Petrographic and geochemical investigation of naturally CO2-free and CO2-flooded sandstones from the Central Pannonian Basin
Authors: Cseresznyés Dóra , Király Csilla , Szabó-Krausz Zsuzsanna , Szamosfalvi Ágnes , Szabó Csaba , Falus György , Czuppon György 
Published: March 2021

&lt;p&gt;We investigated CO&lt;sub&gt;2&lt;/sub&gt;-free and naturally CO&lt;sub&gt;2&lt;/sub&gt;-flooded sandstone samples from a deep saline aquifers formation, which represents potential carbon storage reservoirs. A descriptive geochemical model is also coupled to the laboratory study for the better understanding of geochemical interaction between sandstone and CO&lt;sub&gt;2&lt;/sub&gt;. The studied area is located in the, Western Hungary in the Little Hungarian Plain where one of the largest CO&lt;sub&gt;2&lt;/sub&gt;-producing fields in Europe can be found. In this region, we have the opportunity to compare rocks of the same sandstone formation without CO&lt;sub&gt;2&lt;/sub&gt; (not affected by natural CO&lt;sub&gt;2&lt;/sub&gt; flooding) and naturally CO&lt;sub&gt;2&lt;/sub&gt; flooded sandstone, where the CO&lt;sub&gt;2&lt;/sub&gt; was trapped around 7-4 million years ago. As boreholes sampled not only the parts of the formation, which were flooded by CO&lt;sub&gt;2&lt;/sub&gt; (Mih&amp;#225;lyi-R&amp;#233;pcelak), but also the parts which were not affected at all by this flooding (&amp;#214;lb&amp;#337;).&lt;/p&gt;&lt;p&gt;Besides petrographic observations, scanning electron microscopy and mineral chemistry analyses, X-ray diffraction and infrared spectroscopy were used to determine 7 CO&lt;sub&gt;2&lt;/sub&gt;-free and 6 CO&lt;sub&gt;2&lt;/sub&gt;-flooded samples textural features, mineral compositions and the presence of OH-bearing minerals. We carried out thermodynamic-batch modelling with PHREEQC geochemical modelling software and compared to the laboratory results.&lt;/p&gt;&lt;p&gt;The sandstone samples from the CO&lt;sub&gt;2&lt;/sub&gt; bearing reservoirs contain quartz, mica, kaolinite, K-feldspar and carbonates such as dolomite, calcite, ankerite and siderite. The CO&lt;sub&gt;2&lt;/sub&gt;-free samples also contain chlorite, plagioclase and pyrite and all mentioned above. In the CO&lt;sub&gt;2&lt;/sub&gt;-flooded samples a carbonate phase, dawsonite (NaAlCO&lt;sub&gt;3&lt;/sub&gt;(OH)&lt;sub&gt;2&lt;/sub&gt;) could be also observed in significant amounts (3-16 w/w%). This is an indicator mineral of large amount of CO&lt;sub&gt;2&lt;/sub&gt; inflow in the CO&lt;sub&gt;2&lt;/sub&gt;-water-rock system. In addition, chlorite is apparently missing in the CO&lt;sub&gt;2&lt;/sub&gt;-flooded samples. According to the petrographic observations and X-ray diffraction (XRD) results, it is clear that the plagioclase content is higher (&amp;#8764; 11 w/w%) in the CO&lt;sub&gt;2&lt;/sub&gt;-free samples compared to the CO&lt;sub&gt;2&lt;/sub&gt;-flooded ones (&lt;1 w/w%). The modal amount of K-feldspar is also lower in the CO&lt;sub&gt;2&lt;/sub&gt; flooded reservoir rocks. The lower amount of K-feldspar and plagioclase in the CO&lt;sub&gt;2&lt;/sub&gt;-flooded samples can be explained by precipitation of dawsonite. These minerals can dissolve as a result of CO&lt;sub&gt;2&lt;/sub&gt;-flooding and serve Na&lt;sup&gt;+&lt;/sup&gt; and/or Al&lt;sup&gt;3+&lt;/sup&gt; ion for dawsonite formation.&amp;#160; The amount of the carbonate minerals also reveal systematic differences between the CO&lt;sub&gt;2&lt;/sub&gt;-free and CO&lt;sub&gt;2&lt;/sub&gt;-flooded sandstone, the amount of ankerite is higher (from 6 to 12 w/w%) in the later ones implying that some parts of the ankerite formed after the CO&lt;sub&gt;2&lt;/sub&gt; flooding event.&amp;#160;&lt;/p&gt;&lt;p&gt;The investigation of this unique area provides opportunity to study sandstone before interaction with CO&lt;sub&gt;2&lt;/sub&gt; and after millions of years being in contact with CO&lt;sub&gt;2&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;Acknowledgements:&lt;/p&gt;&lt;p&gt;This research was financed by Hungarian Scientific Research Fund (K131353).&lt;/p&gt;&lt;p&gt;D&amp;#243;ra Csereszny&amp;#233;s&amp;#8217; work is supported by the Cooperative Doctoral Programme granted by The Ministry for Innovation and Technology (ITM), National Research, Development and Innovation Office.&lt;/p&gt;