OIL-IMPREGNATED FLINT IN DANIAN CHALK IN THE TYRA FIELD, NORTH SEA CENTRAL GRABEN

F. Jakobsen*+, H. Lindgreen* and H.P. Nytoft*

*Department of Reservoir Geology, Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.

+ Corresponding author, email: fj@geus.dk

A new model has recently been proposed for the formation of flint in Danian North Sea chalk, according to which nano-quartz spheres crystallized in the marine water column and were then deposited and compacted, ultimately forming flint. This depositional origin implies that apparently massive flint blocks may have a porous structure.  Internal flint porosity is supported by observations on an oil-impregnated flint nodule recovered from the gas and oil zone in the Tyra field, well E-5. Samples from the flint nodule have been investigated – the mineralogy by X-ray diffraction and thermal analysis, and the structure by Atomic Force Microscopy (AFM) and helium porosimetry. In addition, organic-geochemical analyses of the hydrocarbons in the flint nodule were carried out. The structure of the flint, which is composed of nano-size spheres, indicates that significant porosity related to micro-sized pore spaces may be present. Measurements on the flint nodule using standard He-porosimetry demonstrate a porosity of 16.5% which is of the same order of magnitude as that measured in the overlying chalk. The oil extracted from the nodule has a composition similar to that present in other reservoirs nearby. The presence of oil in the flint nodule examined suggests that the porous proto-flint formed part of the reservoir section as both chalk and flint were filled simultaneously with hydrocarbons.

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