Cementation of natural fractures and related influence on permeability and porosity in chalk

Natural fractures occur in chalk from the Kraka Field (Danian Ekofisk Formation, Southern Danish North Sea) and contribute to an increased effective permeability in the reservoir. The majority of fractures are open and act as conduits for hydrocarbons to migrate through. However, some fractures are
cemented and thus act as barriers for fluid flow. Predicting porosity and fluid flow in subsurface carbonate reservoirs is challenging, and with a proper understanding of cementation in fractures these subjects are better understood. Further knowledge on cemented fractures can be useful for hydrocarbon exploration and production.

A number of small-scale, early cemented fractures as well as some cemented shear faults and one cataclastic fault zone with cement are reported from chalk cores in the Kraka Field. The undulating and folded nature of the cemented fractures and the presence of silica cement suggest that they formed relatively early during burial, either prior to or during mechanical compaction of the chalk.

Stable isotope compositions of δ13C and δ18O of the bulk rock and fracture-filling and intraparticle cements in this study illustrate a depletion in both oxygen and carbon isotopic values. This pattern is typical of a burial trend during which an increase in temperature and pressure with depth leads to more negative oxygen values assuming a closed diagenetic system. The negative δ18O values in the early cemented fractures, the cataclastic fault
zone and the brachiopod relative to the bulk rock therefore suggest that they were all cemented at depth, after burial, by precipitation from a hot fluid. The early cemented fractures and the cataclastic fault zone were probably cemented at a similar time and depth, whereas the brachiopod void was cemented later at greater depth; this may be because the fluids would have had difficulties in penetrating the almost closed pore.

The study investigates cementation of natural fractures in reservoir chalk in the Kraka Field, Danish North Sea.
Different types of fractures and cementation are described and isotopic analysis of fracture cement is reported.