The evolution of a model trap in the central Apennines, Italy: Fracture patterns, fault reactivation and development of cataclastic rocks in carbonates at the Narni Anticline

Recent hydrocarbon discoveries in the Southern Apennines of Italy have focu ssed attention on the importance of studying fracturing and cataclasis in c arbonate rocks because of their fundamental impact on reservoir permeabilit y and connectivity The Narni Anticline in the central Apennines consists of a stack of easterly-verging carbonate thrust sheets compartmentalized by e xtensional and strike-slip fault zones. The structure provides afield analo gue for studying the evolution of superimposed fold-and-fault-related fract ures in carbonate reservoir rocks. The fracture pattern at the Narni Anticline developed as a result of three mechanisms. (a) layer-parallel shortening predating folding and faulting; ( b) thrust-related folding and further thrust breakthrough; and (c) extensio nal and strike-slip faulting. Along-strike (longitudinal) fractures develop ed during progressive rollover fault-propagation folding, and their intensi ty depends on the precise structural position within the fold: fracture int ensity is high in the forelimb and low in the crest. The 3-D architecture o f the mechanical anisotropy associated with thrusting folding and related f racturing constrained the location and geometry of subsequent extensional a nd strike-slip faulting. The superimposition in damage zones of a fault-rel ated cleavage on the pre-existing fracture pattern, which is associated wit h layer-parallel shortening and thrust-related folding, resulted in rock fr agmentation and comminution, and the development of cataclastic bands. The evolution of fracturing in the Narni Anticline, its role in constrainin g thrust breakthrough trajectories and the location of extensional and stri ke-slip faults, and the final development of low-permeability cataclastic b ands, will be relevant to studies of known oilfields in the Southern Apenni nes, as well as for future exploration.