Microscopic character of the Lower Cretaceous Tuxen and Sola formations in the Boje-2C well, Danish Central Graben

The Lower Cretaceous Tuxen and Sola formations in the Danish Central Graben (North Sea) constitute pelagic chalks and marlstones, which locally are hydrocarbon-bearing. Based on previous identification of 21 sedimentary facies in the cored Boje-2C well of the Boje Field, 66 samples were analysed through back scattered electron microscopy to delineate how the microscopic character reflects the macroscopic sedimentary facies and to identify indications of diagenesis. For most samples, microscopic characters can clearly be related to the previously identified sedimentary facies. Bioturbation and shear deformation, however, were not consistently reflected in the microscopic character. In all samples, the microtexture is characterised by massive or laminated calcareous mudstone with varying content of mainly kaolinitic clay and only sparse remains of microfossils or shell fragments, whereas particles of quartz silt and kaolinitic claystone silt are common. Porosity is mainly interparticular, typically reaching 25% in the Sola and upper Tuxen formations, and declining with depth to ca. 17% in the lower Tuxen and upper Valhall formations. Diagenesis is reflected in the occurrence of calcite cementation, pyrite and calcite-rimmed dolomite, and some pores may represent moulds after silicious microfossil fragments, which probably sourced the locally observed silicification of the sediment. Further, in clay-poor intervals below the oil zone, a significant diagenetic feature constitutes biogenic calcite mud particles that become increasingly recrystallized and euhedral with depth. It is therefore possible that burial-related crystal ripening was delayed in the oil zone represented by the upper part of the core. In the underlying transition zone, which in the Boje-2C well comprises most of the Tuxen Formation, the degree of recrystallization increases with depth as water saturation increases. Pressure dissolution features were noted in clay-rich intervals, but proper stylolites were not encountered, reflecting the generally high clay content of these marly chalks.