A naturally fractured chalk reservoir in the Ekofisk Formation: characteristics, petrography and stable isotope signature of cemented fractures and faults (The Kraka Field, Danish North Sea)

Abstract

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. This study investigates cemented fractures, faults and a cataclastic fault zone in chalk cores from the Kraka Field. Emphasis is given to small-scale fractures. These are either partially or fully cemented by mineral precipitates, commonly calcite, and have apparent widths of up to 5 mm. One type of fractures has silica cement along the fracture edge and calcite in the central part, while another type only contains blocky calcite cement. The faults have apparent widths up to 5 cm and are partially cemented by calcite. The cataclastic fault zone has an apparent width of 25 cm and is filled with angular fragments of chalk cemented by blocky calcite. Stable isotope analysis of the cement in the fractures and the cataclastic fault zone suggests that they were filled by calcite cement almost simultaneously during burial. The paragenetic sequence of Danian chalk from the Kraka Field is reconstructed and compared with that of other North Sea fields. Based on an investigation of cemented fractures in the Kraka Field, this study provides information on cementation evolution and possible implications for porosity and fluid flow. The majority of Danish