The Cretaceous fluvio-lacustrine Bajo Barreal Formation, known for its large hydrocarbon accumulations, consists of a 1) Lower Member made of floodplain mudstones with isolated channels with an upward increase of sandstone content, and an 2)Upper Member composed by grey and purple mudstones with thicker channel sand bodies. Both stratigraphical units are characterised by the presence of acid volcanoclastics and tuffaceous material mixed with siliciclastic sediments. Previous studies on the Bajo Barreal Formation suggest an active sedimentary input from contemporaneous volcanoclastic material derived from both direct fall-outs or rain-off processes. This material can make up to 15% of host rocks forming a so-called pseudomatrix produced by the disintegration of tuff and d pyroclastic material. In this contribution we present the preliminary results on the reservoir petrography and mineralogy focusing on the characterisation of its volcanic component and related neo-formed mineralization especially considering the clay and zeolite pore infill. The identification of key petrographically distinct stratigraphic units and the detailed quantification of their mineralogical composition (i.e. clay and zeolite content) has an important impact on characterisation of pore-filling material, porosity calculation from density-based wire-line log and the stratigraphic distribution of reservoir properties and hence definition of flow units.