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Petroleum system associated with extensional forced folds with an intermediate decollement level: insights from the Basque-Cantabrian Basin (Spain)

Stefano Tavani, Fabrizio Balsamo, Anna Quintà, Pablo Granado, Eloi Carola, Fabrizio Storti, Stefano Mazzoli, and Josep A. Muñoz

The Plataforma Burgalesa Domain (Basque-Cantabrian Basin) hosts the only onshore productive oilfield in Spain. There, oil accumulated in anticlinal traps developed due to Mesozoic extensional forced folding, with sub-reservoir evaporites having imposed a decoupling between faulted basement rocks and folded cover sequence. Despite the subsequent Pyrenean inversion has partly modified the extensional architectures, and despite the reduced economic impact of the system, availability of seismic cross sections and well data, surface oil shows, and exposures of reservoir and seal levels, make this area an outstanding analogue for plays in extensional setting, like those in the neighbour Mediterranean area, where sub-reservoir evaporites are present.

During forced folding, stretching of the pre-rift sequence overlying the evaporite layer induced pervasive jointing and meso-scale extensional faulting, which enhanced the connectivity of reservoir levels located in the pre-rift package of carbonates, and would also favoured the migration of hydrocarbons upwards into the reservoirs located in the fluvial channels at the lower part of the syn-rift succession. Poorly lithified syn-kinematic sediments, which include multiple seal levels, underwent a less intense stretching, with soft-sediment deformation causing compartmentalization and an overall permeability reduction. Lastly, salt migration associated with incipient extensional forced-folding led to the development of anticlinal traps overlying the major faults. In the central portion of the master fault, high slip rates allowed for the propagation of the fault from the sub-evaporitic basement into the cover sequence during ongoing faulting, which led to the disruption of the previously formed trap. Conversely, in the tip regions of the master fault and along the main transverse faults, such a propagation of the fault into the cover sequence did not occurred, and faults remained confined in the basement. This preserved the integrity of the structural trap and would have facilitated oil accumulation along transverse structures.

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