Fracture network formation in mechanical layers: field and numerical studies in the South Wales coalfield

Layer-scale extension fractures developed in stiff sandstones in mechanical layers were studied in the South Wales coalfield. Field structural data of extension fractures and bedding planes associated with compressive folds which developed at the latest stage of sedimentation in the basin were measured from the southern end to northern end of the basin. The geometrical relationship between the fractures and fold suggests the fractures developed before the folding and during basin burial. Numerical modelling conducted to assess the effect of stress rotation after the folding supports the hypothesis that extension fractures initiated during the basin burial. Fluid overpressure at the time of development of extension fractures which are now filled with mineral was examined with aspect (length/thickness) ratios of the veins. The results show the average fluid overpressure was about 35 MPa as maximum values. Mineral assembly of the mineral veins recognised under microscope suggests the driving mechanism for the fluid overpressure was mainly derived from local stresses that concentrated in the sandstones. Based on the results of the field studies and analytical calculations of fluid overpressure, an appropriate model is required to address extension fracture formation in stiff sandstone layers during basin burial. An analytical model which is simplified version of Bourne model with combination of fluid pressure was built. The analytical solutions indicate extension fractures could develop in stiff layers surrounded by soft material during basin burial. The important factors for the extension fracture formation is only not regional stress field. They can be formed in mechanical layers even in compressive stress field with three compressive regional principal stresses, depending on volume ratio of stiff and soft material, contrast of Young’s modulus, regional stress field and fluid pressure.