Fractures in Chalks and Marls of the Shetland Group in the Gullfaks Field, North Sea

Summary Oil production from the Shetland Group of the Gullfaks Field in the Northern North Sea is controlled by the presence of natural fractures. The fractures and the fracture network have been characterized using core description, thin section and CT analysis. The Shetland Group consist of interbedded chalk, marl and mudstone beds. Fracture distribution is controlled by the mechanical stratigraphy; shear fractures are most developed in the marls and tensile fractures in the chalk. Fracture density tends to be highest in chalks with low porosity. Open fractures are partly filled with calcite cement forming bridges between the fracture walls and/or lining on the fracture surfaces. The distribution of mechanical fracture aperture in 3D has been estimated along the fracture plane. The detailed fracture characteristics will be further used to address the micro-scale sweep efficiency and the geomechanical effects on single and two-phase fluid flow.