Isotope geochemistry of light hydrocarbons adsorbed in Jurassic shales from the Hils syncline, North-west Germany

The Hils syncline is situated in the south-western corner of the eastern part of the Lower Saxony Basin in which oil has been drilled since 1864. The geochemical data of the reservoired hydrocarbons are well known. In the eastern part of the Lower Saxony Basin, i.e. east of the river Weser, the only source rock of the oil is the Posidonia shale. This sediment, rich in organic matter of kerogen type II, was deposited under uniform anoxic conditions during the Toarcian and reached maturity levels in the oil and gas windows. At the rim of the Hils syncline, which extends to about 40km in NW-SE direction, the Posidonia shale crops out and shows the same range of maturation as the deeply buried Posidonia shale in the eastern part of the Lower Saxony Basin as a whole. Five shallow wells with a depth of about 20m were drilled in the Hils syncline to get core samples from the Posidonia shale and from the shales directly overlying. Gaseous hydrocarbons adsorbed in these samples have been analysed and have been compared with the data set of the reservoired hydrocarbons from the eastern part of the Lower Saxony Basin. The results of this study show that: • Gases, adsorbed in the Posidonia shale or neighbouring younger shales of the Hils syncline originated from the Posidonia shale. Their isotopic patterns correspond to the isotopic data of oil-associated reservoir gases from the eastern part of the Lower Saxony Basin. Thus, the Hils syncline can be regarded as a model of the whole eastern part of the Lower Saxony Basin concerning the generation of hydrocarbons. • The generation of the gaseous hydrocarbons in the shallow cores from the Hils area and of most of the oil-associated gases from the reservoirs of the eastern part of the Lower Saxony Basin must have taken place about 88 million years ago according to the thermal history of the Hils syncline. • Migration of the light gaseous hydrocarbons from their source rocks to the reservoirs (secondary migration) did not drastically change the isotopic patterns of these gases, i.e. isotopic patterns of migrated gases carry information on source rock properties. • Considerable amounts of isotopically light methane, ethane, and propane have been observed in many of the gases strongly adsorbed at the cores from the Hils syncline. Isotopically light ethane and propane are unusual.