Mixed-Layer Illite/Smectite Diagenesis in Upper Jurassic Claystones from the North Sea and Onshore Denmark

Claystones of Kimmeridgian-Volgian(-Ryazanian) age are the main source rocks for oil in the Central Trough, North Sea, but they are too immature for oil generation i the Norwegian-Danish Basin. This paper describes the diagenesis of mixed-layer illite/smectite (I/S) in these source rocks as investigated by X-ray diffraction and high-resolution electron microscopy, interpretation for both methods being supported by computer simulations. Randomly-ordered I/S with ~ 70 % illite layers, together with discrete illite, kaolinite, and traces of chlorite and vermiculite, are detrital. The clay mineralogical data suggest the existence of an island at Gassum during part of the Upper Jurassic. Mixed-layer illite/smectite originating from volcanic ash is found in two wells in the Central Trough. The volcanism ay be related to a period of stretching in the North Sea graben system during Jurassic-Lower Cretaceous time. Diagenesis has not altered the I/S in the Norwegian-Danish Basin. In the Central Trough, however, the amount of illite layers in I/S increases from ~ 70 % to ~90 % and the I/S becomes ordered. The volcanic lays have been changed to randomly ordered I/S with ~ 50 % illite layers. These changes occurred in the Central Trough simultaneously with oil generation. Drilling for oil and gas has taken place in the Danish onshore area for fifty years and offshore