The Algarve Pre-salt Sequence (pre-Hettangian) – New insights for hydrocarbon exploration

Abstract

Pre-salt basins are known to be prolific hydrocarbon-bearing provinces independently of their age or sedimentary type (e.g. Brazil, Angola, Middle East). In Portugal, the existence of a widespread Triassic sedimentary sequence overlain by Hettangian evaporites has been known since the late XIX century. Throughout Portugal, this sequence is exposed in several coastal basins. Although excellent reservoir properties have been extensively reported in the Triassic units, particularly in the Algarve Basin, the same cannot be said about source rock presence, which is still speculative. Few exploration wells have penetrated the Triassic pre-salt sequence in Portugal. Nonetheless, Dourada -1 drilled in the offshore Lusitanian Basin, reported overpressured black shales and abundant bitumen in the top Triassic section. Recently two wells drilled the Triassic section in the onshore Lusitanian Basin, both revealing oil and gas shows, but considered sub-commercial. Analysis of geochemical samples collected at these wells are not conclusive. The onshore pre-salt Triassic sequence is characterized by coarse to fine redbeds deposited over Variscan metamorphic basement. It represents the initial basin infill in an intracontinental rift setting related to the Central Atlantic opening, and the diachronous extension related with the Neo-Tethys westwards propagation in the Algarve. The Algarve Basin is located at the heart of the interaction between these two different domains that formed an important triple junction during the Mesozoic. It is commonly accepted that the sedimentary infill of intracontinental rift basins depends mainly on the balance between climate, sedimentary input and accommodation space. In the Portuguese onshore, outcropping Triassic sediments are mainly alluvial to fluvial sediments. Nevertheless, some rare occurrences of possible lacustrine sediments have been reported. Furthermore, several fossils typical of lacustrine to near lacustrine environments, such as large amphibians (Stegalocephalia and Metopossaurus), have been collected throughout the years in Triassic sediments in the Algarve Basin. Consequently, this would increase the probability of having Type I source rock present within Triassic sediments. Furthermore, the deposition of a thick evaporitic unit overlying the Triassic sequence implies an initial marine incursion that could lead to the deposition of intercalated marine black shales, particularly in periodically isolated depocenters, which suggests the possible presence of a Type II source rock. Recently acquired 3D seismic data (PSDM) in the deep-water offshore Algarve, shows the presence of a well-developed pre-salt syn-rift system. Part of it corresponds to large half-graben basins with depocenters thicker than 1500m. These have the necessary conditions to accommodate lacustrine environments. Additionally, RMS amplitude analysis of the pre-salt sequence points to the existence of high amplitude events with channel-like features associated with large structural highs. Several promising leads have been characterized and are presently being de-risked. It is the aim of this paper to revise and present some of the most important geological and hydrocarbon exploration aspects mentioned above together with several examples of analogues that help to understand the importance and potential of this frontier area.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018