Triassic – Early Jurassic (Hettangian), and it is present throughout the Algarve basin. The shallowest evaporitic unit, as per reflection seismic interpretation, is thought to be in between the Jurassic and Cretaceous sequences and its presence is restricted to the eastern side of the Algarve basin.

This shallow evaporitic unit is the main subject of this study, which tries to clarify some aspects about its age, placement, appearance and tectonic history. The interpretation of 2D seismic transects, helped to distinguish this unit from the deeper Triassic one; the sediments between both units are considered to be Jurassic or Triassic.

The secondary objective of this study is to establish the implications for the hydrocarbon potential associated with this evaporitic unit, mainly on the possible Sub Salt Play.

The study is based on previous research and the interpretation of the regional data available in the area, including 2D and 3D seismic surveys, well data, outcrop studies, gravity and magnetics. The main source of information for the seismic interpretation has been the 3D seismic survey, acquired in 2012 by the consortium Repsol/Partex, which covers most of the shallow evaporitic unit. The presence of high velocity carbonates and the structural complexity due to the Cretaceous-Tertiary compression make the interpretation of the salt body very difficult. PSDM processing of the 3D volume has improved the image and the understanding of the evaporitic unit and the sub salt section. It helped with the interpretation, brought some new ideas and also raised some new questions that this study tries to summarize and organize.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015