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K–Ar dating was applied on authigenic potassic minerals which are abundant in sandstones from the south of the Sanfranciscana Basin, Western Minas Gerais State, central Brazil. The Quintinos Member fluvial sandstones (Três Barras Formation, Areado Group) contain significant amounts of authigenic K‐feldspar as microcrystals of adularia and sanidine habits. The ages of these microcrystals cluster into three groups: 106.1 ± 2.2, 89.9 ± 1.9 and 88.8 ± 1.8 Ma (from Albian to Coniacian). The older age of 106.1 ± 2.2 Ma was obtained from the coarse fraction analysed (10–20 µm) that can contain a mixture of detrital potassic minerals (K‐feldspar, muscovite, biotite and illite) and different authigenesis of K‐feldspar (overgrowths and microcrystals). Thus, only the younger ages were interpreted as precipitation of K‐feldspar microcrystals during the Late Cretaceous into the Quintinos Member sandstones. Moreover, these ages can document the formation of microcrystals within a few million years after deposition of the sandstones. The ages of authigenic illite from the Capacete Formation epiclastic sandstones (Mata da Corda Group) range from 88.5 ± 1.9 to 71.5 ± 1.9 Ma (Coniacian–Campanian). These results suggest the timing of the illitization event in these sandstones as well as a synchrony with K‐feldspar authigenesis in the Quintinos Member sandstones. These results are well constrained and are in agreement with stratigraphic, biostratigraphic and radiometric ages previously reported for the Sanfranciscana Basin. Copyright © 2014 John Wiley & Sons, Ltd.

Carbon capture and geological storage are among the most valuable technologies capable of reducing CO2 emissions. Long‐term interactions between CO2 and a reservoir, and the integrity of geological formations, are key factors in the selection of adequate reservoirs for permanent storage. Numerical models of CO2‐water‐rock geochemical interactions are often employed to predict the fate of CO2 stored in a reservoir over time. The Hygiene Sandstone, in the Denver Basin, Colorado, USA, is a geological formation with potential for CO2 storage, and was therefore studied in this work, in which we collected and characterized outcrop samples in order to supply the input parameters for numerical simulations. Four representative thin sections of Hygiene Sandstone outcrops were quantified in terms of detrital constituents, diagenesis, and porosity on the basis of conventional petrography. Sandstone mineralogy included, in decreasing order, quartz, K‐feldspar, muscovite, albite, illite, smectite, kaolinite, poikilotopic calcite, and siderite. Porosity ranged from 4% to 13%. A geochemical modeling study of CO2‐water‐rock interactions performed with two characterized samples and brine data from the Hygiene Sandstones, simulating reservoir conditions, suggested that the mineralogy of the sandstone is quite stable under the conditions that were tested and only minor mineralogical and porosity alterations would occur within a thousand years of storage. © 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.