As a result of a pyroclastic flow occurred during the final rift stage at the Austral Basin, an ignimbrite with a minimum estimated volume of 40 km3 was deposited in the northern zone of Tierra del Fuego. This deposit extends in both Argentine and Chile territory, covering an area of at least 400 km2 and thickness between 100 m to 200 m, showing large-scale tabular geometry. Textural and structural features indicate rhyodacitic, eutaxitic, lapilli-tuff facies with evidence of high degree of weldingand devitrification processes related to a simple cooling unit. Fracture network is characterized by hydrofractures (fluid-assisted-fractures), extensional (Mode I) and to a lesser extent, shear (Mode II) fractures. Geometry, type and structural features of infill and genesis of fracture sets, as well as of crosscut relationships, suggest that hydrofractures and extensional fractures are related to thermal contraction/hydraulic processes occurred during the cooling stage of the ignimbrite unit, whereas the shear fractures are related to post-cooling stages, probably associated with tectonic deformation. Fracture population constitutes a natural fractured reservoir with excellent hydraulic conditions, demonstrated by the high production rates of the wells drilled in the field, reaching up to 400 m3/d of oil. In this contribution, the main characteristics of the petroleum system at the San Martín field are described.