Quantifying Fault Attributes at the Outcrop Scale: a Fault Network Model of Sub-Seismic Normal Faults in a Thinly-Bedded Reservoir Sequence

Three-dimensional (3D) seismic reflection data have revolutionized our knowledge of fault attributes in sedimentary basins. However, the resolution of such data is limited - faults with throws <30 m are not visible in typical 3D seismic profiles. Consequently, there are significant uncertainties in predicting the subsurface location, geometry, and connectivity of small-scale faults/fractures and hence the impact these structures have on reservoir performance. We can address this problem by capturing detailed (cm-resolution) 2.5D digital outcrop models (DOMs) of selected outcrops using terrestrial laser scanning (TLS) techniques. Here, we show an example of a DOM generated from a network of sub-seismic scale, post-sedimentary faults that cut a thinly bedded sandstone/shale sequence of Carboniferous age exposed at Lamberton, SE Scotland, and demonstrate how the DOM data may be used to quantify fault attributes to a remarkable level of detail. The faults are exposed in cross-section and plan view, respectively, in low cliffs and on a wave-cut platform defined by three sub- horizontal sandstone beds. The outcrop is characterized by small fault "scarps" (< 50 cm high) that formed due to erosion of the shale interbeds. We used TLS to capture the detailed surface topography of the three sandstone beds and adjacent cliffs and to build a cm-to m-scale fault network model of the outcrop. Attributes derived from the DOM include fault spacing, displacement-length profiles, 1- and 2-D strain, cumulative throw, fracture intensity, and fault connectivity. Analysis of the DOM shows that the faults are highly segmented and, in some cases, curvilinear in map view, with a mean trace spacing of ca. 30 cm and a mean throw of ca. 4 cm. The fault system is characterized by an abundance of relay ramps and rapid, along-strike changes in fault polarity consistent with a high degree of fault interaction. The relay zones themselves display a wide range of ramp dips and aspect ratios, consistent with the order of magnitude variation in horizontal displacement gradients measured at fault tips, between 0.026-0.26. The high spatial resolution of the Lamberton DOM gives hitherto unrecognised insights into fault and relay ramp geometry within a sub-seismic scale fault network, and has allowed us to evaluate and constrain fault system attributes to a level unachievable by field studies alone.