Outlier Basins on Passive Margins — Cretaceous Outcrop Analogues and Play Concepts for the Central and North Atlantic

Wach, Grant; Pimentel, Nuno; Pena dos Reis, Rui

When new basins are being explored and fields developed along the Central and North Atlantic margin, analogs are often employed to understand the petroleum systems. However, it is not necessarily the most complete section or stratotype that can provide the most relevant information on new exploration and development potential, but the outlier basins. These outliers have significant unconformities that can mark key intervals for reservoir generation and reservoir distribution; and hiatal surfaces that may provide evidence for condensed and sediment starved intervals.

The Wessex-Channel and Lusitanian-Algarve basins have relatively complex geological histories, multiple sources of sediment input, source rock analogues and variable depositional settings. In the Wessex-Channel basins of southern England, minor unconformities, non-sequences (eustatic changes) and variable rates of local tectonic subsidence are exhibited within the basin and are superseded by a major unconformity cutting the Mesozoic section associated with later Cimmerian tectonism, with the unconformity formed in an extensional setting. The deposition of the Lower Greensand marked the end of the late Cimmerian event. In Portugal, both the Lusitanian and Algarve basins show break-up related unconformities, associated with the three-stepped Cretaceous opening of the North-Atlantic on the Western Iberian Margin. Distinct lithological signatures and depositional hiatuses may be detected, but these surfaces do not always correspond to the most significant lithological changes. Those changes are often related with sea level variations and/or progradational/retrogradational cycles, with either an input of siliciclastics (with reservoir potential) or the development of extensive shaly deposits (with source-rock potential).

The link between geological information from outcropping analogues and geophysical data from offshore basins is crucial to understand the complexity of the petroleum systems in passive margins and will help to develop new exploration and production strategies. The key elements in producing effective petroleum systems in the Central and North Atlantic Conjugate margins are the presence of source rock and reservoir. Trap formation and migration are less of a risk with active tectonics along the margin. The Cretaceous sequences of the Wessex-Channel basins of southern England, as well as of the Lusitanian and Algarve basins of Portugal, provide excellent outcrops to examine these stratal surfaces and sequences that may provide us with new play concepts and exploration opportunities.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013