AN ORBITAL CONTROL ON BEEF DISTRIBUTION IN MATURE SOURCEROCK ? (VACA MUERTA FORMATION, NEUQUÉN BASIN, ARGENTINA)
Résumé
In sedimentary basins, source rocks may generate aqueous and hydrocarbon fluids during burial anddiagenesis. This fluid production in very-low permeability sediments is responsible for the development offluid overpressure, and in some cases leads to hydrofracturing processes. Such processes are evidenced by theoccurrence of mineralized fibrous veins distributed parallel to black shale layering in many mature basins,known as “beef” or “bedded parallel fibrous calcite veins”. A link between the beef formation and the totalorganic carbon (TOC) has been suggested since overpressure and hydrofracturing is most likely to be triggeredby a phase transition (solid-liquid) during chemical compaction and thermal maturation of kerogene.In parallel, the organic matter preservation and TOC evolution in marine sediments are highly influencedby eustatic and climatic fluctuations. At short to medium timescales (< 1.106 yr), these fluctuations are cyclicand influenced by Milankovitch cycles, thus illustrating an orbital control on the stratigraphical evolution ofthe organic matter content. Considering a presumed relationship between mechanisms of beef generation andthe TOC as well as a control of orbital parameters on the TOC evolution, we address the following question:is there an orbital control on the beef distribution in source rock?Hereby, we present the preliminary results of a field mission aiming to answer this question andperformed in the Neuquén Basin (Argentina) during March 2017. We conducted fieldwork in the Loconpuéarea where the Late Jurassic Vaca Muerta Formation offers exceptional exposures composed of organic matterrichblack shales interbedded with numerous calcite beef of reliable lateral continuity (100’s meters) andthickness (0.2 to 7 cm). We performed a detailed 100 m thick outcrop logging focusing on the beefstratigraphical position, thickness and continuity. In parallel, we carried out a 10 cm interval shale samplingin order to investigate magnetic susceptibility trends in the stratigraphical record. Knowing that the magneticsusceptibility of sediments is a good indicator of the detrital fraction, we aim to decipher a possible climaticand orbital signal in this Late Jurassic sedimentary record. Using these datasets, we later performed spectralanalyses on the beef stratigraphical distribution, the beef thickness and the magnetic susceptibility signal toevidence a possible control of orbital parameters on beef distribution.