Naturally fractured reservoirs contain networks of fractures that range in size from microscopic (sub-mm) to those visible in satellite imagery (typically 10m-10km in length). Mechanical layering exerts a critical influence in the development of fracture systems within reservoir units, and directly affects fracture orientations, spacing/intensity, segmentation and connectivity. Therefore an understanding of the distribution of mechanical packages in the reservoir, on the scale of individual beds up to entire stratigraphic formations or groups, is essential when representing natural fracture systems in the subsurface.
Outcrop analogues provide critical insight into the mechanical stratigraphy that cannot readily be derived from sub-surface data alone, although image logs or core are very useful to allow outcrop-derived measurements of the thicknesses and proportions of different mechanical packages to be suitably calibrated. In order to calibrate reservoir models we need analysis methods that can acquire data from representative outcrop analogues that span multiple scales, over large areas of exposed bedding surfaces as well as in cross-section bed-cuts.
New geospatial technologies such as terrestrial lidar and photogrammetry (ground-based or from unmanned aerial vehicles; i.e. UAVs or drones) are particularly well suited for capture of the detailed 3D geometry of the outcrop surface, from which quantitative measurements can be derived. This allows rapid acquisition of extensive areas of outcrop data, even from large areas of steep exposure that are unsafe or impractical to access directly.
We illustrate these concepts with field examples incorporating new data from the following areas: fractured carbonate reservoirs at Wadi Bih near Ras Al-Khaimah, UAE; Mesozoic reservoir units from the Kurdistan Region of Iraq; and tight shales from the Cleveland Basin, UK.