A thorough understanding of the chemistry involved in reinjecting heat-depleted geothermal water into poorly consolidated sandstone is vital for the effective design of treatments targeting subsurface rock formations. The intricate chemical interactions occurring within sandstone systems can result in the dissolution of certain minerals and the subsequent precipitation of others, which may significantly contribute to damage within the formation. This process can alter the physical properties of the rock, potentially leading to reduced permeability and other challenges in resource extraction. Thus, it is imperative to monitor not only the concentration of various chemical species present in the geothermal water and sandstone, but also the spatial distribution of these geochemical reactions. By doing so, we can better predict and mitigate their potential adverse effects on rock formations, ensuring the long-term success and efficiency of geothermal energy extraction and other subsurface activities. In this study, we conducted laboratory experiments using both model and natural formation waters, as well as rock samples, to investigate water–rock interactions in a sandstone reservoir in the Szentes area of Hungary. Geochemical models were run with two different thermodynamic databases to simulate laboratory experiments, predict the effects of heat-depleted geothermal water reinjection into the reservoir, and assess predictions of different geochemical databases. Our study shows that calcite dissolves while quartz, kaolinite, and dolomite form. Other mineral reactions, however, remain less certain. The PHREEQC database indicates chlorite dissolution along with the formation of small amounts of feldspars and hematite, whereas the Thermoddem database predicts montmorillonite dissolution and chlorite precipitation. The reservoir porosity and permeability are expected to change over time as a result of mineral reactions. Modeling results, however, indicate negligible porosity changes as the reservoir reaches equilibrium state. The general concept proposed here, which focuses on the geochemical properties of the reinjected water and reservoir, provides a framework for detailed analysis of the geothermal system—a critical step for ensuring sustainable geothermal operations.

A new collection provided some brachiopods assigned to Lacunosella hoheneggeri, Pygope diphya and P. janitor from the lower Tithonian (Micracanthoceras ponti Zone) and lower Berriasian (Calpionella Zone), and from the limestone succession at Zengővárkony, Mecsek Mountains, Hungary. This is the first taxonomic description and photographic documentation of these fossils. We provide reliable documentation of pygopid brachiopods from the Mecsek Mountains, known since 1880, but previously not described. For the moment, three pygopid brachiopod species are reliably reported from the Mecsek Mountains: Pygope diphya, P. janitor and Pygites diphyioides. The Lacunosella hoheneggeri described herein is the first record of Berriasian rhynchonellide brachiopods from the Mecsek Mountains. The lower Berriasian brachiopods may belong to a normal palaeoenvironment, with the usual size distribution.

A new collection at Zengővárkony (Mecsek Mountains, Hungary) provided a rich and diverse but poorly preserved cephalopod-dominated fossil assemblage representing the Kimmeridgian and the lower Tithonian. The material came from mixed scree, soil, and amongst roots affected by weathering processes having been exposed to the elements for a long time. The nautiloid Pseudaganides strambergensis is the first record from the Mecsek Mountains. Due to the weathering, the ammonite fauna consists of mainly fragmentary and dissolved individuals that comprises 528 specimens belonging to 34 species and 30 genera out of which 20 species and 15 genera are reported for the first time from the Mecsek Mountains. The fauna includes specimens of known taxa. No new taxa are introduced. Based on the comparison with other faunas, this assemblage most closely resembles the fauna of the Venetian Alps (Italy). Additional faunal elements include aptychi (Laevaptychus latus, Lamellaptychus murocostatus), belemnites (Hibolithes semisulcatus), and an indetermined brachiopod. The first record of Spiraserpula spirolinites, an encrusting fossil polychaete preserved on the internal mould of a Taramelliceras shell fragment indicates favourable bottom conditions for the epifauna. The presence of Aspidoceras caletanum, Gravesia aff. gigas, and Pseudowaagenia inerme indicates faunal connections with the Submediterranean Province of the Tethys, which is in line with the tectonic and palaeogeographical position of the Mecsek Zone during the Late Jurassic. The ammonite assemblage represents elements of five Tethysian ammonite zones of the Kimmeridgian and Tithonian. The lower Kimmeridgian Herbichi Zone is indicated by Streblites tenuilobatus and Praesimoceras herbichi. The upper Kimmeridgian Acanthicum Zone is indicated by Aspidoceras acanthicum, and the Cavouri Zone by Mesosimoceras cavouri and Aspidoceras caletanum. The upper Kimmeridgian Beckeri Zone is suggested by Hybonoticeras pressulum and Pseudowaagenia inerme. Whereas Gravesia aff. gigas, Lithacoceras aff. siliceum, and Malagasites? denseplicatus are faunal elements characterising the Early Tithonian Hybonotum Zone. Phylloceratid and lytoceratid specimens account only for 12% of the fauna, while the majority of the specimens belong to the Oppeliidae and Ataxioceratidae (60%).

Recent discovery of a previously unknown outcrop in the vicinity of the Zengővárkony lime-kilns (Mecsek Mountains, Hungary) provided a few identifiable upper Oxfordian brachiopods that exhibit a truly Mediterranean (Tethyan) character. Dating of the outcrop is based on a rich ammonite fauna:Benetticeras benettii; Trimarginitesex gr.trimarginatus; Orthosphinctes (Orthosphinctes)ex gr.tizianiclearly indicate the Late Oxfordian. The brachiopod fauna indicates a deep-water marine environment and well-oxygenated sea floor.Nucleata boueiandPygope catulloiare recorded for the first time from the Mecsek Mountains. A pathologic specimen ofPygope catulloiis also recognized. Its ventral valve was injured in an early developmental stage that caused deformation of the left side, which was overgrown by the healthy right side and created an asymmetric adult shell shape. Cause of the injury is unclear but it provides further evidence for subsequent healing of brachiopods after being injured. This is the first description and illustration of Oxfordian brachiopods from the Mecsek Mountains, Hungary. The occurrence of Tethyan originating pygopid brachiopods in the Oxfordian strengthens earlier observations that from the Bathonian/Callovian Tethyan influence became overwhelming in the Mecsek Mountains fauna.Pygope catulloistrengthens records from Algeria that pygopid brachiopods may have occurred very early on the periphery of the Western Tethys.

Fieldwork around the lime-kilns at Zengővárkony (southern Hungary, eastern Mecsek Mts.) has led to the discovery of previously unknown beds of the Hidasivölgy Marl Formation. Based on bio- and lithostratigraphic considerations, here a Valanginian (Hauterivian?) age is assumed. This recently discovered section consists of thin-bedded, greybrownish turbiditic marls and limestones laid down in rhythmic alterations. Excavations of the marl beds have yielded a poorly-preserved, but rich cephalopod fauna. Furthermore, Eutrephoceras ex gr. boissieri has been identified here, and this is the first record of Cretaceous nautiloids from the Mecsek Mountains.