CARBONATE RESERVOIR DIAGENESIS ASSOCIATED TO CONNATE WATER / FRESH WATER INTERACTION WITHIN A THERMAL AQUIFER LOCATED IN THE CENTRAL-NORTHERN PART OF THE MOESIAN PLATFORM (ROMANIA)

• Main Authors: Lucica Niculae, Horia Mitrofan, Constantin Marin
• Format: Article
• Language: English
• Published: Editura Academiei Romane 2020-01-01
• Series: Travaux de l'Institut de Speologie Emile Racovitza
• Subjects: sedimentary basin; connate water; freshwater; diagenesis; calcite retrograde solubility
• Online Access: https://www.speotravaux.iser.ro/19/art01.pdf
• ISSN: 0301-9187; 2067-9033

The Moesian Platform sedimentary basin, which occupies large parts of Romania and of Bulgaria, includes a several hundred meters thick carbonate formation of Late Jurassic – Early Cretaceous age. The regional aquifer hosted by that reservoir formation has been tapped by several deep (up to 3,300 m) wells, drilled in southern Romania during the late 1980s and early 1990s. The main focus of the present study has been to additionally process and interpret previously reported chemistry data concerning groundwater samples which had been collected during the testing of such wells. It accordingly resulted that within the investigated aquifer domain, a significant control on the groundwater chemical properties and on the temperature distribution was exerted by downflowing plumes of cold, meteorically-derived freshwater, which interacted with upflowing plumes of warm, prevalently connate water. In addition, each type of plume was inferred to interact with the host rock in a distinct way, so that two complementary diagenetic processes developed: as downflowing freshwater warmed up – by gradual mixing with connate water – calcite was deposited in the reservoir; and conversely, as upflowing connate water was progressively diluted and cooled by freshwater, the resulting mixture dissolved increasing amounts of calcite. Those processes mirror the specific property of calcite of becoming increasingly soluble, as the aqueous solution progressively cools down (“retrograde solubility”). This overall interpretation is supported by the sampled fluids’ concentrations of Ca2+ and HCO3-, which for the considered wells display reverse correlations with the corresponding inflow temperatures. On the other hand, the calcite saturation index computed for most water samples proved to be close to zero, accordingly indicating that the inferred groundwater flows – either updip or downdip the reservoir – had to be slow, so that enough time was available for chemical equilibrium to be achieved in correspondence to each new thermal status reached by groundwater.