Petroleum Systems of the Lower Saxony Basin, Germany

Abstract The Lower Saxony basin (LSB) is a highly differentiated graben filled with marine, lacustrine, and hypersaline sedimentary rocks of Late Jurassic and Early Cretaceous age. The basin inverted in Late Cretaceous time and was intruded by large laccoliths. Based on the occurrence of three oil types in close association with two source rocks, two petroleum systems are identified in this hydrocarbon province. The LSB Jurassic (!) petroleum system includes the first and oldest source rock, the Posidonia Shale (lower Toarcian), which is present throughout the entire Lower Saxony basin except for the westernmost part. Oils originating from it are characterized by a ?13C value of -29%o (type X). The LSB Lower Cretaceous (!) petroleum system includes the second and younger source rock, the "Wealden" papershale of Berriasian age, which occurs only in the western half of the basin. Its oil is characterized by a ?13C value of -32%c (type Z). A mixture of both oil types may occur (type Y) where both source rocks are present in a thermally mature stage and where reservoir rocks (Valanginian sandstone) are above the Wealden papershale. Reservoir rocks are sandstones and limestones of Late Triassic (Rhaetian) to Early Cretaceous age. The most important reservoirs are Middle Jurassic sandstones, Upper Jurassic sandstones and limestones, and Lower Cretaceous sandstones. Traps are either facies and unconformity traps of pre-Turonian age or structural traps that originated during the Late Cretaceous basin inversion. Two periods of oil generation are recognized. Most oil generated during the first period from both source rocks in the central part of the basin during Albian-Turonian time (120-83 Ma). This oil- except for asphalt traces found-is quantitatively lost, as the traps have been destroyed during the Late Cretaceous (Santonian) inversion phase and the source rocks were overmatured due to additional heating by intrusion of deep-seated Late Cretaceous laccoliths. Recoverable oil accumulations, now concentrated along the northern, western, and eastern borders of the basin, were less affected by these Late Cretaceous events. These oil ocurrences are limited to those areas where the vitrinite reflectance of the source rocks are presently in the range of 0.5-0.9% Ro. This second period of oil generation started with the end of basin inversion and may still be going on (83-0 Ma). From thermal maturity and structural considerations, short and mainly vertically directed migration paths are assumed. Cumulative production and proven reserves indicate that 3-10% of the hydrocarbons generated during the second period of oil formation actually accumulated in adjacent traps.