DIAGENESIS AND PORE-WATER EVOLUTION IN THE KEUPER RESERVOIR, PARIS BASIN (FRANCE)

Keuper (Upper Triassic) fluvial sandstones and nonmarine carbonate roc ks form a major oil reservoir in the western Paris Basin at burial dep ths of approximately 2 km. Early-diagenetic processes comprise red-bed -type diagenesis ( mechanical clay infiltration, iron-oxide precipitat ion) and extensive dolocrete formation both in fluvial channels and in fine-grained overbank sediments. Locally significant paleokarst creat ed vuggy dissolution porosity in the carbonate units and probably also caused leaching of detrital alkali feldspar grains. Oxygen, carbon, a nd strontium isotope analyses of various eogenetic cements indicate a nonmarine pore-water composition. Ferroan carbonates, authigenic albit e and potassium feldspar, quartz, sulfates, sulfides, and clay mineral s formed subsequent to major mechanical compaction. Their isotopic com positions record significant changes in the chemistry of the parent po re water. Cl-Br relationships of the present-day pore water reveal tha t fluids saturated with respect to halite flushed the reservoir during burial. This is documented by the oxygen-isotope composition of the b urial cements. Based on radiogenic dating of illite cements, influx of warm brines into the reservoir most likely occurred during the earlie st Cretaceous. At this time the margins of the basin were uplifted by late Cimmerian tectonism, resulting in an unconformity at the Jurassic /Cretaceous boundary. We suggest that uplift of the Vosges crustal blo ck created a hydraulic head in the eastern part of the basin and estab lished a gravity-driven fluid flow system, displacing interstitial bri nes from the Keuper evaporites from the eastern part towards the weste rn part of the basin. Fluid-inclusion studies indicate that brine-infl uenced cementation occurred at progressively higher temperatures, reac hing peak temperatures of less-than-or-equal-to 140-degrees-C. These h igh subsurface temperatures could not have been caused by advective he at transport, but most likely resulted from a combination of maximum b urial depth, high surface temperatures, and thermal blanketing during the Late Cretaceous. A second gravity-driven fluid flow system was est ablished during the Oligocene by major uplift, and freshwater flushed the Keuper reservoir (mainly from the south), causing brine dilution, The present-day pore water in the study area is still saline (50-100 g /l TDS), and mass-balance calculations indicate that the ratio of basi nal brines to Tertiary meteoric water is about 1:2.