Paleotemperatures inferred from fluid inclusions in diagenetic cements: implications for the thermal history of the Paris Basin

Abstract

Paleofluid characterisation is helpful in reconstructing the thermal histories of sedimentary basins. Paleotemperatures and salinities were obtained from fluid inclusions hosted in diagenetic cements in source and reservoir rocks of the Paris Basin. Samples from limestones and sandstones were taken from drill cores of six boreholes in the central, eastern and southern part of the basin. Toarcian, Hettangian, Keuper, Rhaetian and Dogger strata were sampled in core where carbonate cements are the most abundant. The trapping temperatures inferred from these microthermometric measurements are 25 to 45°C higher than those derived from present-day geothermal gradients calculated from drilling data. However the Dogger limestones have late calcite cements yielding lower values close to the present-day temperatures. Salinities are high, i.e. between 10 to 22 wt% eq. NaCl, suggesting brine circulation from the Keuper into other Mesozoic strata. The former microthermometric results indicate abnormally high temperatures probably due to an increase of the gradient in the whole basin coupled with a higher surface temperature. This increase may have taken place during the Late Cretaceous, in agreement with K/Ar and fission-track dating. Modelling of the basin history shows that, during this period, the sedimentation depth was at a maximum. We propose that those higher gradients could have been caused by the blanketing effect of chalk deposits without any variation of the basal flux