Paleozoic Unconventional Resources - a Frontier Play on the Gondwana Margin of Iberia

Pena dos Reis, Rui B.¹; Pimentel, Nuno; and Fonseca, Paulo
¹[email protected]

The Western Iberian Margin is known for the presence of Mesozoic active petroleum systems, which have been the focus of exploration for decades, both on-shore and onshore. However, the Paleozoic basement should also be looked with interest, especially regarding the presence of unconventional resources, both beneath the Mesozoic basins and in other large outcropping areas. The Western Iberian Paleozoic geology shows several different terranes bounded by broadly NW-SE structures. One of these boundaries corresponds to the Rheic suture defined between the Ossa Morena Zone and the South Portuguese Zone, responsible for putting together a pattern of different basement terranes from both the Avalonia and Gondwana. This configuration controls the geometry of the Paleozoic terranes and sub-basins and also the distribution of their two main potential source-rocks: Silurian graptolitic black-shales and Carboniferous turbidites. Both these units present interesting TOC values and may be considered as unconventional hydrocarbon targets, occurring in large outcropping areas and also below the Mesozoic basins.

Carboniferous units are presently being looked at in S Portugal, as potential shale-gas formations. The predictability of the occurrence of the Carboniferous turbidites and the potential for hydrocarbons generation, are strongly dependent on the detailed knowledge of the structural orientations and boundaries, resulting from the geometry of the collision model for the Rheic closure. The Carboniferous turbidites are clearly more distal and finer westwards and, according to published data, they present TOC values reaching 2%. The thermal maturity is usually high and presents Ro values up to more than 4%, as a result from variscan low-grade metamorphism. Nevertheless, Ro values begin around 2%, with a regional distribution showing increasing maturity towards the NE.

The stratigraphic organization is quite disturbed by thrust fronts with a SW oriented movement and they present thicknesses reaching a few thousands of meters, partially due to those piggy-back variscan thrusts. Expected thicknesses and volumes of the finer grained sedimentary bodies seem interesting enough to justify more detailed studies of these sediments. The probable presence of Carboniferous turbidites below a thin sedimentary cover in some Tertiary basins, as well as in the main Mesozoic basins, also justifies increased interest on these Paleozoic rocks as unconventional hydrocarbon resources.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013