SANDSTONE DIAGENESIS AND EVOLUTION OF THE POROSITY IN A POTENTIAL TRIASSIC SOURCE-ROCK (PRIVAS AREA, ARDECHE)

The detailed geological studies of the French southeast basin (Ardeche area) east of the French Massif central margin allowed to draw the palaeogeographic depositional conditions of the Lower Triassic sandstones and their Jurassic and Cenozoic tectonic setting. This regional knowledge is a good topic in sandstone diagenesis. Six cored bore-holes, representative of the fluvial, transitional and lagunar palaeoenvironments have been studied. The petrographic analysis led to a characterization of the lateral and vertical extension and the relative chronology of the main early diagenetic minerals (feldspars, carbonates, quartz and sulphates), as well as of the substitution phases during burial (dolomite, quartz, baryte). Late weathering of these minerals (calcite and kaolinite) is linked to the Cenozoic tectonic uplift. Our results stress the importance of carbonate and sulphate precipitation during diagenesis of a potential source-rock ; it allows a restoration of the fluid flows during burial. Primary porosity of the fluvial sands is mainly controlled by hydrodynamic deposition; the porosity is partly occulted by pedogenetic carbonates and evaporitic sulphate cements, precipitated during the periods of fluvial hyatuses. A secondary porosity opening and occlusion, is controlled by replacement of the evaporitic sulphates by dolomite, silica, baryte or calcite. This diagenetic stage is attributed to the early burial fluid flow associated with the extensional tectonic stage of early to middle Jurassic age. Pore fluid migrations associated with these successive transformations may be correlated with the illite formation, as shown by K/Ar datations. This first fluid transfer is linked with the late Liassic rifting evolution of the Ardeche margin. The second fluid migration, occurred during early Cretaceous and may be linked to the fluid flow expelled from the basin during the main Mesozoic compaction stage. The fluid flow linked to regional Cenozoic tectonic uplift controlled the late weathering and replacements of feldspar, carbonate and sulphate ; this phase has much increased, locally, the secondary porosity. The petrographic approach to diagenesis is a good tool, allowing to draw qualitative boundaries among the mineral transformations. The access to representative cored bore-holes in a defined area allows a quantification of the porosity of a potential source-rock.