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Geochemical and Fluid Inclusions Analyses of Several Calcite Veins and Matrix Within the Vaca Muerta Formation, Neuquén Basin, Argentina


Bedding-Parallel Veins filled by fibrous calcite (beef or BPV called) are a consequence of fluid-rocks interactions, in particular in sediments of very low permeability such as shales. Concerning the mechanisms for their generation there is no consensus. BPV could be the result of several factors such as: fluid overpressures, tectonic stresses as well as the force of crystallization. However, these veins are interpreted as the result of natural fracturing process. In the Neuquén Basin, Argentina, BPV are common in the main source rock, the Vaca Muerta Formation (VMF). BPV are composed by calcite fibers, which indicated a continuous growth with the fracture opening. Previous studies have shown the presence of solid and fluid hydrocarbons (bitumen, oil and gas) trapped inside crystals and argue a BPV formation during generation and/or primary expulsion of hydrocarbons. Geochemical analysis (O and C isotopes and clumped isotopes) have shown that the aqueous fluid was from marine origin and had a temperature from 90°C up to 170°C. From all of these previous studies, the timing of generation for the BPV of the VMF is estimated from 90 Ma to 5 Ma depending of veins localization in the basin. Nevertheless, these studies are based on BPV randomly selected through the VMF and because of the complexity of the veins network, timing of generation is still not well known. In such a context, we decided to carry out a detailed geochemical characterization of the BPV and shales of the VMF to investigate the PTX conditions of BPV generation. We focus our study on two outcrops: 1) Huncal river and 2) Puerta Curaco. We selected samples localized with millimetric precision in the VMF log. A total of 50 samples of BPV, shales and oblique calcite filled fractures were carefully chosen. We structured our methodology in two parts i) δ18O, δ13C, major elements, traces elements and rare earth elements analysis on BPV, oblique veins and shales, and ii) aqueous and hydrocarbon fluid inclusion by microthermometry analysis on inclusions found in BPV and oblique veins. Our results indicate that the aqueous fluids are derived from shale matrix and thus migrated into a closed-system. Calcite cement in shales next to the BPV are derived from the same fluids. Several fluids, at different PT conditions mineralized the veins. Different BPV groups appear to be developed at several tectonic stages and thus indicate multiple timings of occurrence.