SHEAR VEINS IMPORTANCE IN A DIAGENETIC CONTEXT: A CASE FROM LOS MOLLES FORMATION, NEUQUEN BASIN, ARGENTINA

FORTUNATTI, NATALIA; RAINOLDI, ANA LAURA; FERNÁNDEZ PALMIERI, MARIA DEL MAR

Simposio; XVIII Reunión de Tectónica; 2021

Systematic studies of fractures and veins in oil/gas prone shales provides good-quality data aboutdeformation mechanisms (Gale et al. 2007, 2014). In a diagenetic context, veins contribute to determinecement source material and fluid flow pathways, including hydrocarbon migration (Gale et al. 2014).Detailed knowledge on natural fracture system is required in unconventional shale oil/gas reservoirs inorder to improve hydraulic fracture treatments and enhance storage capacity (Gale et al. 2007).Cemented faults and natural fractures of Los Molles Fm (Pliensbachian-Toarcian) in Chos MalalFold and Thrust Belt (Neuquén Basin), were measured and studied by petrographic, fluorescence andcathodoluminiscence techniques. This contribution is focused in shear veins, very common in layeredsedimentary rocks along bedding planes recording shear displacement of adjacent wall-rock, related tofaulting and/or folding processes (Passchier and Trouw 2005).In the study area, sedimentary section is represented by a thick dark black shale and calcareousmudrock succession, interfingered with light-yellow medium to coarse-grained sandstone layers ofvariable thickness. Shear veins are distributed along the sequence as a common tectonic feature, locateddominantly in fine laminae planes in shales or in the contact between shales and calcareous mudrocks.Shear veins have a 120/15-30° dip direction, according to regional bedding orientation. Thickness variesbetween 5 mm to 3 cm and slickensides were measured in-situ, showing SW-NE direction of motion. Xrayanalysis evidences calcite as dominant cementing mineral, with subordinate quartz (