Utilising Outcrop Analogues to Understand Natural Fracture Systems in Shales

In unconventional hydrocarbon plays, shales can act as a seal, source rock and reservoir. Characterising the geometry and kinematics of natural fracture systems within shales is a critical step in understanding reservoir geomechanics, optimising well plans, and predicting induced seismicity. Because data from boreholes are inherently sparse and of limited spatial extent, and since fracturing in shales is often predominantly sub-seismic in scale, the use of outcrop based studies is critical to provide sufficiently rich datasets.

We illustrate the importance of outcrop analogues using the Cleveland Basin (N. England) as an integrated case study in which knowledge of the sedimentary, diagenetic, burial and tectonic histories are used to understand and predict fracture systems. Field acquisition of fracture data using terrestrial lidar, digital photogrammetry, and detailed fieldwork has allowed the following fracture characteristics to be quantified: orientation, curvature, length, height, aperture, intensity, spatial distribution, and connectivity. These parameters allow a mechanical stratigraphy to be superimposed upon the well-defined bio-stratigraphic sequence of the region.

The study area is 320m x 160m in size, and lies 1.5km from the closest known map-scale fault. The gas-prospective Carboniferous Bowland-Hodder shale unit underlies the study area. Four fracture sets are identified: NNW-SSE (steeply-dipping); WNW-ESE (steeply dipping); NW-SE (curved, more shallowly dipping); and ENE-WSW (curved, more shallowly dipping). Cumulative frequency plots of fracture size and fracture spacings show exponential distributions, and spacing is typically random.