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Jurassic Petroleum Systems in the Western Thetys realm: A case study from the South-western Basque- Cantabrian Basin, Spain

Albert Permanyer, Carlos Beroiz, Gonzalo Marquez, and Jose Luis R. Gallego

More than 60% of original recoverable oil and gas reserves in the world are concentrated in the Tethys realm, associated to Jurassic and Middle-Late Cretaceous prolific source rocks. Whereas the Upper Jurassic and Middle-Late Cretaceous source rock intervals have been largely documented by oil industry and academic researchers, less attention has been paid to the Lower Jurassic Tethyan anoxic events.

Along the Tethys continental paleomargins, early Jurassic anomalously carbon-rich strata (black shales) were detected in stratrigraphic sections from the Alps, Apennines, Central Europe and Tunisian Atlas among others.

In northern Spain, the SW sector of the Basque-Cantabrian Basin presents a well-documented Lower Jurassic (Pliensbaquian to Toarcian) anoxic event, which has been analysed within the context of the Western Tethys realm. This study is based on regional paleogeographical data, geochemical analyses, and basin modelling simulations.

The Lower Jurassic black shales were deposited in subsiding troughs of the Basque-Cantabrian Basin (i.e. Sedano), during an overall transgressive period on a relative quiet tectonic setting. They constitute a productive Petroleum System characterised by a source rock interval with type II kerogen and average TOC of 3.2%. The geochemical analyses of oils, tar sands and source rock extracts indicate good correlations between this source rock and the Ayoluengo and Hontomin oils, and tar sands from the basin margins.

The best reservoir rocks are the Upper Jurassic (Purbeck) sandstones (productive at Ayoluengo oil field) with porosities between 15 and 25% and permeabilities up to 1000 mD. Good sealing capacity in these units is provided by interspaced Purbeck shales.

Petroleum plays and traps are abundant and related to Cretaceous-Lower Tertiary early salt-induced movements or to a late mid-Tertiary compressional phase. Whereas migration seems to be efficient throughout the Sedano trough, key factors controlling the prospectivity of the area are the timing of trap formation versus oil generation/expulsion, and efficient vertical and lateral sealing of the main reservoirs.

Although the Ayoluengo field (20 MMbo recoverable) is the only commercial oil field found in the area, volumetric appraisal estimates indicate that there is still moderate undiscovered hydrocarbon potential in the area.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013