Appraisal of fracture sampling methods and a new workflow to characterise heterogeneous fracture networks at outcrop

Abstract

a b s t r a c tCharacterising fractures at outcrop for use as analogues to fractured reservoirs can use several methods. Four important fracture data collection methods are linear scanline sampling, areal sampling, window sampling and circular scanline sampling. In regions of homogeneous fracture networks these methods are adequate to characterise fracture patterns for use as outcrop analogues, however where fractures are heterogeneous, it is more difficult to characterise fracture networks and a different approach is needed.We develop a workflow for fracture data collection in a region of heterogeneous fractures in a fold and thrust belt, which we believe has applicability to a wide variety of fracture networks in different tectonic settings. We use an augmented circular scanline method, along with areal sampling to collect a range of fracture attribute data, including orientation, length, aperture, spatial distribution and intensity. This augmented circular scanline method more than halves the time taken for data collection, provides accurate, unbiased data that is representative of local fracture network attributes and involves data collection of a wider range of fracture attributes than other sampling techniques alone.

Influence of structural position on fracture networks in the Torridon Group, Achnashellach fold and thrust belt, NW Scotland

“…In order to investigate higher resolution variations in fracture attributes additional sampling sites are selected along linear transects either parallel to the regional transport direction or parallel to the fold hinge. These sampling sites are spaced 10e100 m apart, often on the same bedding plane as neighbouring sampling sites (for a full description of sampling site selection workflow, refer to Watkins et al, 2015).…”

“…for rapid data collection of a range of fracture attributes without incurring any orientation bias (see Watkins et al, 2015; for appraisal of fracture sampling techniques). Sampling circle radii varied depending on fracture intensity; minimum n counts of 30 were required to ensure results were representative (see Watkins et al, 2015).…”

“…Sampling circle radii varied depending on fracture intensity; minimum n counts of 30 were required to ensure results were representative (see Watkins et al, 2015). Circle radii range from 0.25 m in the highest fracture intensity regions to 2 m in very low intensity regions.…”

“…Aperture measurements are likely to have additional errors due to erosion and fracture widening, however a value for this error is not known. A minimum number of 30 n points has been set to ensure fracture set orientation data is representative (see Watkins et al, 2015).…”

Influence of Landscape Coverage on Measuring Spatial and Length Properties of Rock Fracture Networks: Insights from Numerical Simulation

“…airplanes (Reed 1940;Ray 1984), helicopters (Odling 1997), or satellites (Holland et al 2009;Bertrand et al 2015). This technique allows researchers to capture geological features at different length scales, especially those large-scale structures (Maerten et al 2001;Watkins et al 2015). It can also help identify anomalous areas that need further detailed field mapping, and eliminate or reduce the survey tasks in certain regions (Ray 1984).…”