From Gondwana Paleozoic Basement Source-Rocks to Atlantic Mesozoic Rift Reservoirs—Examples From the Western Iberian Margin

Abstract

Exploration on Western Iberian Margin has a long historical track, with over hundred wells drilled both onshore and shallow offshore, in the last few decades, with no commercial discoveries until now. However, production in its Canadian conjugate margins and oil and gas-shows in several wells, open good perspectives for exploration in this region. The potential of Gondwana-related Paleozoic units as source-rocks for conventional hydrocarbons on the overlying Mesozoic basins has been addressed in different regions of the world. In Portugal, the main resistance to this idea is related with the metamorphism that affected the Paleozoic units during the Variscan orogeny. However, “a subsalt (pre-Jurassic) play”, related with Silurian black-shales feeding Triassic redbeds similar to the Algerian Ghadames basin play, has been explored in Portugal since 2005 by Mohave Oil&Gas and Porto Energy. Recently, geochemical data, field observations, structural and geodynamic interpretations, pointed to the potential of other Gondwana-related terranes, as source-rocks for unconventional resources, as well as for conventional resources in the overlying Mesozoic basins. The Silurian graptolitic black-shales of the Central-Iberian Zone, show a good organic content and maturity indications within the gas and frequently even within the oil-window. The presence of these units close to the border of the Lusitanian basin and eventually beneath part of it, have been considered as the source for several gas-shows in Upper Triassic deposits. Also the Carboniferous terranes of the South Portuguese Zone include black-shales as part of distal turbiditic sequences. These lithologies show depleted values of TOC but maturity indications within the gas-window. Although these units do not outcrop in the offshore areas, they may be present beneath the Mesozoic basins, both to the West (Peniche and Porto basins) and to the South (Algarve basin). Regional reconstruction studies point to the probable presence of Carboniferous units in most of the basement of the onshore and offshore basins and may explain the occurrence of gas-shows in several wells, in areas with apparently insufficient Mesozoic overburden. Further studies are necessary to ascertain the nature of the Gondwana related basement rocks and its potential as source-rocks for Atlantic rift related reservoirs in the Western Iberian Margin. However, this situation may be a good lead for exploration in other passive-margin Atlantic basins.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015