PALAEOCIRCULATION IN THE BASIN OF SOUTHEASTERN FRANCE SUB-ALPINE RANGE - A SYNTHESIS FROM FLUID INCLUSIONS STUDIES

In the basin of southeastern France, (sub-Alpine range), circulation o f palaeofluids relates to the geodynamic evolution of the western Teth yan margin. Alpine chain build-up from the Triassic to Present. Recent reconstructions allow distinction of two main provinces: 1) the weste rn border of the basin, where the Jurassic extensional structures have not been affected by Alpine compression; and 2) the central and easte rn parts of the basin, subjected to a partial tectonic inversion durin g Eocene and Miocene compressional events. In the Oxfordian shales, tw o main distinct areas are recognized on both sides of the Digne fault, grading from a diagenetic domain with temperatures lower than 150 deg rees C, corresponding with aliphatic oil in fluid inclusions, to a low grade metamorphism with temperatures up to 250 degrees C and dry gas. Hydrothermal circulations are suspected, but not yet evidenced. Durin g the Oligocene, rising Triassic diapirs were the setting of intense f luid circulation with mineral neoformations. Fluid inclusions indicate a heterogeneous trapping of Na-Ca-K-MgCl solution (up to 35 wt% NaCl equ.) saturated with CO2 +/- N-2 at temperatures up to 350 degrees C, during successive episodes of fluid injection. Such gas may originate from deep sources (pervasive flow of CO2 +/- N-2), but local shallow r eactions can not be rejected. P-T conditions of 250 degrees C-1.5 kbar s are deduced for the eastern diapirs, and 250 degrees C-1.5 kbars to 360 degrees C-2.2 kbars for western diapirs. Geodynamic reconstruction s indicate a palaeoburial depth for the brine of about 10 km correspon ding to a mean thermal gradient of 35 degrees C/km. This is in agreeme nt with the burial depth of Triassic evaporates, exceeding 8 km during Cretaceous, before the main diapir rise in the Oligocene. Mesozoic se diments were investigated by drilling near the Uzer fault (Balazuc). W e recognize major diagenetic events involving mineral transformations at temperatures up to 150 degrees 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. Accord ing to its tectonic characteristics, this episode may relate to fault reactivation during the Eocene. In conclusion, the configuration of th e basin and its western border allows trapping of connate fluids benea th the Triassic evaporate, and their mixing with deeper water. All the se fluids circulated between the Hercynian crystalline basement and th e Triassic evaporates, across Lias and middle Jurassic strata during t he Middle Cretaceous, in infra-Triassic sedimentary layers. The estima ted depth during this period is about 10 km. In the basin, a normal ge othermal 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. Copyright (C) 1996 Elsevier Science Ltd