Diagenesis and alteration of subsurface volcanic ash beds of the Vaca Muerta Formation, Argentina

Abstract

The Vaca Muerta is a marine source rock with excellent petroleum potential containing a large range of heterogeneities such as natural fractures filled with fibrous calcite (“beef”), concretions and ash beds. An exhaustive diagenetic study of ash beds samples was carried out, based on petrographic, mineralogical and geochemical investigations on five wells from the Neuquén Basin with different burial (1.1–1.6% VRo Eq). Ash beds usually display two facies, the clay-rich and the calcite-cemented one, both with complex alterations. The clay-rich ash beds are characterized by predominance of illite/smectite assemblage, added mainly to albite, pyrite and Fe–Ti oxides. They are interpreted to correspond to the first, denser and less porous ash emitted by the volcanoes, with a high amount of micro-crystals transported over relatively short distances. The calcite-cemented ash beds consist mostly of argillitized glass shards and pumice fragments cemented by calcite. They may represent late eruptive stage, wind-blown tephra deposited at greater distance from the volcanic source, with higher initial porosity and lower crystal content favoring early calcite cementation. Diagenetic alteration has completely modified the initial minerals of both facies, which makes it difficult to establish the precursor mineral composition. However, geochemical analyses demonstrate that both facies display well marked calc-alkaline magmatic signature (andesite, dacite/rhyodacite), and similar conditions of alteration during burial.