Oblique normal faulting along the northern edge of the Majella Anticline, central Italy: Inferences on hydrocarbon migration and accumulation

Along the northern edge of the Majella anticline, in a large quarry originally excavated by ancient Romans, we studied the deformation mechanisms and internal architectures of faults characterized by both normal and lateral components of slip. These oblique normal faults, which crosscut Miocene carbonate grainstones, are associated with hydrocarbons in the form of tar. Within the faults, tar is present in the breccia of the fault cores, as well as in the surrounding fractured and faulted damage zones. Outside of them, tar is found within the porous carbonate beds flanking the oblique normal faults. We propose a conceptual model of fault nucleation and development. In this model, incipient faulting was characterized by shearing of the pre-existing pressure solution seams and formation of two main sets of tail pressure solution seams. With ongoing faulting, exhumation, and growth of the Majella anticline, the main deformation mechanism switched to predominant opening-mode failure in the form of tail joints and veins within the evolving oblique normal faults. This processes allowed the linkage of isolated slip surfaces across contiguous carbonate beds, and the development of isolated pods of fragmented rocks along the evolving faults. Brecciation and cataclasis localized only along the main slip surfaces of the oblique normal faults, forming a zone of intense deformation, the fault core, surrounded by less-deformed fractured, faulted and fragmented carbonates of the damage zone.Tar distribution was used as a proxy for fault and fracture permeability. Well-developed oblique normal faults, as a whole, form combined barrier-conduit structures to fluid flow. The cataclastic rocks, if continuous along the fault cores, form seals for cross-fault fluid flow, whereas the fault breccia and the surrounding carbonate damage zones act as conduits to fluid flow. Less-evolved oblique normal faults may form either distributed or localized conduits for fluid flow, depending on the presence (distributed) or absence (isolated) of fragmented carbonates around the conductive slip surfaces. Due to their orientation, these faults enhance the overall fault parallel fluid flow, which is thus prominent in the damage zones of the larger faults. The fundamental structural elements with greater tar content are sub-parallel to the current σhmax of central Italy, reflecting the possible control exerted by the stress state on the overall fault and fracture permeability. At a larger scale, as suggested by the structural location of the study quarry, hydrocarbons are channeled primarily within the releasing jogs of interacting oblique normal faults.