ABSTRACT

Keuper (Upper Triassic) fluvial sandstones and nonmarine carbonate rocks form a major oil reservoir in the western Paris Basin at burial depths of 2 km. Early-diagenetic processes comprise red-bed-type diagenesis (mechanical clay infiltration, iron-oxide precipitation) and extensive dolocrete formation both in fluvial channels and in fine-grained overbank sediments. Locally significant paleokarst created vuggy dissolution porosity in the carbonate units and probably also caused leaching of detrital alkali feldspar grains. Oxygen, carbon, and strontium isotope analyses of various eogenetic cements indicate a nonmarine pore-water composition. Ferroan carbonates, authigenic albite and potassium feldspar, quartz, sulfates, sulfides, and c ay minerals formed subsequent to major mechanical compaction. Their isotopic compositions record significant changes in the chemistry of the parent pore water.

Cl-Br relationships of the present-day pore water reveal that fluids saturated with respect to halite flushed the reservoir during burial. This is documented by the oxygen-isotope composition of the burial cements. Based on radiogenic dating of illite cements, influx of warm brines into the reservoir most likely occurred during the earliest 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 block created a hydraulic head in the eastern part of the basin and established a gravity-driven fluid flow system, displacing interstitial brines from the Keuper evaporites from the eastern part towards the western part of the basin. Fluid-inclusi n studies indicate that brine-influenced cementation occurred at progressively higher temperatures, reaching peak temperatures of <= 140°C. These high subsurface temperatures could not have been caused by advective heat transport, but most likely resulted from a combination of maximum burial depth, high surface temperatures, and thermal blanketing during the Late Cretaceous. A second gravity-driven fluid flow system was established 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 basinal brines to Tertiary meteoric water is about 1:2.