Possible absence of small faults in the Gullfaks Field, northern North Sea: implications for downscaling of faults in some porous sandstones

Abstract

Faults and fractures have been studied in more than 6 km of cores from the Gullfaks Field, northern North Sea, and compared to fault populations determined by stratigraphic correlation of well logs and seismic data. Statistical analysis indicates a power-law correlation between displacement and fault frequency for displacements down to 5–10 m. Observations of deformation bands, with displacements ranging from 1 mm to 10 cm, perfectly fit the low-end extension of this power-law model. However, integrated use of well-log correlation data and core data indicates that few faults exist with displacement between ∼20 cm and 5 m. If this data gap is real and representative for other sandstones, uncritical downscaling of seismic-scale fault sizes for use in oil reservoir description or strain estimation may yield erroneous results. The possible gap may be related to the high density and coalescent nature of (larger) faults, excluding the tip portions of many faults and thus the likelihood for wells to intersect low-displacement tip regions. Deformation bands, on the other hand, seldom develop displacements much in excess of ∼10 cm, thus providing a distinct population of displacement data in the lower end of the displacement scale which, by coincidence, falls close to the extrapolated trend of the well data in log–log space.