Diagenesis and Primary Migration in Upper Jurassic Claystone Source Rocks in North Sea

Abstract

Carbonate bands are common in Central graben Kimmeridgian (Upper Jurassic) claystone that is mature for oil generation. X-ray diffraction, thermal analysis, Mossbauer spectroscopy, thin-section studies, scanning microscopy, and electron microprobe investigations showed that the bands are composed of claystone fragments, coarse grains of dolomite, quartz, and siderite, and diagenetic layer-silicates, all cemented to a certain degree by ankerite. The diagenetic layer-silicates are kaolinite being neoformed into muscovite. In claystones adjacent to the carbonate bands, ankerite fills pores about 50 µm in diameter; however, claystone further away from the bands contains little ankerite. Gas adsorption with N2, Kr, and H2O together with microscopy showed porosity in the claystone is composed of a large amount of micropores and mesopores, possibly with inlet constrictions, and a few macropores with diameters of about 50 µm. In the carbonate bands, microporosity and mesoporosity are probably located in the claystone fragments, whereas several pores with diameters of about 50 µm are present in the carbonate matrix. The presence of claystone fragments, fluid release structures, and large amounts of diagenetic carbonates indicates that carbonate bands formed through fracturing, possibly as a result of overpressuring. Porosity parameters indicate that fluid transport occurred by diffusion in the claystone matrix to claystone macropores and further through macropores in the carbonate bands, out of the source rock. The diagenesis of carbonates and clay minerals in the carbonate bands and in the claystone points to two migration stages: an early migration of fresh, neutral fluids, and a later migration in connection with hydrocarbon migration of saline-alkaline fluids.