Evolution of porewater composition through time in limestone aquifers: Salinity and D/H of fluid inclusion water in authigenic minerals (Jurassic of the eastern Paris Basin, France)

Abstract

Past water circulations can significantly reduce the porosity and permeability of marine limestones. This is particularly the case in the Middle (Bathonian / Bajocian) to Upper (Oxfordian) Jurassic limestones from the eastern border of the Paris Basin. The knowledge of the timing, the temperature and composition of paleowaters is essential to model the hydrological evolution in this area where the Callovian-Oxfordian claystones are studied for the storage of nuclear wastes. In this way, fluid inclusions hosted in low-temperature (<60 °C) authigenic calcite, quartz and celestite crystals were analyzed by Raman spectroscopy and mass spectrometry to determine the chlorinity and D/H ratios. Chlorinity measurements (mmol Cl per liter of water) in fluid inclusions trapped in authigenic crystals during the late Jurassic / early Cretaceous period revealed unexpected high values, up to 3800 mmol•l-1 , indicating that brines were involved in some of the diagenetic crystallization processes. By contrast, fluid inclusions in calcite cements of Cenozoic age within the Oxfordian limestones have low Cl concentration (less than 150 mmol•l-1), thus showing that a dilution event caused by water infiltrations during the Cretaceous uplift of this part of the basin has flushed out the original saline porewater. By coupling D of fluid inclusion with  18 O of calcite crystals, we estimate that calcite precipitation occurred at temperatures between 25 and 53 °C. The

Basin‐scale thermal and fluid flow histories revealed by carbonate clumped isotopes (Δ47) – Middle Jurassic carbonates of the Paris Basin depocentre

“…Such observations suggest that periods of active tectonics in the basin favoured fluid flows, leading to the precipitation of carbonate cements in the remaining pore space of the Middle Jurassic reservoirs. This geodynamic control on cementation was previously demonstrated in the Upper Jurassic carbonates of the eastern Paris Basin, located at ca 500 km from the studied area, which were mainly affected by meteoric cementation (Buschaert et al ., ; Gonçalvès et al ., ; Vincent et al ., ; Brigaud et al ., ; André et al ., ; Carpentier et al ., ; Blaise et al ., ).…”

Origin of 87Sr enrichment in calcite cements in Jurassic limestones (Eastern Paris Basin, France)

“…The Triassic fluids were highly diluted in the Upper Jurassic limestone porewaters. Consistently, Cl concentration is low in calcite fluid inclusions from the Gondrecourt graben and Upper Jurassic boreholes (less than 150 mmol l −1 , Blaise et al, 2015).…”

“…At that time, the eastern part of the Paris Basin was already inverted, allowing the incursion of meteoric waters. The consistently high δ 18 O values reflects a meteoric origin at relatively low temperature (Blaise et al, 2015). It shows that the Upper Jurassic limestone acted as an opened hydrologic system, allowing large scale fluid-flows under high water-rock ratios.…”

“…The younger generation matches the ID-TIMS U-Pb age of 33.5 ± 2.8 Ma obtained on vug-filling calcite in the Upper Jurassic limestone in wells EST205 and HTM102 (Pisapia et al, 2017). Calcite infillings along the Gondrecourt graben and in the Upper Jurassic limestone vugs also have similar fluid inclusions δD composition (Blaise et al, 2015). Therefore, they share probably a common origin, i.e., they were precipitated from the same fluids simultaneously, during the Eocene-Oligocene transpressional to extensional regime period.…”

“…The conceptual diagenetic model proposed by Carpentier et al (2014) highlights the role of propagating horizontal stress field in the opening of micro-fissures, which contributed to fluid flows and calcite precipitation. To better constrain the nature of paleo-fluids, Blaise et al (2015) measured the δD value of aqueous fluid inclusions trapped in authigenic calcite crystals. This method confirmed the meteoric origin of fluids that precipitated calcite both in the UJL vugs and in the breccias and fractures along the Gondrecourt Graben ("AB7" site, Augeville locality, Figure 1).…”