Quantifying a critical marl thickness for vertical fracture extension using field data and numerical experiments

In fractured reservoirs characterized by low matrix permeability, fracture networks control the main fluid flow paths. However, in layered reservoirs, the vertical extension of fractures is often restricted to single layers. In this study, we explored the effect of changing marl/shale thickness on fracture extension using comprehensive field data and numerical modeling.
The field data were sampled from coastal exposures of Liassic limestone-marl/shale alternations in Wales and Somerset (Bristol Channel Basin, UK). The vertical fracture traces of more than 4000 fractures were mapped in detail. Six sections were selected to represent a variety of layer thicknesses. Besides the field data also thin sections were analyzed. Numerical models of fracture extension in a two-layer limestone-marl system were based on field data and laboratory measurements of Young's moduli. The modeled principal stress magnitude σ₃ along the lithological contact was used as an indication for fracture extension through marls. Field data exhibit good correlation (R²=0.76) between fracture extension and marl thickness, the thicker the marl layer the fewer fractures propagate through. The model results show that almost no tensile stress reaches the top of the marl layer when the marls are thicker than 30 cm. For marls that are less than 20 cm, the propagation of stress is more dependent on the stiffness of the marls. The higher the contrast between limestone and marl stiffness the lower the stress that is transmitted into the marl layer. In both model experiments and field data the critical marl thickness for fracture extension is ca. 15-20 cm.
This quantification of critical marl thicknesses can be used to improve predictions of fracture networks and permeability in layered rocks. Up-or downsampling methods often ignore spatially continuous impermeable layers with thicknesses that are under the detection limit of seismic data. However, ignoring these layers can lead to overestimates of the overall permeability. Therefore, the understanding of how fractures propagate and terminate through impermeable layers will help to improve the characterization of conventional reservoirs.