Source rocks of the German Central Graben

Abstract

Abstract Jurassic to Lower Cretaceous formations are the most important source rocks for hydrocarbons in the southern Central Graben area. Within the Dutch Central Graben, the Lower Jurassic Posidonia Shale Formation is the most prolific source for hydrocarbons, while the Upper Jurassic to Lowermost Cretaceous Bo Member of the Farsund Formation plays this role in the Danish Central Graben. Oil and gas discoveries in the Norwegian, Dutch and Danish part of the Central Graben proved a prolific petroleum system. Despite limited success of hydrocarbon exploration in the German part of the Central Graben, various indications suggest migration of gas out of active thermogenic source rocks that are closely related to potential shallow gas accumulations. These indications include the restricted location of the bright spots above the Jurassic graben system, the occurrence of gas chimneys underneath these accumulations and geochemical data from offshore The Netherlands, which support contribution of thermogenic gas. The purpose of this study is to assess the potential for generating thermogenic hydrocarbons from Upper Triassic to Lower Cretaceous formations. For this reason, prominent source rocks of the Southern North Sea, i.e. the Posidonia Shale Formation and the Upper Jurassic to Lower Cretaceous “Hot Shales” (namely Clay Deep Member offshore The Netherlands and Bo Member offshore Denmark), were mapped in detail. These and other potential formations from the Late Triassic to the Early Cretaceous are integrated into an existing petroleum system model of the Northern German North Sea, which was modified for the new requirements. The results showed that the main source rocks of the southern Central Graben, the Posidonia Shale Formation, and the Upper Jurassic – Lower Cretaceous “Hot Shales” are insignificant as sources for commercial hydrocarbon accumulations within the German Central Graben area. That is either due to marginal occurrence or to low maturity. However, Lower to Upper Jurassic formations such as marine claystones of the Aalburg Formation and the lower parts of the Kimmeridge Clay Formation, as well as coaly intervals of the Central Graben Subgroup, are likely to have generated, and still generate, hydrocarbons.