Abstract The spatial distribution of extensional strain in interbedded mudstones and carbonates from around Kimmeridge Bay in southern England is examined using a variety of line samples. Normal faults and tensile fractures (veins) from the same deformation event show displacements ranging over 6 orders of magnitude. The relative contribution of these structures to the overall extension varies, with large faults (>10 m heave) accommodating about 65%, smaller faults (1–10 m heave) about 25% and veins less than 10% of the overall extension. The heterogeneity of fracture density and strain can be quantified from cumulative plots by applying a non-parametric method based on Kuiper's test. Both the degree and statistical significance of strain heterogeneity can be determined and are shown to be scale-dependent. Thin veins accommodate a fairly constant background strain across the region, whilst thick veins and small faults take up localized higher strains in damage zones around larger faults. Fault-strain is relatively homogenously distributed across the region. The faults and veins do not share the same scaling relationship. Thus, this study shows that it is not possible to simply extrapolate fracture frequencies and strain from fault scale to vein scale, and that the heterogeneity of extensional strain is scale dependent.

Abstract The Kimmeridge Oilfield is the oldest commercial discovery of petroleum in the Wessex Basin. The field was discovered in 1959. The discovery well was completed for production and that same well continues to produce today, more than forty years after discovery. More than three million barrels of oil have been produced. Oil is trapped in fractured, Middle Jurassic, Cornbrash limestones in an anticline. The anticline is a product of Alpine inversion tectonics. Moreover, the Kimmeridge accumulation is unusual insofar as it is the only inversion structure to contain a commercial quantity of petroleum and the only producing field in the hanging wall of the Purbeck Disturbance.