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Exploration Pathways for Western Portugal – an Integrated Look to the Lusitanian and the Peniche Basins

Abstract

Introduction - The Lusitanian Basin is one of the Western Iberian Margin sedimentary basins related with the opening of the North Atlantic. These basins have their counterparts in the eastern Canada Jeanne D'Arc and Whale Basins, as part of the Iberia-Newfoundland conjugate margins complex. In the Canadian basins, the intense exploration led to several good production and development results, but the Iberian basins did not have, so far, similar positive results.

The Lusitanian Basin corresponds to the most proximal margin, whereas the Peniche basin (deep offshore) is located at the distal margin. Both sedimentary infills show similarities, related with its contiguous basin evolution. The same major tectono-sedimentary packages, erosion and inversion events may be recognized in both basins.

This presentation deals with the evolution of the mainly onshore Lusitanian Basin and its petroleum systems, in order to establish an analogue for other nearby offshore basins, aiming to contribute to a better regional framework for exploration in the North-East Atlantic region.

Petroleum Systems - From the analysis of the petroleum system elements and its articulation in space and time, three main petroleum systems may be considered in the Lusitanian Basin (Fig.1): i) A pre-salt petroleum system, sourced by meta-sedimentary Palaeozoic rocks feeding Upper Triassic siliciclastic reservoirs and sealed by the Hettangian evaporitic clays; ii) A second petroleum system with Lower Jurassic source-rocks, namely the Sinemurian and Pliensbachian organic-rich marls, feeding mainly Cretaceous siliciclastics, using diapiric structures as migration pathways; iii) A third petroleum system with Upper Jurassic transitional to coastal marine Oxfordian source-rocks, feeding mainly Upper Jurassic fractured limestones and fluvio-deltaic to turbiditic siliciclastics. A fourth potential petroleum system may be considered, related to Cenomanian-Turonian marine marls, locally maturated and feeding tertiary siliciclastics, acting as reservoir and seal.

Both the Lusitanian and Peniche basins present sufficient maturation for the Lower and Upper Jurassic source-rocks. The presence of those source-rocks, as well as reservoirs and seals, is proven for the Lusitanian basin but it is, for the moment, speculative for the Peniche basin. Petroleum systems charts show similarities but improved subsidence in the deep offshore Peniche basins contributed to earlier and more intense maturation. This situation may also alow for the maturation of an eventual Late Cretaceous source-rock, which would feed thicker tertiary siliciclastics, with good reservoir and sealing properties.

Conclusions -

  1. 1.The Lusitanian Basin is well known outcropping basin and may be used as an analogue to the Peniche Basin.
  2. 2.The petroleum systems in both basins may be approached within the same geodynamic and stratigraphic framework.
  3. 3.Basement nature and structure seem to have strong influence on located subsidence and increased overburden areas.
  4. 4.Different positions within the North Atlantic opening margin, promoted different rfiting, overburden and maturation timings in both basins.
  5. 5.Globally, maturation in the deep offshore Peniche Basin is expected to be: i) more recent; ii) more intense, comparing with the onshore basins.
  6. 6.The influence of inversion issues must be stressed on any approach to the Peniche Basin. Although it helped in creating folds- and fault-related migration pathways, its impact on seal integrity may have been critical.
  7. 7.De-risking strategies for the West Iberian margin offshore basins must include basin and petroleum systems analysis based on detailed outcrop studies.