Petroleum generation and migration in the ‘Tight Gas’ area of the German Rotliegend natural gas play: a basin modelling study

The Northwest German Basin is an important hydrocarbon province with considerable reserves of natural gas accumulated at its centre. To unravel the thermal and maturation history of the Carboniferous source rocks, particularly gas generation and migration, a combined petrological–geochemical and numerical basin modelling study was performed. Generally two main phases of maturation can be assigned to the Triassic–Late Jurassic and latest Cretaceous–Present intervals. Maturity data reflect the latter event and are consistent with a variable heat flow of 60–63 mW m−2 during Cretaceous–Tertiary burial which accounts for considerable late gas generation. Vitrinite reflectance data and maturity modelling show the Top pre-Permian strata in the northern part of the basin to be in the gas window at present, with a rather uniform maturity of VRr = 1.5% to 2.0% (at 4600–5100 m). However, the important top coal marker is at different depth levels and reveals a more complex coalification pattern, suggesting a strong structural control on maturation of source rocks. This effect is influenced locally by the high thermal conductivity of large salt bodies in the overburden. A significant delay in gas generation from source rocks at elevated horst blocks can be observed. The generation and migration of pre-Westphalian gas started during Late Carboniferous times, when much of the gas was lost from the basin due to ineffective seals. With ongoing burial, gas migration from Westphalian source rocks started in Early Triassic times within Permian graben areas, but was actually delayed until the Late Cretaceous at highly elevated horst blocks. The gas from early migration phases was replaced almost entirely by successively younger Westphalian gas.