The Vaca Muerta Formation (Late Jurassic—Early Cretaceous): History, Stratigraphic Context and Events of this Emblematic Unit of the Neuquén Basin, Argentina

Héctor A. Leanza
Departamento de Geología, Museo Argentino de Ciencias Naturales—CONICET

The first informal description of what actually is known as the Vaca Muerta Formation was due to Bodenbender, who in 1892 described in the Salado river valley in southern Mendoza, highly bituminous shales, with fluid oil in nucleus of ammonites. Early studies carried out by German paleontologists based on Bodenbender’s collections, namely Beherendsen and Steuer, allowed to determine the Tithonian age of these shales. Since then, ammonite workers and oil prospectors were extremely interested in this unit. The first studies along the Dorsal de Huincul area carried out by Windhausen and Keidel, which derived in the discovery of oil producing wells in 1918, where based on stratigraphical, structural and paleontological studies involving the so called Tithonian “Margas bituminosas”. But it was not until 1931 when Charles E. Weaver first established the Vaca Muerta Formation in the geological literature. Actually this unit as a whole is stratigraphically placed in the Lower Mendoza Group, and extends from a catastrophic and fast flooding surface above the Tordillo and/or Quebrada del Sapo Formations up to the intra-Valanginian unconformity. Its age span, based on ammonite studies, is encompassed between the late Lower Tithonian to the Lower Valanginian. Source rock for excellence in the Neuquén basin, later turned on a first class unconventional reservoir, the lowermost part of the Vaca Muerta Formation, expands paleogeographically more than any other unit in the Neuquén basin, and the higher TOC values are precisely located near the base of the unit. Coeval marine nearshore silicoclastic and carbonatic rocks (Carrín Cura, Picún Leufú, Quintuco Fms.) and fluvial units (Bajada Colorada Fm.) are recorded at the southeastern and eastern basin margins, displaying clinoforms with a prograding pattern towards the basin depocentre. Other events, as lithographic limestones bearing a splendid variety of fossils (Los Catutos Member), and turbidites (Huncal Member) indicating an abrupt seaflor topography near the Jurassic/Cretaceous arc at the Chilean territory, are recorded within the Vaca Muerta Formation. Other interesting fact worth to remark is the great quantity of beef in shales spread out in the Vaca Muerta For-mation around Pliocene to recent volcanoes (e.g. Tromen, Auca Mahuida), those favoring a thermal influence for oil matu-ration. Well developed microfracture systems, allowing oil migration as well, are particular features that can be indeed observed by means of the beefs in field outcrops, allowing indirectly a better understanding of the subsurface fracturing of the unit.

AAPG Search and Discovery Article #90165©AAPG 2012 GEOSCIENCE TECHNOLOGY WORKSHOP, 2-4 December 2012, Buenos Aires, Argentina