On the occurrence and formation of open fractures in the Jurassic reservoir sandstones of the Snøhvit Field, SW Barents Sea

Abstract

Open fractures were described in core and Formation Micro Image (FMI) image logs in the Jurassic sandstones of the Tubaen, Nordmela and Sto formations in the Snohvit Field, and 3D fracture network properties analysed using Computer Tomography (CT)scanning in selected core samples. The most frequent open fracture type is short stylolite-related fractures (F1), but longer open fractures are also present, with no obvious relationship to stylolites (F2). The F1 fracture densities are related generally to the clay content of the host rock, which controls the occurrence and spacing of the stylolites. The fractures are steep, with a N–S-dominant strike azimuth and significant spread. Although, generally, the F1 fractures are short, a percolating and 3D connected open fracture network across the core was found in most of the CT-scan samples. Open fractures were also found in the damage zone of a lately reactivated fault. The formation of the open fractures in the Snohvit Field is related most likely to thermoelastic processes during removal of overburden in late Tertiary time. The presence of open fractures may influence reservoir flow, particularly in intervals containing a high frequency of stylolites and in the damage zones of reactivated faults.