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Timing of Opening and Cementation of Bedding-Parallel and Vertical Fractures, Vaca Muerta Formation, Argentina


Fluid inclusions are a key to unlocking the history of fracture growth. When correlated with burial and thermal history models, fluid inclusion temperatures provide evidence for timing of fracture opening and cementation. The black shales of the Neuquén Basin, Argentina, contain widespread horizontal, bedding-parallel, fibrous calcite-filled fractures, or “beefs”. The antitaxial cements of the beefs are zoned showing various opening stages. In addition to the beefs one set of horizontal, beef-parallel microfractures, and four sets of vertical opening-mode fractures have been. Set E-W shows evidence of reactivation, suggested by the presence of early microfractures that are crosscut by late blocky ankerite and calcite. Set NE-SW shows evidence for fracture porosity, lined by thin veneers of idiomorphic calcite crystals that grew into open space. All fractures contain coexisting aqueous fluid and hydrocarbon gas inclusions. Microthermometry of fluid inclusions in the horizontal fractures shows homogenization temperatures (Th) that range from ~ 175°C to 208°C, and fluid salinities of ~15 wt% NaCl equivalent. Ice melting temperatures suggest that the fluid present in the inclusions contains substantial amounts of CaCl2 (±MgCl2) beside NaCl. The later horizontal fractures have Th ranges from~165°C to 183°C, and the fluid salinities similar to the inclusions in beef. The inclusions in the vertical fractures are small, and show overall Th ranges from ~ 142°C to 161°C. The gas inclusions are liquid at room temperature and homogenize at ~ -75°C. This suggests that the gas is primarily methane dominated, with possibly low ethane and/or propane concentration. Temperatures of aqueous inclusions coexisting with hydrocarbon inclusions correspond to the trapping temperature, coinciding with fracture opening and cementation. Temperature and cement texture correlations, as well as crosscutting relations suggest the following timing of fracturing: beefs formed between 94 and 62 Ma around maximum burial, during Late Cretaceous. This is followed by Set E-W vertical fractures. The late horizontal fractures that crosscut Set E-W formed at ~60 Ma; Sets NE-SW and N-S vertical fractures formed at ~ 56-60 Ma, during the Paleocene. Set E-W set is probably reactivated at this time.