Towards a Quantitative Definition of Mechanical Units: New Techniques and Results from a Field Test Site

Bertotti, Giovanni¹, Nico J. Hardebol¹, Stefan M. Luthi², Jose Taal² (1) Vrije Universiteit Amsterdam, Amsterdam, Netherlands (2) Delft University of Technology, 2628 RX Delft, Netherlands

Physical properties of reservoirs are strongly influenced by distributed fracture fields. Outcrop studies are often used to determine them but have provided unsatisfactory results as the definition of mechanical units, i.e. groups of layers displaying constant fracture patterns (specifically spacing) is difficult and typically carried out in a qualitative manner. We have developed an innovative methodology to acquire and process fracture patterns in outcrops and to derive its mechanical stratigraphy. The method makes full use of GIS capabilities and allows for direct digital acquisition in the field leading to time-efficient acquisition. Data are processed with a newly developed routine which permits an objective description of changes of fracture characteristics along the stratigraphy of the outcrop. The operator is then able to decide the most suitable mechanic stratigraphy. Field data from a test site in the Karoo Basin (South Africa) demonstrate that mechanical interlayers, i.e. layers devoid of fracturing, are not the rule and that variations in fracture density and spacing often occur in a continuous manner without the interposition of joint-free layers. The data also show a negative, though weak correlation between spacing distance and bed thickness. This is in contrast with data from the literature but compatible with what is observed in scientific boreholes drilled in the same area. This downhole validation of surface fracture fields strongly supports the use of outcrop analogues for predicting subsurface fracture distributions.