Grain assemblages and diagenesis in the Vaca Muerta Formation (Jurassic-Cretaceous), Neuquén Basin, Argentina

Abstract Mudrocks of the Vaca Muerta Formation (Tithonian-early Valanginian; Neuquen Basin, Argentina) contain grain assemblages of mixed extrabasinal and intrabasinal derivation. Mudrocks that contained, at deposition, relatively more abundant carbonate and siliceous biogenic debris display cementation by calcite and quartz that occluded pores and enhanced rock stiffness, perhaps acting to retard further pore loss by compaction. Another notable diagenetic feature in this mudrock succession is grain replacement that includes carbonate replacement of biosilica and feldspar and replacement of calcitic and aragonitic skeletal debris by calcite, dolomite, quartz, albite, chlorite, and pyrite. Organic matter is present as both detrital and diagenetic components that can in some cases be discriminated petrographically. The extensive chemical reorganization of Vaca Muerta mudrocks in diagenesis can be attributed to the high reactivity of the primary grain assemblage. Petrographic evidence suggests a chemical system in which silicate, carbonate, and organic matter components mutually participate in a complex set of reactions in which silicates provide acid buffering that controls the precipitation of carbonate. Despite the prominent role of chemical diagenesis in the post-depositional history of the Vaca Muerta, compaction is the major cause of porosity decline in these mudrocks. While the relatively small size of the sample set in this study (19 samples) does not permit lithologic heterogeneity to be assessed in the context of basin-scale stratigraphy, the observed variety of mudstones suggests that a stratigraphically representative sample set would need to be much larger than the one used here.