Extensional fold-related sub-seismic fracturing in the Armeña rollover (Pyrenees). Mesozoic and Cenozoic carbonates of the Neo-Thetys: old and new concepts for petroleum explorations

The Armeña rollover is an outstandingly exposed extensional fault-related fold in the southern Pyrenees. It involves more than 3 km of Upper Cretaceous wedge shaped syn-kinematic sediments overlying Jurassic to Lower Cretaceous pre-kinematic strata, detached above an Upper Triassic salt layer. Although the rollover was positively inverted during the Pyrenean orogeny, its first order extensional architecture and, above all, the associated syn-extensional fracture pattern are well preserved, making this structure one of the few exposed analogues of reservoirs located in extensional rollovers. An integrated macro- and mesostructural study was carried out in the Armeña rollover, in order to get insights about the relationships between fold and fracture pattern evolution.

Two sets of bed-perpendicular joints occur in both hanging-wall and foot-wall of the NE-dipping Armeña fault, being oriented about parallel (longitudinal) and perpendicular (transverse) to the fault, respectively. The frequency of longitudinal joints increases towards the fault. Such an increase is progressive in the central portion of the rollover, while at its lateral termination two panels of highly and poorly fractured strata are well-recognizable, being divided by a very narrow transitional area. These panels coincide with the SW- and NE-dipping limbs of the drag-like syncline that occurs only in the lateral termination of the rollover.

Our study points out that fracture densities varies in the different structural positions of the fold, which is expected to influence fluid flow and accumulation. In particular, we conclude that widespread longitudinal jointing occurred as part of the regional extensional framework and affected the entire rock volume with a rather constant density. Laterally decreasing fault-propagation rate caused the development ofthe drag-like syncline at the lateral termination of the rollover. There folding of strata above the slowly propagating fault’s tip was ensured by development of further longitudinal joints that subsequently, during the later stage of fault-propagation, underwent shearing.