Palaeocirculation in the basin of southeastern France sub-alpine range: a synthesis from fluid inclusions studies

Abstract

In the basin of southeastern France, (sub-Alpine range), circulation of palaeofluids relates to the geodynamic evolution of the western Tethyan margin. Alpine chain build-up from the Triassic to Present. Recent reconstructions allow distinction of two main provinces: 1) the western border of the basin, where the Jurassic extensional structures have not been affected by Alpine compression; and 2) the central and eastern parts of the basin, subjected to a partial tectonic inversion during Eocene and Miocene compressional events. In the Oxfordian shales, two main distinct areas are recognized on both sides of the Digne fault, grading from a diagenetic domain with temperatures lower than 150°C, corresponding with aliphatic oil in fluid inclusions, to a low grade metamorphism with temperatures up to 250°C and dry gas. Hydrothermal circulations are suspected, but not yet evidenced. During the Oligocene, rising Triassic diapirs were the setting of intense fluid circulation with mineral neoformations. Fluid inclusions indicate a heterogeneous trapping of NaCaKMgCI solution (up to 35 wt% NaCl equ.) saturated with CO 2 ± N 2 at temperatures up to 350°C, during successive episodes of fluid injection. Such gas may originate from deep sources (pervasive flow of CO 2 ± N 2), but local shallow reactions can not be rejected. P-T conditions of 250°C-1.5 kbars are deduced for the eastern diapirs, and 250°C−1.5 kbars to 360°C 2.2 kbars for western diapirs. Geodynamic reconstructions indicate a palaeoburial depth for the brine of about 10 km corresponding to a mean thermal gradient of 35°C/km. This is in agreement with the burial depth of Triassic evaporates, exceeding 8 km during Cretaceous, before the main diapir rise in the Oligocene. Mesozoic sediments were investigated by drilling near the Uzer fault (Balazuc). We recognize major diagenetic events involving mineral transformations at temperatures up to 150°C. This can be explained by greater burial during the Cretaceous with a post-Cretaceous erosion amounting to about 1.5 km. Late circulation of low temperature CaMgNaBa rich solutions in all the series is indicated by intense fracturing. According to its tectonic characteristics, this episode may relate to fault reactivation during the Eocene. In conclusion, the configuration of the basin and its western border allows trapping of connate fluids beneath the Triassic evaporate, and their mixing with deeper water. All these fluids circulated between the Hercynian crystalline basement and the Triassic evaporates, across Lias and middle Jurassic strata during the Middle Cretaceous, in infra-Triassic sedimentary layers. The estimated depth during this period is about 10 km. In the basin, a normal geothermal gradient account for oil and gas generation. Low temperature subsurface aqueous fluids did circulate along the basin border in the Palaeogene during an episode of fault reactivation.