Noble gases constrain the origin, age and fate of CO2 in the Vaca Muerta Shale in the Neuquén Basin (Argentina)

Györe, D. , Pujol, M., Gilfillan, S. M.V. and Stuart, F. M. (2021) Noble gases constrain the origin, age and fate of CO2 in the Vaca Muerta Shale in the Neuquén Basin (Argentina). Chemical Geology, 577, 120294. (doi: 10.1016/j.chemgeo.2021.120294)

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Unconventional hydrocarbon resources such as shale oil/gas and coal-bed methane have become an increasingly important source of energy over the past decade. The Vaca Muerta Shale (Neuquén Basin, Argentina) contains the second largest technically recoverable quantity of shale gas in the world. Exploitation of the play has been complicated by elevated concentrations of CO2 in several fields, the origin of which is currently poorly understood. Elevated CO2 levels are consistently encountered when deep-rooted faults in the Auquilco Evaporite Formation, present below the Vaca Muerta Shale, overlap with shallower faults that propagate from the top of evaporites into the shale, indicating a sub-evaporate origin of the CO2. Here we report new isotopic analysis of CO2-rich gases from two producing fields. CO2 concentrations increase with C1/(C2 + C3) values (4.8–33.5) and fractionation of δ13CCO2 (−0.9 to −7.7‰), suggest that CH4 have been displaced by CO2 which entered the shale after hydrocarbon maturation. The noble gas composition (3He/4He of 3.43–3.95 RA, where RA is the atmospheric ratio of 1.399 × 10−6, 21Ne/22Ne of 0.0310–0.0455, 20Ne/22Ne of 9.89–10.52, 40Ar/36Ar of 2432–3674 and CO2/3He 6.8–20.2 × 107) of the gases is consistent with mixing of magmatic CO2 with crustal hydrocarbon-rich gases and provides evidence for the loss of significant CO2. Using inverse modelling techniques, we determine that the magmatic gas has a 3He/4He of 3.95–4.08 RA, CO2/3He of 8.8–16 × 108 and 20Ne/22Ne of 12.13−0.10+0.08, 21Ne/22Ne of 0.074−0.003+0.004. Based on the radiogenic He and Ne this is consistent with a depleted asthenosphere mantle source, which has been trapped in the crust since 6.0–22.8 Ma. This is significantly younger than Late Cretaceous maturation of the hydrocarbon source rocks. Mantle melting as a result of asthenosphere upwelling induced by the collision of the South Chile Ridge and the Chile Trench at ~14 Ma is the most likely source of the CO2. Gases from below the shale contain two air saturated water-derived noble gas components, distinguished on the basis of 20Ne†/36Ar, 84Kr/36Ar, 132Xe/36Ar ratios. These are consistent with early and late stage open system Rayleigh fractionation of groundwater-derived noble gases. We find evidence that these mix with the magmatic component prior to entering the Vaca Muerta and mixing with an adsorption derived gas retained in the source kerogen.

Item Type:Articles
Additional Information:The research was funded through the ‘Isotope Coupling and Classical tools: Applications to Reservoir and Exploration (ICCARE)’ project, sponsored by Total Exploration and Production (Total EP).
Keywords:Subsalt CO2, isotope tracing, mass spectrometry, stable isotope, noble gas isotope, unconventional, shale gas.
Glasgow Author(s) Enlighten ID:Gyore, Dr Domokos and Stuart, Professor Fin
Authors: Györe, D., Pujol, M., Gilfillan, S. M.V., and Stuart, F. M.
Subjects:Q Science > QE Geology
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Chemical Geology
ISSN (Online):1872-6836
Published Online:30 April 2021
Copyright Holders:Copyright © 2021 Elsevier B.V.
First Published:First published in Chemical Geology 577: 120294
Publisher Policy:Reproduced in accordance with the publisher copyright policy

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