Natural hydraulic fractures in the Wessex Basin, SW England: Widespread distribution, composition and history

Bedding-parallel veins of fibrous calcite (‘beef’) are historical in the Wessex Basin. The veins are common in Mesozoic mudstones and shales, especially of Liassic to Mid-Cretaceous ages. Cone-in-cone structures, which consist of multiple nested cones, are also well developed within the ‘beef’. To investigate the distribution and the context of formation of ‘beef’ in the basin, we have made several field studies and analysed numerous samples. The veins are widespread vertically and horizontally in the sedimentary sequence, but they are especially common near or within potential source rocks for petroleum or near major tectonic faults. The internal structures of some ‘beef’ veins have revealed that they formed during Late Cretaceous to Tertiary compressional inversion of the basin. The typical composition for ‘beef’ is of calcite, with some pyrite and fragments of shale. However, inclusions or patches of hydrocarbons (liquid or solid) occur within calcite crystals or between fibres, respectively. According to some previous studies, as well as ours, ‘beef’ veins of the Wessex Basin represent natural hydraulic fractures, which formed as a result of fluid overpressure. This may have resulted in part from chemical compaction of petroleum source rocks, during Late Cretaceous to Tertiary times. Indeed, source rocks at outcrop in the Wessex Basin could be more mature than previously thought and the ‘beef’ veins may be good markers of maturation. •‘Beef’ veins (natural hydraulic fractures) are widespread in the Wessex Basin.•‘Beef’ veins are distributed within Mesozoic shales or mudstones.•Hydrocarbons are present within the fibres of ‘beef’ veins.•The development of ‘beef’ has been synchronous with hydrocarbon generation.•The formation of ‘beef’ has taken place during inversion of the Wessex Basin.