The geochemistry of formation waters in the molasse basin of upper Austria

The geochemistry of formation waters in the Molasse basin of Upper Austria has been investigated to ascertain the extent of meteoric water replacement of the connate interstitial fluids in these sediments. The chemistry, isotopic composition, and dissolved gas contents of the groundwaters and of oil and gas associated brines have been determined. The most superficial sediments of the basin, the Innviertel (Miocene), have been completely flushed by meteoric waters within the last 200 ka. The underlying Hall and Puchkirchen formations (Miocene/Oligocene) form gas reservoirs for biogenic methane, and the associated formation waters are chemically and isotopically modified connate brines of the original marine deposition. In the northeastern part of the basin, the connate brines of the deeper sediments (Cretaceous/Jurassic) have been partially or completely replaced by meteoric waters, whereas in the south of the basin these sediments contain high salinity fluids which are substantially of connate origin. These conclusions are supported by the stable isotope composition of the various brines. Oil-associated brines from the Eocene sediments contain large amounts of dissolved radiogenic⁴⁰Ar, which suggests that the oils have migrated from high-temperature environments. This is in contrast with the Puchkirchen, for which the observed absence of radiogenic⁴⁰Ar suggests that the biogenic methane has been formed in situ. The⁴He contents of these brines and of the Cretaceous/Jurassic groundwaters are, however, less than those in the overlying Puchkirchen formation and suggest that He has been removed from the deeper sediments as a result of flushing by meteoric water. The ratios of dissolved methane and nitrogen to argon increase with increasing ammonium content of the formation waters. All of these parameters may be used as indices for the maturation of the system. The heavy noble gases, Kr and Xe, are abnormally abundant in the dissolved gases, and this is attributed to geochemical concentration of these gases by adsorption onto shales in the sediment sequence. The overall geochemical situation confirms the existence of separate hydraulic systems with little interconnection in the several overlying geological horizons.